HVAC Load Calculations Guide

FMI Energy Conference 2006

HVAC Load Calculations Perry Morrow & John Parker Brandt Engineering, Inc. Richmond, Virginia

Key Concepts to Consider as we look at Load Calculations • • • •

Interior Design Criteria Exterior Design Criteria Kitchen Hoods and Make Up air Walls that aren’t as well insulated as they appear.

Why are Supermarket loads Unique?

Why is moisture control important?

Illustration provided by Todd Smith of Munters Corporation.

Where does the moisture come from? • Infiltration • Ventilation • Makeup Air • Miscellaneous Loads

Makeup Air • There have been many attempts at using tempered makeup air. This one appears the most successful; however, some of the humid air introduced here doesn’t go out the hood.

Image Courtesy of Food Service Technology Center publication Design Guide 2 - Optimizing Makeup Air

Summer Weather Data – Richmond, VA & Phoenix, AZ

Data From 2005 Ashrae Fundamentals CD

HVAC Load Basic Terms •

Zone: A single area with similar thermal and use characteristics.

• •

Envelope Loads or Skin Loads: Loads associated with the Roof, Walls, Windows, Doors, Floor and Partitions.

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Infiltration: Air that leaks into a building through the building envelope.

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Humidity: The quantity of water vapor present in air. It can be expressed as an absolute, specific or a relative value.

Humidity Measurement Terms •

Absolute humidity is the actual mass of water vapor present in the air water vapor mixture. The absolute humidity may be expressed in pounds of water vapor (lb).

•

Specific Humidity Ratio or Humidity Ratio is the ratio between the actual mass of water vapor present in moist air - to the mass of the dry air. Humidity Ratio is normally expressed in pounds of water vapor per pound of dry air or in Grains of moisture per pound of dry air. There are approximately 7000 grains in a pound.

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Relative Humidity: The ratio of the amount of water vapor in the air at a specific temperature to the maximum amount that the air could hold at that temperature, expressed as a percentage.

•

Dew Point temperature: The temperature below which moisture will condense out of air. - Air at a given Humidity Ratio has a constant Dew Point. If air is cooled below this point, moisture condenses out thus changing its humidity ratio. Definitions courtesy of wikipedia.org and .engineeringtoolbox.com

Relative Humidity vs. Humidity Ratio

As a given mass of moisture laden air is heated or cooled the air volume changes but the moisture does not. Thus there is a change in Relative Humidity, without a change in actual water content. The mass of both water and air remain unchanged, so the humidity ratio (Grains/Lb ) does not change. If the air continues to cool, the relative humidity will reach 100% and water will begin to condense on surfaces at 63oF so this is the Dew Point of the air.

Load Calculation Methods • Manual Calculations • Spreadsheet • Specialized Software: Elite Software - Chvac - Commercial HVAC Loads Carrier - Hourly Analysis Program Trane - Trace family of programs Department of Energy – ENERGYPlus, BLAST and DOE-2. Text based software with graphic interfaces sold by 3rd party software vendors.

Loads the New Way

Screen Capture from Carrier Hourly Analysis Program © Carrier Corporation

HVAC Load Basics Although HVAC load calculation includes a significant amount of number crunching, it begins with digging out the answers to a number of questions. A lack of good information up front can mean bad answers at the end of the process. Garbage In = Garbage Out

HVAC Load Questions 1. 2. 3. 4.

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6.

1

Establish allowable Indoor Temperature and Humidity Range. Determine Outdoor Weather Conditions to use. Choices are 0.4%, 1% and 2% Ashrae data or Other. Determine mass and insulation value of all building envelope components (Walls, Roof, Glass, Doors…). Determine all internal load components (Anything that consumes electricity, gas, solid fuel or food. Also anything that has energy piped to it or away from it.). Calculate heat flow into and out of each zone, at summer and winter outdoor weather conditions, varying temperature and sun location for each hour of the day and each day or month of the year. Repeat steps 4 & 5 for every zone (thermally similar space of interest) in the building.

ASHRAE Indoor Air Temperature and Humidity Recommendations • … The industry chose 75°F db and 64°F wb (55% rh, 57.5°F dew point{71.8 grains}) as summer design conditions. This is the ambient condition at which refrigeration load for food store display refrigerators is normally rated. Store humidity is one of the most critical variables that can affect performance of display refrigerators and refrigeration systems. Excerpt from 2006 ASHRAE Refrigeration Handbook, Chapter 46

Refrigerated Case Loads 110%

Percent Load

100% 90% 80% 70% 60% 30

40

50

60

70

36.7

42.3

48.5

53

57.2

80

Humidity Ratio (gr./LB)

60.8 Dew Point (oF)

Data From 2003 ASHRAE Applications Handbook 2.3

Building HVAC Cost vs. Store Dewpoint $34,000

In Miami $3,000 A/C energy COST if “setpoint” is 50 versus 58 dewpoint

$30,000 $28,000 (excludes auxilary heat cost)

Energy Cost to operate A/C for 1 year

$32,000

$26,000 $24,000 $22,000 $20,000

In Baltimore $400 A/C energy COST if “setpoint” is 50 versus 58 dewpoint

$18,000 $16,000 $14,000 $12,000 $10,000 $8,000

In Miami: 90% of a year the store needs humidity control

40 42 44 46 48 50 52 54 56 58 60 Store Dewpoint Setpoint BALTIMORE

MIAMI

Building HVAC Cost vs. Store Dewpoint

In Baltimore: 45% of a year the store needs humidity control

Excerpt from 2002 FMI Presentation by Lee Churchill of Seasons-4 Inc.

