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PDH Course M226
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Psychrometric Chart Fundamentals and its application to HVAC Troubleshooting By: Timothy D. Blackburn, PE, MBA
Course Content Introduction If HVAC problems and challenges are to be properly diagnosed and designed, it is essential that the Psychrometric chart and psychrometrics in general be clearly understood. This course will review the essential elements of psychrometrics (the behavior of mixtures of air and water vapor under varying conditions of heat) and the chart that represents it. With this knowledge, you will be able to understand many of the HVAC challenges that are certain to arise, as well as anticipate problems before they occur and incorporate in your design. Like many self-study courses, you will only get as much from it as you put into it. Plot each step on your own psychrometric chart. Be certain you understand the basics before studying the examples. Try to solve the examples for yourself. This is not the type of course you can just read through and take the Quiz – it will take effort. But if you do your best, you should be able to pass the Quiz with ease – whether beginner or advanced. Disclaimer – Nothing in this course should be considered consulting engineering for your specific application – each situation requires individual analysis.
Definitions and Concepts It is important to understand the primary concepts and definitions before we begin our study. For some of you, this will be a refresher, and for others an introduction. There are other courses that cover the fundamentals in more detail, but the following are the minimum essentials.
Page 1 of 38
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HVAC – Heating, Ventilation, and Air conditioning
Psychrometrics –The behavior of mixtures of air and water vapor under varying conditions of heat
Enthalpy = Total heat in the air = Sensible plus Latent heat
Sensible Heat – Changes in temperature that do not alter the moisture content of air
Latent Heat – Related to level of moisture in the air
BTU (British Thermal Unit) – The amount of heat that must be added to or subtracted from a pound of water at 60oF to affect a temperature change of 1oF
BTUH or BH – BTU’s per hour
MBH – 1000 BTUH
Ton
1 Ton equals the amount of heat needed to melt 1 ton of ice in one day
12,000 BTUH’s
Drybulb Temperature – The temperature reading given by a dry thermometer that gives a direct indication as to the sensible heat content of air
Wetbulb Temperature – The temperature reading from a wetted bulb that gives a direct indication as to the total heat content of air
Dew Point Temperature – Temperature at which air will begin to release moisture.
Relative Humidity (RH)
The actual amount of moisture in the air expressed as a percentage of the amount of moisture the air is capable of holding.
More technically:
The amount of water vapor in the air divided by the amount of water vapor the air can hold (at the same temperature and pressure.)
Page 2 of 38
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The ratio of the air's vapor pressure to its saturation vapor pressure.
Example: An air sample that is at 50% RH is holding half the moisture it is capable of holding at the same temperature (at dew point or saturated.)
RH is inversely relational to temperature for the same moisture level (grains of moisture per pound of dry air) – warm air can hold more moisture
RH is what we sense
High RH: Sticking, mold
Low RH
Affects electronics, promotes static
Low RH air is seeking saturation, absorbing moisture wherever it can
Specific Humidity or Humidity Ratio
The weight of the water vapor in each pound of dry air
Typically grains of moisture/pound of dry air
Grain = 1/7000 pound
Density – Unit weight of dry air at a given temperature and moisture content, #/ft3
Specific Volume – Space occupied by dry air at a given temperature and moisture content (the reciprocal of density), ft3/#
The Psychrometric Chart Template Pause and print off the next page. Make multiple copies, as you will need them in the course. As noted previously, if you are to benefit from the course, you must plot each step for yourself. There are no shortcuts in learning this material. The smaller example charts are not intended for you to plot on, but rather instruction – use a clean larger copy to plot each example.
Page 3 of 38
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PSYCHRO M ETRIC CHART
75
70
Sea Level
www.PDHonline.org
80
85
90
300 1.9
BARO METRIC PRESSURE 29.921 inches of Mercury
290 65
10
0
280 17.0
16.0
60
1.8
270
95
1.7
260
95
95 250 1.6 240 BU
LB
90
TE
AIR
50
DR Y
- °F RE AT U PE R T EM ION RAT SAT U
9 0%
-25
-20
-15
-10
-5
0
5
10
120
100
% 80
90
15
15
20
20
25
25
30
35
45
50
% 70
.9 75 .8 70
.6 65
60
60
70
.5
.4
50
65
10
70
65
45
%
30 %
75
ID H UM T IV E
80
IT Y
20 10
85
90
95
60
50 .3
40 35
.2
30 25 20
.1
55
10 0 -40
100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 50
DRY BULB TEMPERATURE - °F
Linric Company Psychrometric Chart, www.linric.com
75
.7
50
%
20
60
1
60
30
55
80
80
55
RELA 10%
40
1.2
1.1
70
%
35 30
85
80
%
20
12.0
-30
40
30 25
10
130
40 45 13.0
0
5
140
70
40
50
40 35
10
150
85
1.3
DEW POINT - °F
OF ND POU ER - BT UP EN T HA L
PY
55
45
0
160
IR
50
5
170
A DRY
60
5
0
180
14.0
10
-5
200
F
110
65
55
-10
-°
lb OF
65
15
-15
E
³/ ME ft
75
60
-20
210
U VOL
75
20
-25
R
CIFIC
80
90 1.4
U
SPE
35
70
15
AT
15.0
80
1.5
220 R
190
85
30
-5
PE
85
40
25
M
90
45
90
230
15
20
Page 4 of 38
25
30
35
40
45
ENTHALPY - BTU PER POUND OF DRY AIR
ET
VAPOR PRESSURE - INCHES OF MERCURY
W
HUMIDITY RATIO - GRAINS OF MOISTURE PER POUND OF DRY AIR
95
55
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The Psychrometric Chart – an Overview
Before we proceed further in our study, let’s learn or refresh regarding the Psychrometric chart. The following is a summary of the major elements on the chart. Do not continue to the next section until you can find the following on the chart.
Constant Drybulb Temperature: Vertical Lines
Constant Dew Point and Humidity Ratio: Horizontal Lines
Constant Wetbulb temperature: Upward left sloping lines
Relative humidity: Curving lines (100% line is the saturation curve or correlates with Dew Point)
Constant specific volume, ft3/# of dry air: Nearly-vertical sloping lines
Enthalpy or total heat, BTU/pound of air: Staggered scale left of saturation curve and left sloping lines
Humidity Ratio: Right hand scale, grains of moisture/pound of dry air
Saturation Curve: 100% RH Curve (or the point at which an air mixture can hold no additional moisture at a given temperature); temperature on the curve is the Dew Point
The following is a chart with the above noted (note: this is a standard curve is at Sea Level).
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PSYCHROMETRIC CHART
75
70
Sea Level
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80
85
90
300 1.9
Heavy Text Bullet Slide Sample BAROMETRIC PRESSURE 29.921 inches of Mercury
290
65
10
0
280
130 120
90%
6
70 %
100
5
3
20
25
30
35
45
50
55
60
60 50
65
.9 75 .8 70
70
65
80
30 20 10
90
95
.6
.5
.4 60
50
50
Y MIDIT E HU
85
45
.3
40 35
.2
30 25 20
.1
55
10 0 -40
100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 50
DRY BULB TEMPERATURE - °F
Linric Company Psychrometric Chart, www.linric.com 10
15
70
65
60
60
1
75
75
.7
0%
V ELATI 10% R
40
1
DEW POINT - °F
30
80
80
55
2
20%
1.2
1.1
70
%
15
15
20
90
85
80
%
35
25
7
85
1.3
20
Page 6 of 38
25
30
35
40
45
ENTHALPY - BTU PER POUND OF DRY AIR
140
% 30 13.0
10
12.0
10
5
5
20
4
45 40
30 25
0
150
4
40
35 0
0
60
50
40
0
160
70
65
55
5
-5
170
110
80 %
65
45
-10 -5 -25 -20 -15
80
75
55
-30 -25 -20 -15 -10
8
75
180
70
50
10
80
30
10
5
190
°F TEM PER ATU RE SAT U
RA T ION
6
35
200
VAPOR PRESSURE - INCHES OF MERCURY
AIR DRY OF OUN D ER P Y-
BTU P
40
90 1.4
210 -° F
85
85
1.5
220 RA TU RE
90
45
90
230 TE M PE
HUMIDITY RATIO - GRAINS OF MOISTURE PER POUND OF DRY AIR
90
15
15
BU LB
50
60
-5
240
IR RY A OF D
8.
