Experiment 1 Heat Pump Abstract
The heat pump experiment was performed to collect temperature, pressure, flow rate, and power data from the Hylton Air Air and Water Water Heat Pump. These data were used to calculate heat transfer in the system and overall performance values of the heat pump system. The experiment bean by ad!ustin the heat pump to the correct operatin parameters. The amount of power re"uired to operate the compressor was then recorded. Temperature, pressure, and mass flow rate were determined for b oth refrierant and water at multiple locations on the heat pump. These values v alues were read from either a diital display or an analo display dependin on what was to be measured. The data recorded was then used, alon with a pressure#enthalpy diaram and a temperature table of $#1%& refrierant, to determine the enthalpy. enthalpy. 'nowin the enthalpy at each stae of the heat hea t pump cycle it was possible to determine the heat transfer in the evaporator, condenser, and compressor. compressor. The calculated enthalpies were analy(ed and determined to reasonable for the operatin conditions. c onditions. )liht differences can be contributed to a small amount of heat bein lost to the surroundins of the heat pump. There may also have been minor errors in ta*in readins from the analo e"uipment.
1
Objective
The purpose of the heat pump experiment was to determine the performance values of the Hylton Air and Water Pump )ystem. This was done by ta*in readins at the four basic components of the system+ the compressor, condenser, expansion valve, and evaporator. The readins were ta*en so it could be determined where wor* was put in and where heat was added or removed. These values allowed for the calculation of the heat pump efficiency and the coefficient of performance. Introduction
Heat pumps are devices that move heat in a direction that is opposite of spontaneous flow. This means that a heat pump moves heat from a location with a cooler temperature to another location with a warmer temperature. The term heat pu mp can be used to describe a device that heats or cools a iven location. Heat pumps can be used in a variety of applications includin refrieration, air conditionin, and heatin. The function of heatin or coolin is determined by the conditions of the environment and where the heat is released by the heat pump.
The heat pump unit covers the followin areas of heatin and refrieration enineerin # -amiliari(ation with the basic construction of heat pump. omponents of thermal enines, heat pumps and refrieration system -amiliari(ation with cyclic processes Wor*in with p#h diarams # /asics of refrieration enineerin. Theory
The basic functions of a thermodynamic cyclic process. 0n a thermodynamic cycle process a service medium e.. $ 1%&a2 passes throuh various chanes of state in a pre#set se"uence. The chanes of state are repeated cyclically, so the service medium repeatedly returns to its initial state. That is why the process is termed a cyclic process -i. 12. hane of state refers to compression, expansion, h eatin or coolin+ # ompression means absorption of mechanical enery # Expansion means dischare of mechanical enery # Heatin means absorption of thermal enery heat # oolin means dischare of thermal enery n o i s s e r p m o C
Heat absorption
3 Heat discharge
n o i s n a p x E
-iure 1.1 + yclic process for the $efrieration 0n a chane of state without heat dischare is termed an isentropic chane of state the specific entropy remains constant2, a chane of state is termed an adiabatic chane of state. 0n pure compression and expansion without heat dischare or absorption isentropic or adiabatic respectively2, the necessary mechanical enery W1#3 for the chan e of state from state 1 to state 3 is calculated as W1
3
$
m
1
T1
T3 2
12
or W 1
3
$
m
R
1
p1v1
p 3 v 3 2
32
pv
T , from the e"uation of state. Where, 4 the isentropic index m 4 the mass of the as.
-or isochoric heatin or coolin i.e. same volume, but increasin or decreasin temperature2 the followin applies for input or output heat "uantity 51#3 51#3 4 m vT3 6 T12
%2
v is the specific heat capacity of the as under observation at constant volume. A distinction between two types of specific heat cap acity+ #
Heatin from T1 and T3 causes the pressure increase, the volume remain constant+
#
The heatin brins about an increase in volume, the pressure remains constant+ p
v
-rom the specific heat capacity the isentropic expansion+ . p
4 .v
&2
0n reality, ideal ases are practically never encountered. The observation of chanes of state with li"uids or vapours as with common service produ cts e.. refrierant2 for heat pumps is much more complicated, and uses other state variable such as enery or enthalpy, with the aid of caloric state e"uations.