Rack Energy Cost vs. Store Dewpoint $110,000 $108,000 $106,000 $104,000 $102,000 Cost to Operate 3 Racks (annual)

In Miami $18,000 rack energy savings if “setpoint” is 50 versus 58 dewpoint

$100,000 $98,000 $96,000 $94,000 $92,000

Baltimore, MD Miami, FL

$90,000

Salt Lake City

$88,000

Pittsburg, PA

$86,000

Houston, TX

$84,000 $82,000 $80,000

In Baltimore $8,000 rack energy savings if “setpoint” is 50 versus 58 dewpoint

$78,000 $76,000 $74,000 $72,000 $70,000 40

42

44

46

48

50

52

54

56

58

60

Store Dewpoint SETPOINT


HVAC + Rack Energy Cost vs. Store Dewpoint $130,000

Cost to Operate 3 Racks & A/C (annual)

$125,000

$120,000

$115,000

$110,000 Baltimore, MD

$105,000

Miami, FL $100,000

Conclusion to study: 50 F dewpoint is an optimum setpoint

$95,000

$90,000

$85,000

$80,000 40

42

44

46

48

50

52

54

56

58

60

Store Dewpoint SETPOINT



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2


ASHRAE Weather Data • 2 Types of Summer Data • Cooling Dehumidification DB / MCWB

&

DP / MCDB

# of Hours Exceeding Summer Data – 0.4% 35 hr/Yr – 1% 88 hr/Yr – 2% 175 hr/Yr

• Types of Winter Data # of Hours Exceeding Winter Data – 99% 88 hr/Yr – 99.6% 35 hr/Yr

2005 Ashrae Example Weather Data

Dehumidification Hours by City • Based on 74oF/55% RH • ( 67.5 Gr/lb ) Internal Set point

Hours by City 7,000 to 9,000 6,000 to 6,999 5,000 to 5,999 4,000 to 4,999 3,000 to 3,999 2,000 to 2,999 1,000 to 1,999

Illustration provided by Munters Corporation.

0 to 999

2005 Richmond, VA Summer Weather

0.4% 1.0%

Carrier Hourly Analysis Program – Opening Screen

HAP – Weather Input


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Typical Wall Constructions

Illustration from 2001 Ashrae Fundamentals Handbook, Chapter 25


HAP – Wall Construction

Similar Calculation for 6” Metal Studs with R-19 insulation, No Insulated Sheathing

Simplified parallel Path calculation for Example purposes. See 2005 Ashrae Fundamentals, Chapter 25 Modified Zone Method for less conservative method.

Insulated CMU Walls R-12 or R-5 ? •

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•

•


CMU walls exhibit similar thermal bridging to the Stud Wall example with the addition of heat flow parallel to the wall surface around the insulated cores. One insulating product’s published data indicates a 12” CMU wall with their product including air film coefficients provides R-12.5. The fine print indicates that more detailed calculations are available. We requested the calculations, they indicate R=9.6. Often for structural reasons, one in four of the CMU cores are grouted solid and have rebar added. This added thermal bridging brings the wall assembly down to approximately R=5.1 Note that the problem is not with the insulating material, but with the fact that there are numerous paths around the insulation.

Carrier Hourly Analysis Program – Opening Screen

•

•

Information describing the Roof, Windows, Doors and any overhangs (Canopies) are entered in the library so they will be available as we define spaces. When equipment will be operated, when the lights are programmed to be on, and estimated hourly occupancy rates are entered under schedules.

HAP – Space General

Ventilation Air The minimum quantity of ventilation air required is set by the building code in most areas. In most spaces, this is defined as X cfm / person. In some spaces, usually ones that are sparsely populated, ventilation rates may be set on a cfm / sq. ft. (floor area) basis. In general, people exhale a predictable quantity of CO2 as they breathe. The CO2 level can be monitored and used to control outdoor air dampers to reduce the amount of Ventilation air during lightly occupied hours.


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Internal Loads Dry (Sensible Heat)

Mixed

• Lights • Equipment:

(Contain Sensible & Latent Components)

– Self Contained Display Cases – Slicers, Registers, Computers

• Hooded Equipment – Broilers – Ranges – Ovens

• People • Steam table • Holding Cabinets • Coffee Brewer • Case Credits

HAP - Internal

Hooded Loads – Where Applicable

• Spreadsheets or pads of paper are good ways to total the various components that make up the Miscellaneous portion of the internal load.