W ET
E ft³/lb LU M
7.
55
VO CIFIC SPE
6.
1.6 95
15.0
5.
95 250
ALP
3. 4.
95
1.7
260
95
14.0
Constant dry bulb temperature Constant Dew Point and Humidity Ratio Constant wet bulb temperature Relative humidity (100% line is the saturation curve or Dew Point) Constant specific volume, ft3/# of dry air Enthalpy or total heat, BTU/pound of air Humidity ratio or grains of moisture per pound of dry air Saturation curve – 100% humidity line or the point at which an air mixture can hold no additional moisture at a given temperature; 25 temperature on the curve is 20 the dewpoint
ENT H
1. 2.
17.0
16.0
60
1.8
270
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Latent versus Sensible Changes
In the next graph, let’s look at changes in Latent versus Sensible changes. Latent changes move in the “Y” axis (associated with moisture content changes), and Sensible changes move in the “X” axis (associated with temperature but not moisture content changes.)
PSYCHROMETRIC CHART
75
70
Sea Level
80
85
90
300
Heavy Text Bullet Slide Sample BAROMETRIC PRESSURE 29.921 inches of Mercury
1.9
290
65
10 0
280
17.0
16.0
60
1.8
270
95
1.7
260
95
95 250 1.6 240 ET
90
ND OF DRY AIR
TUR E-
°F
POU PER
ERA
BTU
ION T
Y-
EMP
AIR DRY
SAT URA T
lb OF
EN T HAL P
ft³/ UME
Sensible
90%
5
10
12.0
0
10
5
0
-5
15
15
20
20
25
30
30
35
80%
90
40
45
50
% 70
80 1
.9 75 .8 70
65
65
60 %
70
10
70
.6
.5
55 60
.4
45
%
30 20
75
80
85
90
10
95
60
50 .3
40 35
.2
30 25 20
.1
55
10 0 -40
100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 50
DRY BULB TEMPERATURE - °F
Linric Company Psychrometric Chart, www.linric.com
75
65
60
70
Y MIDIT VE HU ELATI 10% R
60
80
.7
80
20%
55
1.2
1.1
DEW POINT - °F
15
30 13.0
5
0
-5 -25 -20 -15 -10
10
25
70
50 %
45
40
35
25
110
40
35
20
120
85
50
40
-5
130
85
1.3
% 40
50
5
-30 -25 -20 -15 -10
60
55
45
30
140
14.0
15
55
150
100
65
60
50
160
VOL
20
10
170
CIFIC
75
70
65
180
SPE
80
75
25
Latent
200
15.0
80
90 1.4
210 -° F
190
85
1.5
220 RE
85
40
35
PE RA TU
90
45
90
230 TE M
50
30
0
BU LB
15
20
Page 7 of 38
25
30
35
40
45
ENTHALPY - BTU PER POUND OF DRY AIR
W
VAPOR PRESSURE - INCHES OF MERCURY
95
55
HUMIDITY RATIO - GRAINS OF MOISTURE PER POUND OF DRY AIR
Sensible versus Latent: Sensible changes are horizontal, and Latent changes are vertical
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Relationship of Dewpoint, RH, Wetbulb, and Drybulb The following chart provides an example to determine Dewpoint, RH, Wetbulb, or Drybulb if only two of the criteria are known. Drybulb is read with a typical thermometer. When I was in engineering school, we determined Wetbulb by using a sling psychrometer. In it was a Drybulb, plus another thermometer with a wet gauze. The Wetbulb reading was affected by the moisture content in the air; the lower the air moisture content, the faster the gauze evaporated and the cooler the temperature reading. Today, electronic instrumentation is generally used.
PSYCHROMETRIC CHART
75
70
Sea Level
80
85
90
300 1.9
Heavy Text Bullet Slide Sample BAROMETRIC PRESSURE 29.921 inches of Mercury
290
65
10 0
280
Relationship of Dewpoint, RH, Wetbulb, and Drybulb: Example: What is the Wet bulb temperature of 70oF air at 50% RH?
250 1.6
90%
100
80 %
70
90
20
25
30
35
40
45
50
70 %
60
.8 70
65
65
60
.6
.5
.4
20
10
10
Y MIDIT E HU LATIV % RE
85
90
95
60
50 45 DEW POINT - °F
30
80
.3
40 35
.2
30 25 20
.1
55
10 0 -40
100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180
DB
50
DRY BULB TEMPERATURE - °F
Linric Company Psychrometric Chart, www.linric.com 10
15
70
55
%
75
75
65
60
50
20%
70
80
.7
0%
30
55
.9 75
60
%
30
1
70
%
15
15
20
4
45 40
80
50
50
1.2
1.1
80
RH
55
35
25
60
50
85
20
Page 8 of 38
25
30
35
40
45
ENTHALPY - BTU PER POUND OF DRY AIR
IR
120
85
1.3 VAPOR PRESSURE - INCHES OF MERCURY
°F ERAT UR TEM PE ION RAT SAT U
A DRY
130
HUMIDITY RATIO - GRAINS OF MOISTURE PER POUND OF DRY AIR
AIR DRY ER P BTU P YENT HAL P
140
110
65
60
13.0
12.0
10
20
15 10
5
5
150
40
35 30 25
0
160
14.0
WB DP
0
0
80
75
65
40
0
170
lb OF
20
5
-5
80
75
45
-5 -25 -20 -15 -10
180
ft³/ UME VOL
35
55
-30 -25 -20 -15 -10
200
CIFIC SPE
40
90 1.4
210 -° F
190
85
1.5
220 UR E
85
OU ND
OF
45
90
230 TE M PE RA T
70
10
10
240 BU LB
90
30
15
5
W ET
90
25
-5
95
50
What is the Dew Point? (DP) Answer:
95
55
58.43oF
50.53oF
95
1.7
260
95
15.0
Answer:
17.0
16.0
60
1.8
270
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Impact to Relative Humidity from Sensible Changes
The following chart illustrates the impact to Relative Humidity from changes in temperature (when moisture content remains constant.) You will note that Temperature and Relative Humidity are inversely relational – the higher the temperature, the lower the RH. The lower the temperature, the higher the RH.
PSYCHROMETRIC CHART
75
70
Sea Level
80
85
90
300
Heavy Text Bullet Slide Sample BAROMETRIC PRESSURE 29.921 inches of Mercury
1.9
290
65
10 0
280
17.0
16.0
60
1.8
270
95
1.7
260
95
95 250 1.6
120
°F PER ATU RE TEM ION 90%
100
80%
70
90
25
30
40
% 70
1
.9 75 .8 70
45
50
55
60
65
.6
.5
60
55 .4
%
30 20
10
10
Y MIDIT E HU LATIV % RE
75
80
85
90
95
60
50 45
.3
40 35
.2
30 25 20
.1
55
10 0 -40
100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 50
DRY BULB TEMPERATURE - °F
Linric Company Psychrometric Chart, www.linric.com 10
70
65
60
60
20%
70
75
70
50 %
65
80
.7
DEW POINT - °F
30
35
80
80
%
20
25
1.1
15
20
Page 9 of 38
25
30
35
40
45
ENTHALPY - BTU PER POUND OF DRY AIR
130
1.2
VAPOR PRESSURE - INCHES OF MERCURY
AIR DRY POU PER Y-B TU ENT HAL P
SAT URA T
IR RY A OF D
15
15
20
30 13.0
20
12.0
10
140
40 45
35
25
10
150
85
% 40
50
40
30
5
160
85
1.3
50
40 35 0
5
170
ft³/lb UME VOL
55
5
0
180
14.0
60
50 45
0
200
110
65
60
55
0
F
IFIC PEC
65
10
-5
75
90 1.4
210 -°
70
15
-10 -5 -25 -20 -15
80
75
25
-30 -25 -20 -15 -10
80
1.5
220 RE
S 15.0
40
35
TE M PE RA TU
190
85
90
230
85
ND
OF
45
20
15
BU LB
90
30
Answer: 84.85%
10
240
90
What if we cool to 55oF instead?
5
W ET
50
Answer: 25.98%
-5
95
55
HUMIDITY RATIO - GRAINS OF MOISTURE PER POUND OF DRY AIR
Sensible Changes: Example: If we heat 70oF at 50% RH to 90oF without adding moisture, what is the new RH?