%
0n a heat pump the cyclic process is run throuh an reverse order. The direction of heat flow is also in reversed. $efer the Temperature vs entropy diaram in -i.3. A compressor compresses the vapour, whereby mechanical enery Win is absorbed 1 6 32. 0n the condenser the 5out is drawn off from the refrierant at the same temperature2 and the medium is li"uefied 3 6 %2. 0n an expansion valve pressure is relieved from the li"uid refrierant, thereby coolin it down % 6 &2. An evaporator evaporates the refrierant, with heat absorption & 6 12. T
3 %
&
1
s s 1 4 s 3 -i. 1.3+ T#s diaram for the refrieration cycle.
7efined and measured variables+ &
t 4 Time in seconds m 4 water "uantity per water vessel php 4 Pressure upstream of the condenser plp 4 Pressure at the inlet into the compressor Thp 4 Temperature of water bein heated from thermometer Tlp 4 Temperature of water deliverin heat from thermometer Thb 4 Temperature of refrierant at upstream of the condenser from aue Tcb 4 Temperature of refrierant at the inlet into the compressor from aue The resultin heat delivered to the water bein heated between two state 1 at the beinnin2 and state 3 at the point of measurement2 is Qout
4
mC P T hp 3
T hp1
3.12
with p 4 &.18 *9:* ' 6 specific heat capacity of water.
The output heat power useful heat flow2 is thus Qout
;
Q out
4
t
3.32
The input power is composed of the input mechanical Pin 1&< W to the compressor2 and the heat power drawn from the cold water in the vessel 5 in the case of a refrierator the cold output2.
Apparatus
=
# Heat pump demonstrator #Thermometer #Timer
Procedure
>
The two vessels were filled with water with the room temperature and the mass of water and water temperature with two laboratory thermometers in each vessel were measured. The compressor was switched on. The measured values were recorded and plotted on the wor* sheet for 1, 1%<, 1&<, 1=< seconds. # The ompressor was switched off for 1< minutes and the procedure was repeated as above for each readin for findin the averae value.
ResultCalculation
?
Water vessel content+ =<< ml
@o. 1 3 % & =
Time sec2 < 3&< &< ?3< 8><
Php bar2 1. <.> <.> <. <.8
Plp bar2 < %& %> &< &3
Thp 2 < %& %> &< &3
Tlp 2 < #1> #1= #13 #13
Thb 2 3>.3 3>.% 3?.3 38 %1.3
Tcb 2 3>.3 3=. 3%.8 33 3<.&
Table 1+ Beasured variables )tae )ec2 < 1#3&< 3&1#&< &1#?3< ?31#8><
0nterval )ec2 < 3&< 3&< 3&< 3&<
php bar2 3.<< ?.= .= 8.< 1.<<
Co php Pa2 =.%<1< =.?8? =.8%%= =.8813 >.3==%
plp bar2 1.< <.>< <.>< <.< <.8<
m 4 <.= * p 4 &81<9:*' Pin 4 13< Watt )ample calculation+
ompressor,
)tae )ec2 < 1#3&< 3&1#&< &1#?3< ?31#8><
0nterval )ec2 < 3&< 3&< 3&< 3&<
Win41&< W x tinterval 9oule2 < 3<< 3<< 3<< 3<<
Thp3#Thp1, '2 <.<< %&.<< 3.<< &.<< 3.<<
)ample calculation+
Qout
m C p T hp3 t
T hp12
ondensor 5out 9oule2 <.<<<< 38>.?81? 1?.&=% %&.81>? 1?.&=%
Co plp Pa2 &.3==% %.??3 %.??3 %.8<%1 %.8=&3
ompressor,
)tae )ec2 < 1#3&< 3&1#&< &1#?3< ?31#8><
Win41&< W x tinterval 9oule2 < 3<< 3<< 3<< 3<<
0nterval )ec2 < 3&< 3&< 3&< 3&<
Tlp1#Tlp3, '2 <.<< 1>.<< #1.<< #%.<< <.<<
Evaporator 5in 9oule2 < 1%8.>>>? #.?383 #3>.1?= <.<<<<
Enthalphy *9:*2
)ample calculation +
Qin
m
t
C p 1T lp1
T lp3 2
3?% To convert temperature to '
)tate 1, before compression 3, after compression %, after condensation
Pressure bar2
Temp. o2
Temp. '2
<.8<
3<.&
38%.&
1.<<
%1.3
%<&.3
1.<<
#
3?%
&, before evaporation <.8< # 3?% The temperatures after condensation and before evaporation are not re"uired. The enthalpy values and temperatures were obtained from $1%&a p#h raph )ample calculation
W134h3#h1
514h1#h&
534h%#h3
DPref
)ample calculation+ COP
Q hp
COP hp
out
W in
h% h3
h3 h1
COP
ref
COP ref
Q in W in
h1 h3
h& h1
8
DPhp
1<
!iscussion