Case Credit - Summary

• Case Credits can be calculated a number of different ways that will be discussed in up coming slides.

Non-Hooded Loads and Totals

Case Credit Calculation Simplified Method

• We have seen this fairly simple method recommended by at least one refrigerated case manufacturer and more than one client. Although relatively simple, the results appear to be fairly accurate.

Case Credit Calculation Ashrae Method

Ashrae publishes a table listing Sensible and Latent Case Credit values on a BTU/Foot basis for various case types. Some case manufacturers have this type of data available for their specific cases.

Case Credit Calculation BTU / Ft & Return Air

One client has taken the BTU/LF or Ashrae method a step further. This calculation bases the case credits on a btu/lf method and then assigns only a portion of this credit to the space. Where under case returns are utilized, the remainder of the credit is assigned to the unit cooling coil.

HAP - Wall

HAP – Roof

HAP - Infiltration

Infiltration • From Carrier HAP 4.20a - Help File – Items in the CFM/sqft column define infiltration in terms of airflow per unit of exterior wall area. Because infiltration occurs through exterior walls - especially through windows and doors in these walls - rule of thumb infiltration rates are sometimes tabulated in this CFM/sqft format.

• From: Ashrae 2005 Fundamentals – Chapter 27, Page 23 – NONRESIDENTIAL AIR LEAKAGE – …Typical air leakage values per unit wall area at 0.30 in. of water are

0.10, 0.30, and 0.60 cfm/ft2 for tight, average, and leaky walls, respectively.

Wall Construction Tight Average Leaky

Infiltration rate (cfm/ft2) 0.10 0.30 0.60

HAP - Floor

HAP – Partition Input

• Partitions are internal walls, ceilings or floors that separate conditioned spaces from un-conditioned or partially conditioned spaces. Machine rooms and Receiving Areas can be partition loads to the adjacent spaces. These walls should be insulated.

HAP – Partition U Value Calculation

•

Use the built in Wall and Roof property calculation portions of the program to calculate the U-Value for any partitions. Manually transfer this data to the partition tab of the space input form.


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Systems

1

• After all spaces to be conditioned have been entered, we can begin grouping them together into zones and systems. • Only spaces with similar thermal and occupancy characteristics should be grouped together as a zone. There will be one thermostat per zone generally • The systems usually correspond to air handling units. These can be split systems, central station air handling units or rooftop units.. • A typical rooftop unit or split system can only serve one zone, and is controlled by one thermostat. So in this case each system consists of only one zone. • There are other system types that allow one unit to serve multiple zones, but in our experience, they are not often applied to supermarkets.

HAP – System Input

General

HAP – System Components

Systems

2

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When we were describing spaces, we entered information that will give us the total outside air required for ventilation.

•

In stores with cooking, this is only part of the outside air story. You need make-up air for hoods and other exhaust sources. The software is aware of this, so all you need to do is enter this value as an exhaust quantity as you describe the system.

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If the zones where the exhaust loads occur are open to the main sales area and each other you have a decision to make. Where to allocate the make up air.

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You can either design for one large semi-custom unit capable of handling most of the outside air or multiple semi-custom units.

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If the zones are not open to one another, you have no choice. Make Up air must be provided in the system where it is actually exhausted.

Building Air Balance

HAP – Zone Components

Load Software Air System Sizing Summary Output

Notice Warning

Load Software Air System Design Load Summary

Heating Load Calculation – “Case Credits” • Heating loads are calculated for the maximum required heating from the system without taking credit for any internal lights, equipment and people. External heating credits such as solar effects are also omitted. Because the software ignores miscellaneous loads while calculating the heating load, the “case credits”, which in heating mode are an additional heating load not a credit, they must be added to the required unit capacity manually. • If sufficient heat from the refrigeration rack is reclaimed by the mechanical systems, it may not be necessary to increase the unit heating capacity to offset this added heating load. But be careful, often the reclaim heat is not available.

Summary •

Humid Climate

129.4 Total Tons 0.67 Coil SHR

•

- Pie

Dry Climate


Summary


– Stacked Bar


Take Home Points • Infiltration / Transmission Load thru Walls. • Interior & Exterior Design Conditions • Costs of Controlling vs. Not controlling space humidity.

Credits Presentation includes excerpts from: • ASHRAE 2003, 2005 and 2006 Handbooks • Screen Outputs from Carrier Hourly Analysis Program HAP version 4.20a • Humidity illustrations provided by Todd Smith of Munters Corporation. • Rack Energy Cost vs. Store Dewpoint data provided by Lee Churchill of Seasons-4 Inc.

Software Resources • Load & Energy Simulation Software – – – –

Carrier Software - Hourly Analysis Program US Dept of Energy - EnergyPlus and DOE-2 Elite Software - Chvac - Commercial HVAC Trane - Trace Family of software

• Psychometric Calculation Tools – PsyCalc98 - Linric Company – PsyFunc is a collection of psychometric functions for Excel sold by Linric Company.

HVAC Load Calculations Guide

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