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Dehumidification As we saw in the previous section, when the temperature cools the RH increases. What happens when the RH is 100%? It rains. As cools, it eventually reaches its dewpoint and moisture begins to appear. That is what happens when you have a glass of iced tea, and there is moisture on the outside – the air in the immediate vicinity of the glass cools to the point it is fully saturated, and out comes moisture. In the following example, we continue on with the previous example to examine what happens when we continue to cool.
PSYCHROMETRIC CHART
75
70
Sea Level
80
85
90
300
Heavy Text Bullet Slide Sample BAROMETRIC PRESSURE 29.921 inches of Mercury
1.9
290
65
10 0
280
17.0
16.0
60
1.8
270
95
1.7
260
95
95 250 1.6 240 BU LB
90
TE
AIR
50
DRY OF
190
°F
OUN D
RAT UR
E-
ER P
TEM PE
BTU P Y-
180
80
170 160
80
150 140
75
130
DRY
SAT URA TIO N
ALP
200
lb OF
ENT H
F
ft³/ UME VOL
35
-°
CIFIC SPE
40
120
AIR
30
75
70 90%
110 100
70
80%
65
25 20
90
70
14.0
%
65
60
80 1
.9 75 .8 70
65
45
-5
0
10
5
0
-5
5
15
10
12.0
0
-5 -25 -20 -15 -10
10
5
20 15
15
20
20
25
25
30
30
35
40
45
30 13.0
35
25
-30 -25 -20 -15 -10
40
30
50
%
30
20%
20
Y MIDIT VE HU ELATI 10% R
55
60
65
.5
.4
45 40
40 35 0
.6
70
75
80
85
90
95
10
60
50
DEW POINT - °F
5
.3
40 35
.2
30 25 20
.1
55
10 0 -40
100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 50
DRY BULB TEMPERATURE - °F
Linric Company Psychrometric Chart, www.linric.com 10
70
55
% 40
50
75
65
60
50 45
80
.7
60
%
55
1.2
1.1
50
50
85
70
60
60
55 10
85
1.3
80
%
15
90 1.4
210
85
85
1.5
220 UR E
15.0
Answer: 50.53oF is the Dew Point. Cooling below that point removes water
M PE RA T
90
45
90
230
15
20
Page 10 of 38
25
30
35
40
45
ENTHALPY - BTU PER POUND OF DRY AIR
W ET
VAPOR PRESSURE - INCHES OF MERCURY
95
55
HUMIDITY RATIO - GRAINS OF MOISTURE PER POUND OF DRY AIR
Dehumidification: In the previous example, what happens when I cool below 50.53oF?
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Humidification In the previous example, we learned that we can de-humidify the air by over cooling it. But to add humidity we must have an external source of moisture. The following example illustrates that we can plot the results of adding moisture on the Psychrometric chart.
PSYCHROMETRIC CHART
75
70
Sea Level
80
85
90
300
Heavy Text Bullet Slide Sample Note: Steam BAROMETRIC PRESSURE 29.921 inches of Mercury
10 0
280
85
80
TEM ION
80
75
30
75
200 190 180 170 160 150 140 130 120
70
% 70
45
0
10
5
0
-5
5
10
12.0
0
-10 -5 -25 -20 -15
20 15
15
20
20
25
25
30
60
Steam
30
40
45
50
1
.9 75 .8 70
60
65
70
10
75
70
.7 65
.6 65
60
.5
.4
E LATIV % RE
75
80
20
ITY HUMID
85
10
90
95
60
50 45
.3
40 35
.2
30 25 20
.1
55
10 0 -40
100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 50
DRY BULB TEMPERATURE - °F
Linric Company Psychrometric Chart, www.linric.com
80
55
30
20%
55
80
60
%
10
35
1.1
50
30
13.0
35
25
-30 -25 -20 -15 -10
40
1.2
70
40
40 35 30
85
% 40
50
5
85
1.3
80
%
45
0
60
55 Evap.
50
50 %
55 10
10
90 14.0
15
5
70
65
60
15
100
80%
65
25 20
-5
110 90%
55.86oF
90 1.4
210
F
15
20
Page 11 of 38
25
30
35
40
45
ENTHALPY - BTU PER POUND OF DRY AIR
°F PER ATU RE -
220
IR RY A OF D
SAT URA T
-°
1.5
VAPOR PRESSURE - INCHES OF MERCURY
AIR
85
ND
OF
DRY
90
POU PER Y-B TU ENT HAL P
UR E
90
230
ft³/lb UME VOL
35
TE M PE RA T
IFIC PEC
What is the new DP?
BU LB
1.6 240
S 15.0
Answer: 60.81%
W ET
250
HUMIDITY RATIO - GRAINS OF MOISTURE PER POUND OF DRY AIR
90 50
95
increases enthalpy maintaining temperature; evaporation maintains enthalpy and lowers temperature 95
55
40
95
1.7
260
95
45
1.8
270
DEW POINT - °F
Humidification: At 70oF and 50% RH, what is the new RH when we add 12 gr/lb of moisture?
17.0
16.0
60
Answer:
1.9
290
65
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PDH Course M226
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Enthalpy Changes Enthalpy represents the total heat in the air, a summation of Sensible and Latent. Enthalpy is measured in BTU/# dry air, and can be determined if you know at least two primary chart parameters. As well, when you condition the air, there is a change in enthalpy that can be calculated. The following example illustrates how Enthalpy changes can be calculated, as well as changes to enthalpy.
PSYCHROMETRIC CHART
75
70
Sea Level
80
85
90
300
Heavy Text Bullet Slide Sample BAROMETRIC PRESSURE 29.921 inches of Mercury
1.9
290
65
10 0
280
17.0
16.0
60
1.8
270
95
1.7
260
95
95 250 1.6
°F PER ATU RE TEM ION SAT URA T
90%
80%
70
90
15
15
20
25
30
30
35
40
% 70
1
.9 75 .8 70
45
50
55
60
65
.6
.5
60
55 .4
%
30 20
10
10
Y MIDIT E HU LATIV % RE
75
80
85
90
95
60
50 45
.3
40 35
.2
30 25 20
.1
55
10 0 -40
100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 50
DRY BULB TEMPERATURE - °F
Linric Company Psychrometric Chart, www.linric.com 10
70
65
60
60
20%
70
75
70
50 %
65
80
.7
DEW POINT - °F
25
80
80
%
12.0
10
20
30
13.0
20
15
100
40 45
35
25
10
120
1.2
1.1
% 40
50
40
30
5
130
85
50
40 35 0
5
140
14.0
55
5
0
150
110
65
60
50 45
0
160
85
1.3
15
20
Page 12 of 38
25
30
35
40
45
ENTHALPY - BTU PER POUND OF DRY AIR
AIR DRY POU PER Y-B TU ENT HAL P
65
55
0
170
70
10
-5
180
IR RY A OF D
75
75
15
-10 -5 -25 -20 -15
200
ft³/lb UME VOL
80
30
60
-30 -25 -20 -15 -10
F
IFIC PEC
80
90 1.4
210 -°
190
85
1.5
220 UR E
S 15.0
35
TE M PE RA T
85
ND
OF
45
90
230
90
40
25
10
BU LB
90
20
5
240
50
Answer: 33.6825.33 = 8.35 BTU/Lb Dry Air
-5
W ET
VAPOR PRESSURE - INCHES OF MERCURY
95
55
HUMIDITY RATIO - GRAINS OF MOISTURE PER POUND OF DRY AIR
Enthalpy Changes: Example: What is the change in enthalpy when you go from 70F/50% RH to 80F/60% RH?
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PDH Course M226
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Heating Cycles Heating cycles can be graphically illustrated on the chart. The following illustrates a typical heating cycle of an HVAC system.
PSYCHROMETRIC CHART
75
70
Sea Level
80
85
90
300
Heavy Text Bullet Slide Sample BAROMETRIC PRESSURE 29.921 inches of Mercury
1.9
290
65
10 0
280
17.0
16.0
60
1.8
270
95
1.7
260
95
95 250 1.6
120
90%
110 70
100
80 %
5 (SA)
70 %
90
20
25
30
35
45
50
60
.8 70
65
65
10
60
70
.6 65
60
.5
.4
70
40
4 (SA)
%
E LATIV % RE
75
80
30 20
ITY HUMID
85
10
90
95
60
50 45
.3
40 35
.2
30 25 20
.1
55
10 0 -40
100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 50
DRY BULB TEMPERATURE - °F
Linric Company Psychrometric Chart, www.linric.com
75
.7
50
0%
20%
55
75
60
10
40
.9
80
55
30
1 (OA)
20
1
70
%
30
80
50
4
1.2
1.1
80
%
15
15
25
85
15
20
Page 13 of 38
25
30
35
40
45
ENTHALPY - BTU PER POUND OF DRY AIR
°F PER ATU RE TEM ION SAT URA T
130
85
1.3 VAPOR PRESSURE - INCHES OF MERCURY
AIR DRY ER P BTU P YALP ENT H
140
AIR DRY
12.0
10
10
150
lb OF
35
25 20
50
13.0
40
30
55
45
35 0
5
160
ft³/ UME VOL
40
5
170
14.0
5
0
75
65 2 (RA) 60
50
5
CIFIC SPE
65
3 45(MA)
0
200
180
75
55
0
F
70
10
-5
80
30
15
-5 -25 -20 -15 -10
80
90 1.4
210 -°
190
85
1.5
220 UR E
15.0
40
35
TE M PE RA T
85
OUN D
OF
45
90
230
90
60
-30 -25 -20 -15 -10
BU LB
90
25
10
240
50
20
15
W ET
DEW POINT - °F
Outside Air (1) is mixed with room Return Air (2) and results in Mixed Air (3). Mixed Air is heated up to (4). Humidity may be added to (5), the Supply Air or via space humidifiers. The room absorbs heat (and moisture with ventilation or special process, etc.) resulting in Return Air (2).
-5
95
55
HUMIDITY RATIO - GRAINS OF MOISTURE PER POUND OF DRY AIR
Heating Cycle
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PDH Course M226
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Cooling Cycle Similar to the previous example, the Cooling Cycle can also be plotted on the Chart as follows.
PSYCHROMETRIC CHART
75
70
Sea Level
80
85
90
300 1.9
Heavy Text Bullet Slide Sample BAROMETRIC PRESSURE 29.921 inches of Mercury
290
65
10 0
280
17.0
16.0
60
1.8
270
95 1.7
260
95
95 250 1.6 240
90
AIR
50
URE
- °F
POU PER
RAT
Y-
150 140
75
130
DRY
SAT U
80
lb OF
R AT ION T
ALP
160
³/ ME ft
EM PE
BTU
80
170
U VOL
35
180
CIFIC SPE
ENT H
200
15.0
40
120
AIR
30
75
70 90%
110
70 %
%
40
35
25 5
0
10
5
0
-5
5
10
12.0
0
-5 -25 -20 -15 -10
20 15
15
20
20
25
25
30
30
20% 10%
35
40
.9 75 .8 70
45
50
55
60
65
70
80
40 30
85
90
20 10
95
65
10
15
70
.6
65 60
.5
.4
60
50 45
.3
40 35
.2
30 25 20
.1
55
10 0 -40
100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180
50
DRY BULB TEMPERATURE - °F
Linric Company Psychrometric Chart, www.linric.com
75
55
ITY HUMID TIVE RELA
75
80
.7
60
230%(RA) 13.0
40
30
1
% 40
45
35
0
80
50 50
1.2
1.1
70
3 (MA)
%
55
4 (SA)
5
85
50
50 45
-30 -25 -20 -15 -10
60
90
85 1.3
80
60
55
10
1 (OA) 14.0
15 10
15
70
65
60
-5
100
80 %
65
25 20
90 1.4
210 -° F
190
85
1.5
220
85
ND
OF D
RY
90
45
90
230 TE M PE RA TU RE
20
Page 14 of 38
25
30
35
40
45
ENTHALPY - BTU PER POUND OF DRY AIR
BU LB
DEW POINT - °F
Outside Air (1) is mixed with room Return Air (2) and results in Mixed Air (3). Mixed Air is cooled and dehumidified to saturation to (4). The room heat and moisture is transferred to to the Return Air (2).
W ET
VAPOR PRESSURE - INCHES OF MERCURY
95
55
HUMIDITY RATIO - GRAINS OF MOISTURE PER POUND OF DRY AIR
Cooling Cycle
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PDH Course M226
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Try it yourself! (A Pop Quiz)
(This isn’t the official course Quiz)
Now that you have reviewed (or learned) the basics of psychrometrics and the chart, give a try to answer the following (Print off this page and the Chart at the beginning of the course; circle the best answer):
For an air mixture of 74F and 45% RH, determine the following:
1.What is the dewpoint ? a)51 b)41 c)65 2.What is the enthalpy? a)11 BTU/# Dry Air b)27 BTU/# Dry Air c)31 BTU/# Dry Air 3.How many grains of moisture/# dry air? a)33 b)40 c)56 4. What is the specific volume in cf/# dry air?
Page 15 of 38
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a. 13.6 b. 11.2 c. 12.1 5.What is the wetbulb temperature? a.50 b.60 c.74
Page 16 of 38
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Pop Quiz Answers The following are the correct answers: 1-a; 2-b; 3-c; 4-a; 5-b. How well did you do? If you struggled with this, the following are the Charts that give a step-by-step illustration – compare it to your chart.
PSYCHROMETRIC CHART
75
70
Sea Level
80
85
90
300
Heavy Text Bullet Slide Sample BAROMETRIC PRESSURE 29.921 inches of Mercury
1.9
290
65
10 0
280
17.0
16.0
60
1.8
270
95
1.7
260
95
95 250 1.6 240
90
AIR
50
DRY OF
°F
OUN D
PER ATU RE -
ER P
80
ION
ALP
TEM
Y-
BTU P
190
80
75
160 150 140 130
AIR DRY
30
170
lb OF
SAT URA T
180
ft³/ UME VOL
ENT H
200
CIFIC SPE
35
F
15.0
40
75
120
70 90%
110
20
70 %
%
35
0
10
5
0
-5
5
10
12.0
0
-5 -25 -20 -15 -10
10
20 15
15
20
20
25
25
30
30
40
45
50
%
30
20%
55
60
.9 75 .8 70
65
65
10
70
.6 65
60
.5
.4
70
E HU LATIV % RE
75
80
85
20
Y MIDIT
10
90
95
60
50 45
.3
40 35
.2
30 25 20
.1
55
10 0 -40
100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 50
DRY BULB TEMPERATURE - °F
Linric Company Psychrometric Chart, www.linric.com
75
.7
50
0%
10
35
1
80
55
30 13.0
40
30 25
4
45
35 0
80
60
40
40
1.2
1.1
70
%
50
5
85
50
55
45
-30 -25 -20 -15 -10
60
50
85
1.3
80
60
55 10
5
14.0
15
15
90
65
60
-5
100
70
80 %
65
25
90 1.4
210 -°
85
85
1.5
220 UR E
90
45
90
230 TE M PE RA T
15
20
Page 17 of 38
25
30
35
40
45
ENTHALPY - BTU PER POUND OF DRY AIR
BU LB
VAPOR PRESSURE - INCHES OF MERCURY
W ET
HUMIDITY RATIO - GRAINS OF MOISTURE PER POUND OF DRY AIR
95
55
DEW POINT - °F
Step 1: Plot the conditions – pick a point at 74oF DB and 45% RH
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PDH Course M226
PSYCHROMETRIC CHART
75
70
Sea Level
www.PDHonline.org
80
85
90
300
Heavy Text Bullet Slide Sample BAROMETRIC PRESSURE 29.921 inches of Mercury
1.9
290
65
10 0
280
17.0
16.0
60
1.8
270
95
1.7
260
95
95 250 1.6 240
90 50 AIR DRY OF
°F
OUN D
PER ATU RE -
ER P BTU P
TEM
Y-
ION
ALP
130
AIR DRY
SAT URA T
ENT H
lb OF
140
120
90%
110 100
80 %
70
90
15
20
20
25
25
30
40
45
70 %
60
1
.9 75 .8 70
65
60
70
.6
.5
.4
50
65
10
70
65
60
% %
30
E HU LATIV % RE
75
80
85
20
Y MIDIT
10
90
95
60
50 45
.3
40 35
.2
30 25 20
.1
55
10 0 -40
100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 50
DRY BULB TEMPERATURE - °F
Linric Company Psychrometric Chart, www.linric.com
75
.7
50
20%
55
80
80
55
%
50
1.2
1.1
60
10
35
85
80
%
15
30
85
1.3
70
30 13.0
10
20
12.0
10
5
5
ft³/ UME VOL
45 40
35
25
0
150
14.0
40 30
0
160
40
35 0
-5
75
40
50
5
-5 -25 -20 -15 -10
60 55
45
-30 -25 -20 -15 -10
80
65
55 50
15
CIFIC SPE
65
25
10
10
170
70
15
5
200
180
75
60
0
80
30
20
-5
F
15.0
•Read Humidity Ratio on the right, 56.37 grains/pound of dry air
-°
190
85
90 1.4
210
85
40
35
220 UR E
90
45
1.5
DEW POINT - °F
•Read Dewpoint on the left at the 100% saturation curve, 51.34oF
90
230 TE M PE RA T
15
20
Page 18 of 38
25
30
35
40
45
ENTHALPY - BTU PER POUND OF DRY AIR
BU LB
VAPOR PRESSURE - INCHES OF MERCURY
W ET
HUMIDITY RATIO - GRAINS OF MOISTURE PER POUND OF DRY AIR
Step 2: Draw a horizontal line.
95
55
www.PDHcenter.com
PDH Course M226
PSYCHROMETRIC CHART
75
70
Sea Level
www.PDHonline.org
80
85
90
300
Heavy Text Bullet Slide Sample BAROMETRIC PRESSURE 29.921 inches of Mercury
1.9
290
65
10
0
280
17.0
16.0
60
1.8
270
95
1.7
260
95
95 250 1.6 240 BU LB
90
AIR
50
OF D
RY
90
45
190
°F
OUN D
PER ATU R
E-
ER P
80
TEM
Y-
ION
ALP
170 160
80
75
30
150 140 130
Y AIR
SAT U
RAT
35
180
R OF D ft³/lb
ENT H
200
UME VOL
BTU P
40
75
120
70 90%
110
80
50
70 % 55
45 5
40
45
-5
0
10
5
0
-5
5
15
10
12.0
-10 -5 -25 -20 -15
10
5
0
20 15
15
20
20
25
25
30
30
35
40
45
50
60 %
30 20
ITY UMID VE H ELATI 10% R
60
65
1
.9 75 .8 70
10
70
65 .6 65
60
.5
70
.4
75
80
85
90
10
95
60
50 45
.3
40 35
.2
30 25 20
.1
55
10 0 -40
100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 50
DRY BULB TEMPERATURE - °F
Linric Company Psychrometric Chart, www.linric.com
75
.7
50
20%
55
80
80
55
% 30 13.0
25
-30 -25 -20 -15 -10
40
35
1.2
1.1
60
40
35
85
70
% 40
50
85
1.3
80
50 %
60
55 10
90 14.0
15
30
70
65
60
0
100
%
65
25 20
90 1.4
210 -° F
85
85
1.5
220 UR E
CIFIC SPE
At the saturation curve Wet Bulb = 60oF
PE RA T
15.0
Enthalpy = 26.57 BTU/# dry air
90
230 TE M
15
20
Page 19 of 38
25
30
35
40
45
ENTHALPY - BTU PER POUND OF DRY AIR
ET
VAPOR PRESSURE - INCHES OF MERCURY
W
HUMIDITY RATIO - GRAINS OF MOISTURE PER POUND OF DRY AIR
95
55
DEW POINT - °F
Step 3: Draw diagonal lines and read
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PDH Course M226
PSYCHROMETRIC CHART
75
70
Sea Level
www.PDHonline.org
80
85
90
300 1.9
Heavy Text Bullet Slide Sample BAROMETRIC PRESSURE 29.921 inches of Mercury
290
65
10 0
280
17.0
16.0
60
1.8
270
95
1.7
260
95
95 250 1.6 240 BU LB
90
AIR
50
RY OF D ND
190
URE
- °F
POU PER
200
RAT
80
EM PE
Y-
160 150 140
75
130
AIR DRY
SAT U
80
30
170
lb OF
R AT ION T
ALP 35
180
ft³/ UME VOL
BTU
-° F
CIFIC SPE
40
75
120
70 90%
110
5
10
70 %
%
15
15
20
20
25
30
30
35
40
75 .8 70
65
45
50
%
30 20
ITY UMID VE H ELATI 10% R
55
60
65
.6
.5
.4
70
75
80
85
90
10
95
60
50 45
.3
40 35
.2
30 25 20
.1
55
10 0 -40
100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 50
DRY BULB TEMPERATURE - °F 10
70
65
60
0%
20%
Linric Company Psychrometric Chart, www.linric.com
75
.7
DEW POINT - °F
25
.9
80
55
30 13.0
0
10
5
0
-5
15
12.0
-5 -25 -20 -15 -10
10
4
45 40
35
25 20
1
60
40
35 30
80
70
% 50
40
1.2
1.1
50
5
0
85
50
55
45
-30 -25 -20 -15 -10
60
50
85
1.3
80
60
55 10
5
14.0
15
0
90
65
60
-5
100
70
80 %
65
25 20
90 1.4
210
85
85
1.5
220
15.0
ENT H
TE M PE RA TU RE
90 45
90
230
15
20
Page 20 of 38
25
30
35
40
45
ENTHALPY - BTU PER POUND OF DRY AIR
W ET
VAPOR PRESSURE - INCHES OF MERCURY
95
55
HUMIDITY RATIO - GRAINS OF MOISTURE PER POUND OF DRY AIR
Step 4: Draw a line parallel to the specific volume = 13.6 ft3/# dry air
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HVAC Troubleshooting Examples Now lets consider some specific HVAC troubleshooting examples. Before looking at the charts provided with the answers, try to plot the information yourself. If you don’t do this, you will not be able to follow the examples. Some of the examples will be more challenging than others – if you don’t have a HVAC background, you likely will struggle at some points. Don’t be overly alarmed. If you have a strong grasp to this point, you are well on your way and passing the Quiz should be no problem. The goal for the student is to be able to use the Psychrometric chart to solve virtually any HVAC problem/challenge that involves air temperature/moisture issues.
HVAC Troubleshooting Example #1 You have a complaint of moisture condensing in a cooling chamber in a packaging line. You find you need to cool the chamber to 40oF. The desired room temperature is 70oF.
A. What relative humidity in the space is needed?
Answer: To keep the condensation from occurring, keep the RH below 100% (say 90%) at the cooling chamber temperature (40oF). Plot this condition, and draw a horizontal line that intersects 70oF and read the RH, which is 30.15% (Note: This is very low and uncomfortable for occupants – consider a local environment)
B. Answer: The lowest discharge temperature of the airhandler is 52oF. Can you satisfy the conditions needed above?
No. Plot from 52oF and 90% RH horizontally to 70oF, and the minimum RH expected is 47.51% >> 30.15% - condensation will occur. (To lower further would require more expensive systems.)
Page 21 of 38
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PDH Course M226
PSYCHROMETRIC CHART
75
70
Sea Level
www.PDHonline.org
80
85
90
300 1.9
Heavy Text Bullet Slide Sample BAROMETRIC PRESSURE 29.921 inches of Mercury
290
65
10 0
280
17.0
16.0
60
1.8
270
95
1.7
260
95
95 250 1.6 240 BU LB
90
AIR
50
RY OF D ND
190
URE
- °F
POU PER
200
RAT
80
EM PE
Y-
160 150 140
75
130
AIR DRY
SAT U
80
30
170
lb OF
R AT ION T
ALP 35
180
ft³/ UME VOL
BTU
-° F
CIFIC SPE
40
75
120
70 90%
110
5
10
70 %
%
15
15
20
20
25
30
30
35
40
75 .8 70
65
45
50
%
30 20
ITY UMID VE H ELATI 10% R
55
60
65
.6
.5
.4
70
75
80
85
90
10
95
60
50 45
.3
40 35
.2
30 25 20
.1
55
10 0 -40
100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 50
DRY BULB TEMPERATURE - °F 10
70
65
60
0%
20%
Linric Company Psychrometric Chart, www.linric.com
75
.7
DEW POINT - °F
25
.9
80
55
30 13.0
0
10
5
0
-5
15
12.0
-5 -25 -20 -15 -10
10
4
45 40
35
25 20
1
60
40
35 30
80
70
% 50
40
1.2
1.1
50
5
0
85
50
55
45
-30 -25 -20 -15 -10
60
50
85
1.3
80
60
55 10
5
14.0
15
0
90
65
60
-5
100
70
80 %
65
25 20
90 1.4
210
85
85
1.5
220
15.0
ENT H
TE M PE RA TU RE
90 45
90
230
15
20
Page 22 of 38
25
30
35
40
45
ENTHALPY - BTU PER POUND OF DRY AIR
W ET
VAPOR PRESSURE - INCHES OF MERCURY
95
55
HUMIDITY RATIO - GRAINS OF MOISTURE PER POUND OF DRY AIR
Example #1
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PDH Course M226
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HVAC Troubleshooting Example #2
You would like to humidify a space that has a 100% outside air unit to 50% RH in the winter, and would like to keep the discharge RH from exceeding 80% (ignoring any latent gains in the space). However, you get frequent low humidity alarms in the winter. Why?
First, check to see if the Psychrometrics are possible.
Plot the conditions off the coil, 52oF discharge at 80%, and draw a horizontal line to 70oF. The maximum RH possible is 42.23% << 50% - system will not reach conditions under some winter outside air possibilities.
Note: It may be possible to raise the discharge temperature to absorb more moisture and/or raise permitted RH after the coil.
Page 23 of 38
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PDH Course M226
PSYCHROMETRIC CHART
75
70
Sea Level
www.PDHonline.org
80
85
90
300 1.9
Heavy Text Bullet Slide Sample BAROMETRIC PRESSURE 29.921 inches of Mercury
290
65
10 0
280
17.0
16.0
60
1.8
270
95
1.7
260
95
95 250 1.6 240 BU LB
90
AIR
50
RY OF D ND
190
URE
- °F
POU PER
200
RAT
80
EM PE
Y-
160 150 140
75
130
AIR DRY
SAT U
80
30
170
lb OF
R AT ION T
ALP 35
180
ft³/ UME VOL
BTU
-° F
CIFIC SPE
40
75
120
70 90%
110
5
10
70 %
%
15
15
20
20
25
30
30
35
40
75 .8 70
65
45
50
%
30 20
ITY UMID VE H ELATI 10% R
55
60
65
.6
.5
.4
70
75
80
85
90
10
95
60
50 45
.3
40 35
.2
30 25 20
.1
55
10 0 -40
100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 50
DRY BULB TEMPERATURE - °F 10
70
65
60
0%
20%
Linric Company Psychrometric Chart, www.linric.com
75
.7
DEW POINT - °F
25
.9
80
55
30 13.0
0
10
5
0
-5
15
12.0
-5 -25 -20 -15 -10
10
4
45 40
35
25 20
1
60
40
35 30
80
70
% 50
40
1.2
1.1
50
5
0
85
50
55
45
-30 -25 -20 -15 -10
60
50
85
1.3
80
60
55 10
5
14.0
15
0
90
65
60
-5
100
70
80 %
65
25 20
90 1.4
210
85
85
1.5
220
15.0
ENT H
TE M PE RA TU RE
90 45
90
230
15
20
Page 24 of 38
25
30
35
40
45
ENTHALPY - BTU PER POUND OF DRY AIR
W ET
VAPOR PRESSURE - INCHES OF MERCURY
95
55
HUMIDITY RATIO - GRAINS OF MOISTURE PER POUND OF DRY AIR
Example #2
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PDH Course M226
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HVAC Troubleshooting Example #3 On a day when outside air reached 80F/60RH, we noticed we were not maintaining space conditions. Assume we need 52F discharge (saturated at 90% RH) to maintain space conditions with 10,000 cfm supply air. We did not maintain space conditions that day. We installed a 25T chiller for the 100% outside air unit. Why is the system not working?
To diagnose the problem, first check the loads. Enthalpy needing to be removed is 33.68-20.52 = 13.16 BTU/lb dry air
Calculate BTUH = cfm * 4.5 * (h1 – h2) = (10000)(4.5)(13.16) = 592,200 BTUH
To convert to Tons of cooling, = 592,200/12,000 = 49.35 Tons >25 – chiller was undersized.
PSYCHROMETRIC CHART
75
70
Sea Level
80
85
90
300 1.9
Heavy Text Bullet Slide Sample BAROMETRIC PRESSURE 29.921 inches of Mercury
290
65
10 0
280
17.0
16.0
60
1.8
270
95
1.7
260
95
95 250 1.6 240 ET
BU
LB
90
AIR
50
RY ND OF D
RAT U
RE
- °F
POU PER
EMP E ION T
Y-
75
160 150 140
IR RY A OF D
SAT URA T
170
E ft³/lb L UM IC VO
80
30
180
IF PEC
BTU
200
S 15.0
80
75
130 120
70 90%
110
% 70
14.0
55
85
80 1
.9 75 .8 70
65
40
40
45
35 35
25 -5
10
5
0
5
15 10
5
0
-5
10
20
12.0
0
-5 -25 -20 -15 -10 -30 -25 -20 -15 -10
40
30
15
15
20
20
25
25
30
30
35
40
45
30 13.0
0
50
%
30
20%
20
ITY UMID VE H ELATI 10% R
55
60
65
70
.6
.5
.4
75
80
85
90
10
95
60
50 45
.3
40 35
.2
30 25 20
.1
55
10 0 -40
100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 50
DRY BULB TEMPERATURE - °F
Linric Company Psychrometric Chart, www.linric.com 10
70
55 60
DEW POINT - °F
5
75
65
60
% 40
50
80
.7
50 45
1.2
1.1
70
50 %
60
55
85
1.3
80
60 %
15
50
90
65
60
10
100
70
80%
65
25 20
90 1.4
210 -° F
190
85
1.5
220 UR E
85
40
35
TE M PE RA T
90
45
90
230
15
20
Page 25 of 38
25
30
35
40
45
ENTHALPY - BTU PER POUND OF DRY AIR
W
VAPOR PRESSURE - INCHES OF MERCURY
95
55
HUMIDITY RATIO - GRAINS OF MOISTURE PER POUND OF DRY AIR
Example #3
EN T HAL P
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PDH Course M226
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HVAC Troubleshooting Example #4
You have an airhandler supplying 25,000 cfm. You bring in 5,000 cfm outside air (OA) and relieve the same. You know you need to return 74F/50%RH air and have the supply 52F/80% RH to maintain space conditions. However, you are getting space temperature alarms. The contractor provided a 800 MBH coil. What is the problem? What are possible solutions?
First, plot the conditions for the OA and RA to get the differences in enthalpy for both. Use outside design day conditions of 95F DB/76F WB.
Calculate the load = (5000)(4.5)(39.37-19.62) + (20000)(4.5)(27.56-19.62) = 1158975 BTUH/1000 = 1159 MBH
The coil can only provide 800 MBH, which is under 1159 MBH on a design day
To solve this, increase coil capacity or decrease outside air if possible
Page 26 of 38
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PDH Course M226
PSYCHROMETRIC CHART
75
70
Sea Level
www.PDHonline.org
80
85
90
300 1.9
Heavy Text Bullet Slide Sample BAROMETRIC PRESSURE 29.921 inches of Mercury
290
65
10 0
280
17.0
16.0
60
1.8
270
95
1.7
260
95
95 250 1.6 240 ET
BU
LB
90
AIR
50
RY ND OF D
RAT U
RE
- °F
POU PER
EMP E
BTU
ION T
Y-
75
160 150 140
IR RY A OF D
SAT URA T
EN T HAL P
170
E ft³/lb L UM IC VO
80
30
180
IF PEC
OA
200
S 15.0
80
75
130 120
70 90%
110
% 70
55 55
85
80 1
.9 75 .8 70
65
40
40
45
35 35
25 -5
10
5
0
5
15 10
5
0
-5
10
20
12.0
0
-5 -25 -20 -15 -10 -30 -25 -20 -15 -10
40
30
15
15
20
20
25
25
30
30
35
40
45
30 13.0
0
50
%
30
20%
20
ITY UMID VE H ELATI 10% R
55
60
65
70
.6
.5
.4
75
80
85
90
10
95
60
50 45
.3
40 35
.2
30 25 20
.1
55
10 0 -40
100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 50
DRY BULB TEMPERATURE - °F
Linric Company Psychrometric Chart, www.linric.com 10
70
55 60
DEW POINT - °F
5
75
65
60
% 40
50
80
.7
50 45
1.2
1.1
70
50 %
60
50
85
1.3
80
60 %
15
10
90
65
60
14.0
RA
100
70
80%
65
25 20
90 1.4
210 -° F
190
85
1.5
220 UR E
85
40
35
TE M PE RA T
90
45
90
230
15
20
Page 27 of 38
25
30
35
40
45
ENTHALPY - BTU PER POUND OF DRY AIR
W
VAPOR PRESSURE - INCHES OF MERCURY
95
55
HUMIDITY RATIO - GRAINS OF MOISTURE PER POUND OF DRY AIR
Example #4
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PDH Course M226
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HVAC Troubleshooting Example #5
For some reason, when outside conditions dropped to 30F/30%RH for the first time, an area fed by a 20,000 cfm 100% outside air unit went into alarm. The wintertime setpoint for RH in the space was 30% at 72F. The humidifier was designed to deliver 300 pounds of moisture per hour. What could the problem be?
Plot the conditions and determine the grains of moisture/# dry air for both conditions. Then, calculate the required amount of moisture.
Moisture needed = (g1-g2)*cfm*4.5/7000
Moisture needed = (34.96-7.23) g/# DA * 20000 cfm * 4.5 7000 G/# =356 #/hour
Therefore, the humidifier is undersized (or slow down the fan/cfm if possible)
Page 28 of 38
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PDH Course M226
PSYCHROMETRIC CHART
75
70
Sea Level
www.PDHonline.org
80
85
90
300 1.9
Heavy Text Bullet Slide Sample BAROMETRIC PRESSURE 29.921 inches of Mercury
290
65
10 0
280
17.0
16.0
60
1.8
270
95 1.7
260
95
95 250 1.6 240 BU LB
90
45 °F TUR EERA
ER P
EMP ION T
Y-
160 150 140
lb OF
75
130
AIR DRY
SAT URA T
170
ft³/ UME VOL
80
30
180
CIFIC
BTU P
200
SPE
EN T HAL P
-° F
15.0
80
75
120
70 90%
110 100
70
80 %
65
25 20
90
80 1
.9 75 .8 70
65
45
35 35
25
-5
10
5
0
5
15 10
5
0
-5
10
20
12.0
0
-10 -5 -25 -20 -15 -30 -25 -20 -15 -10
40
30
15
15
Linric Company Psychrometric Chart, www.linric.com
20
20
25
25
30
30 13.0
0
4
30
40
45
50
20%
55
60
65
.5
.4
70
45
Humidity Ratio =40 35 34.96 30
%
10%
35
.6
20
IDITY HUM TIVE RELA
75
80
85
90
95
100 105 110 115 120 125 130 135 140 145 150
60
50
0%
40
40
30 25 20
10 Humidity Ratio =10 0 7.23 -40 155 160 165 170 175 180
.3
.2
55 .1
50
DRY BULB TEMPERATURE - °F Space Setpoint
OA 10
70
55
DEW POINT - °F
50
5
75
65 60
50 45
80
.7
50
55
1.2
1.1
60
%
60
50
85
70
60
55
10
85 1.3
80
%
15
14.0
70 %
65
60
90 1.4
210
190
85
1.5
220 RA TU RE
85
40
35
TE M PE
90
OUN D
OF DRY AIR
50
90
230
15
20
Page 29 of 38
25
30
35
40
45
ENTHALPY - BTU PER POUND OF DRY AIR
W ET
VAPOR PRESSURE - INCHES OF MERCURY
95
55
HUMIDITY RATIO - GRAINS OF MOISTURE PER POUND OF DRY AIR
Example #5
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PDH Course M226
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HVAC Troubleshooting Example #6
You have a “floating” discharge temperature from 52 to 60oF. Your design space conditions are 70/50%RH. Do you need to override the “floating” discharge to control upper humidity?
First, plot the conditions of the highest discharge temperature and space conditions.
Note that the moisture content at saturation for the discharge when at 60F is higher than the room conditions. It must be lower to absorb moisture. Try It Yourself: Plot for yourself (not shown) the Humidity Ratios. You will find that a lower Humidity Ratio is needed than the supply discharge conditions can supply. Obviously, that means the RH will not be maintained as desired since there will be too much moisture in the air coming into the room.
Therefore, an overriding dehumidification cycle is needed if space conditions are to be maintained.
(Note: In good practice, “floating” is typically based on outside air dew point and the above is usually not a problem.)
Page 30 of 38
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PDH Course M226
PSYCHROMETRIC CHART
75
70
Sea Level
www.PDHonline.org
80
85
90
300 1.9
Heavy Text Bullet Slide Sample BAROMETRIC PRESSURE 29.921 inches of Mercury
290
65
10 0
280
17.0
16.0
60
1.8
270
95
1.7
260
95
95 250 1.6 240 BU LB
90
90
45
RAT URE -
°F
POU PER
80
75
170 160 150 140 130
AIR DRY
30
180
lb OF
SAT URA TIO NT
Y-
EM PE
BTU
200
ft³/ UME VOL
ENT HAL P
-° F
CIFIC SPE
80
75
120
70 90%
110
5
10
70 %
%
15
15
20
20
25
30
30
35
40
75 .8 70
65
45
50
%
30 20
ITY UMID VE H ELATI 10% R
55
60
65
.6
.5
.4
70
75
80
85
90
10
95
60
50 45
.3
40 35
.2
30 25 20
.1
55
10 0 -40
100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 50
DRY BULB TEMPERATURE - °F 10
70
65
60
0%
20%
Linric Company Psychrometric Chart, www.linric.com
75
.7
DEW POINT - °F
25
.9
80
55
30
13.0
0
10
5
0
-5
15
12.0
-5 -25 -20 -15 -10
10
4
45 40
35
25 20
1
60
40
35 30
80
70
% 50
40
1.2
1.1
50
5
0
85
50
55
45
-30 -25 -20 -15 -10
60
50
85
1.3
80
60
55 10
5
14.0
15
0
90
65
60
-5
100
70
80 %
65
25 20
90 1.4
210
190
85
1.5
220
85
40
35
TE M PE RA TU RE
15.0
ND OF DRY A
IR
50
90
230
15
20
Page 31 of 38
25
30
35
40
45
ENTHALPY - BTU PER POUND OF DRY AIR
W ET
VAPOR PRESSURE - INCHES OF MERCURY
95
55
HUMIDITY RATIO - GRAINS OF MOISTURE PER POUND OF DRY AIR
Example #6
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PDH Course M226
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HVAC Troubleshooting Example #7 You design a production space to 72F/50%RH. After moving in, the operators wish to lower the setpoint to 68F while maintaining the 50% RH in the summer. Is this a problem? What must be done?
First, plot the conditions of the original and desired conditions. Then draw a line to the 90% saturation line and drop down to see what the new discharge temperature will be . Clearly, the discharge temperature must be lowered if the RH is to be maintained. Can the system handle this? Is the chilled water temperature low enough? PSYCHROMETRIC CHART
75
70
Sea Level
80
85
90
300
Heavy Text Bullet Slide Sample BAROMETRIC PRESSURE 29.921 inches of Mercury
1.9
290
65
10 0
280
17.0
16.0
60
1.8
270
95 1.7
260
95
95 250 1.6 240 BU
LB
90
AIR
50
DRY
200
PER ATU RE -
160
TEM ION
80 75
140 130
Y AIR
30
150
F DR ³/lb O ME ft
SAT URA T
170
U VOL
Y-
180
CIFIC
ER P
°F
OUN D
OF
F
SPE
80
75
120
70
90%
110
70 %
55
45
5 45
-20
-15
-10
0
10
5
0 -5
5
10
12.0
0
-5 -25 -20 -15 -10
10
20
15
15
20
20
25
25
30
30
35
40
45
50
55
60
65
.9 75 .8 70
10
70
65 .6
65 60
.5
.4
60
30 20
10
10
Y MIDIT E HU LATIV % RE
75
80
85
90
95
60
50 .3
40 35
.2
30 25 20
.1
55
10 0 -40
100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180
50
DRY BULB TEMPERATURE - °F
Linric Company Psychrometric Chart, www.linric.com
75
.7
45
20%
70
1
80
55
% 30 13.0
35
25
-30 -25
40
30
80
50 40
40 35
0
1.2
1.1
60
% 40
50
85
70
%
50
85 1.3
80
50 %
60
55
10
5
14.0
15
15
90
65
60
-5
100
70
80 %
65
25 20
90 1.4
210 -°
190
85
1.5
220 RE
15.0
BTU P
PE RA TU
85
40
35
M
90
45
90
230 TE
15
20
Page 32 of 38
25
30
35
40
45
ENTHALPY - BTU PER POUND OF DRY AIR
ET
VAPOR PRESSURE - INCHES OF MERCURY
W
HUMIDITY RATIO - GRAINS OF MOISTURE PER POUND OF DRY AIR
95
55
DEW POINT - °F
Example #7
ALP
ENT H
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PDH Course M226
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HVAC Troubleshooting Example #8
You construct a room 20’ x 30’ at design conditions 70oF/50%RH and 2800 cfm supply air. After washing the room with 100oF water, the operators do not dry it and there are humidity alarms. Assume the evaporation rate is 0.075#/ft2/hr (this can be determined by tables not included herein). Are there problems with the HVAC system?
First plot the conditions at 70oF/50%RH (Humidity Ratio = 54.68 gr/#).
Then calculate the new conditions by determining the new Humidity Ratio after evaporation (you can consider it like the humidifier example earlier, where #/hr of moisture = (g1-g2)*cfm*4.5/7000) Solve for the unknown “g.”
((g1 – 54.68)*2800*4.5)/(7000) = 0.075*30’*20’; solving for g1 = 79.7 gr/#
Plot the new condition from 79.7 gr/# to 70oF and determine the new RH which is 72.47% resulting from the evaporation.
How long before the water evaporates away?
Based on evaporation rates alone, you can solve for the amount of evaporation per inch/hr, or
Time/inch = 62.4 #/ft3/(0.075 * 12) > 69 hours per inch, or over 4 hours for 1/16”
Actually much longer as water cools
What can we do to keep from having alarms/RH above 50%?
Quickly dry/squeegee the space as soon as possible.
Lower the discharge temperature. To determine how much is required, determine the required moisture holding capacity of the discharge air. First, calculate the maximum Humidity Ratio allowed, or
Humidity Ratio Allowed = (Max. HR allowed) – (amount to be absorbed) = 54.68-(79.7-54.68) = 29.7gr/#
Page 33 of 38
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PDH Course M226
www.PDHonline.org
Plot a line to the 90% saturation curve and draw line down to the DB temperature, which is 37.48oF, or the required discharge temperature. Note the line now slopes up to the new 70/50 setpoint that takes into account absorbing the evaporation. Likely, this will not be possible without changing coils and/or the chilled water system. PSYCHROMETRIC CHART
75
70
Sea Level
80
85
90
300
Heavy Text Bullet Slide Sample BAROMETRIC PRESSURE 29.921 inches of Mercury
1.9
290
65
10 0
280
17.0
16.0
60
1.8
270
95
1.7
260
95
95 250 1.6 240
90
AIR
50
DRY
PER ATU RE
ER P
- °F
OUN D
OF
190
80
TEM
Y-
ION
ALP
160
80
75
150 140
30
130
Y AIR
SAT U
170
R OF D
RAT
35
180
ft³/lb UME VOL
BTU P
200
CIFIC SPE
40
75
120
70 90%
110
70 %
%
%
45
35 35
25 10
5
10
12.0
0
15 10
5
0
-5
20 15
15
20
20
25
25
30
30 13.0
40
30
%
30
%
40
45
50
30
20% 10
35
55
60
65
70
1.2
1.1 80 1
.9 75 .8 70
65
g1
10
75
70
.6 65
60
.5
g2
.4
HU ATIVE % REL
75
80
85
20
Y MIDIT
10
90
95
60
50 .3
40 35
.2
30 25 20
.1
55
10 0 -40
100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 50
DRY BULB TEMPERATURE - °F
Linric Company Psychrometric Chart, www.linric.com
80
.7
45 40
40 0
60 50
5
85
55
40
50
85
1.3
70
50
55
45
-30 -25 -20 -15 -10
60
50
5
80
60
55 10
0
90 14.0
15
-10 -5 -25 -20 -15
70
65
60
-5
100
80 %
65
25 20
90 1.4
210 -° F
85
85
1.5
220 RA TU RE
15.0
ENT H
TE M PE
90
45
90
230
15
20
Page 34 of 38
25
30
35
40
45
ENTHALPY - BTU PER POUND OF DRY AIR
BU LB
VAPOR PRESSURE - INCHES OF MERCURY
W ET
HUMIDITY RATIO - GRAINS OF MOISTURE PER POUND OF DRY AIR
95
55
DEW POINT - °F
Example #8
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PDH Course M226
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HVAC Troubleshooting Example #9
A Stability Chamber (used in the Pharmaceutical industry to hold products at tightly controlled temperature and humidity conditions) experienced a humidity excursion – the relative humidity dropped below the setpoint of 40% to 36% RH. The conditions in the chamber are set for 25C/40% RH. At the time of the excursion, the temperature also increased to 82F. What caused the drop – could the humidifiers be malfunctioning?
Plot the conditions. Note that 25C = 77F. Note that the rise in temperature required more capacity from humidifiers to reach same RH setpoint.
Therefore, the problem was a result of change in temperature versus under-sized humidifier.
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PDH Course M226
PSYCHROMETRIC CHART
75
70
Sea Level
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80
85
90
300
Heavy Text Bullet Slide Sample BAROMETRIC PRESSURE 29.921 inches of Mercury
1.9
290
65
10
0
280
17.0
16.0
60
1.8
270
95
1.7
260
95
95 250 1.6 240 ET
BU LB
90
AIR
50
RY
TEM ION RAT
80
75
30
150 140 130
Y AIR
SAT U
160
75
120
70 90%
110
55
45 40
45
0
10
5
0
-5
5
15
10
12.0
-10 -5 -25 -20 -15
10
5
0
15
15
20
20
25
25
30
30
35
40
45
50
%
30 20
ITY UMID VE H ELATI 10% R
60
1
.9 75 .8 70
65
10
70
65 .6 65
60
.5
.4
70
75
80
85
90
10
95
60
50 45
.3
40 35
.2
30 25 20
.1
55
10 0 -40
100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 50
DRY BULB TEMPERATURE - °F
Linric Company Psychrometric Chart, www.linric.com
75
.7
50
20%
55
80
80
55
30
13.0
-5
-30 -25 -20 -15 -10
40 35
25
60
40
35
20
70
% 40
50
5
60 %
50
1.2
1.1
50
60
55 10
85
80
%
15
14.0
70 %
65
60
30
90
80
20
0
70
%
65
25
100
85
1.3
DEW POINT - °F
YALP
170
R OF D ft³/lb
35
180
UME VOL
PER ATU R
E-
ER P
°F
OUN D
OF D
200 190
80
90 1.4
210 -° F
CIFIC SPE
40
BTU P
UR E
85
85
1.5
220 RA T
15.0
ENT H
PE
90
45
90
230 TE M
15
20
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25
30
35
40
45
ENTHALPY - BTU PER POUND OF DRY AIR
W
VAPOR PRESSURE - INCHES OF MERCURY
95
55
HUMIDITY RATIO - GRAINS OF MOISTURE PER POUND OF DRY AIR
Example #9
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Using on-line data or software You might have had difficulty getting the significant figures that the examples illustrated. That is because I used software to determine the accurate values. It is often difficult to acquire the accuracy needed by plotting on the chart. Software is available, often free online, to accurately calculate various psychrometric conditions. This course utilized linric.com – a website providing all psychrometric tools. Using such software, we can get more accurate data and at different altitudes above sea level. As well, it avoids cumbersome conversions. However, the Psychrometric chart remains useful to understand graphically the array of parameters. The following is an example screenshot.
Course Summary This course presented an overview of the fundamentals of psychrometrics and use of the psychrometric chart. Specific examples were provided for typical HVAC troubleshooting problems that have occurred in the Instructor’s experience. It is essential that students have fundamental understanding of the subject matter if they are to successfully design to avoid problems, as well as diagnose HVAC problems when they occur.
Page 37 of 38
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Acknowledgments I would like to thank Mr. Bill Weaver, PE, (Engineers Plus, Richmond, Viginia) for his assistance in reviewing my math and content of a slide show that resulted in this course. I would also like to thank Mr. Jim Judge, PE, Linric Company Owner, for the use of his version of the Psychrometric chart.
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