DP or coefficient of performance is a measure of the efficiency of a heat pump. The heat pump used in the experiment had a DP reater than 1 which is %.3> and that means at this condition, %.3> *9 of heat enery could be extracted from the system with the input of 1*9 of wor*. Efficiency can never be reater than 1 but here DP is above 1 because DP is not a percentae itFs !ust a coefficient so by definition it should be reater than 1 Althouh the collection of data, and the calculations were performed with the utmost of care, the values obtained from the actual refrierator will not be e"ua l to those of the ideal refrierator. The differences may be associated with the accuracy of the measurin e"uipment, and the methods used, which would impact the calculated DP values. 0n the calculations, the data used were actual field measurements. However, the calculations used in the classroom are performed with the ideal refrierator, where reasonable assumptions are made. 0n an effort to ma*e calculations easier, one of the assumptions made is that there are no irreversibilityFs, which will result in a reater DP values. Thus, discussion will also include the evaluation of the apparatus for areas where possible sources of error may have arisen. "easurement E#uipment The measurement e"uipment used in the laboratory consisted of the thermometer for measurin temperature /ourdon aes to measure pressure of the refrierant, analoue voltmeter and ammeter to measure voltae and amperae into the apparatus.
/oth the condenser and the evaporator refrierant temperature were measured with a thermometer. /oth devices had an openin from the top where the thermometer could be placed into the openin with a diameter slihtly reater than that of the thermometer and a depth of less than the thermometer lenth. The thermometer is never in contact with the refrierant, but is separated by the inner wall of the device. Thus, the heat transfer must occur between the refrierant, inner wall of the device, and the poc*et of air surroundin the thermometer, to have an effect on the thermometer. Thus, reduced and increased temperatures result, with direction dependin on the temperature of the environment. The same variation occurs in the measurement of the inlet and outlet temperatures of the flow water for both the heat transfer devices. To countermeasure this affect, the use of thermometer that is actually in the fluid to be measured will eliminate the above. To ensure accurate data, the thermometer must be read correctly. The thermometer fluid must be allowed to settle prior to ta*in a measurement. The readin of the measurement is also critical since the temperature must be read from the bottom of the meniscus. 0t should be noted that to read the temperature e of the condenser and the evaporator, the thermometer must be removed from the openin of the device to read the scale. Which would have allowed the temperature to try to return to room temperature. )ince the scale was readable for the water temperature, removal of thermometer was not re"uired for them. incorporatin the thermometer into the fluid without disturbin the fluid flow2 and allowin the temperature scale to readable without removin it, p erhaps a diital readout of the temperature. Pressure of the refrierant was obtained from the analoue /ourdon ae. To ensure accurate readins, the readins should be ta*en directly in front of the ae, and not from an anle. The same is true of the 11
voltae and current ae. To ensure accurate readins aes should be calibrated in accordance with the manufacturers recommendations. $ariations in Actual and Ideal %Textboo&' Results
As mentioned earlier, differences from the actual and the ideal results of a refrieration cycle are d ue to the assumptions made. alculations with the ideal refrieration cycle include the followin, 0rreversibilityFs within the evaporator, condenser, and compressor are inored @o frictional pressure drops $efrierant flows at constant pressure thouh the two heat e xchaners )tray heat losses to the surroundins are inored ompression process is isentropic Dn the other hand, the data used for the calculations were from actual field measurements ta*in into consideration irreversibilityFs, pressure drops due to friction, non constant pressure across heat exchaners2, but the calculations were made with the above assumptions except the compression process was not assumed to be isentropic. Thus the entropy difference across the compressor at state points one and two reflects the entropy enerated due to the irreversibilityFs. Thus the actual enthalpy and temperature at state two will always be reater than the enthalpy and temperature at state 3 for the ideal process isentropic2. This results in a reater amount of wor* to be applied to the wor*in fluid in comparison to the isentropic case. )ince DP is defined as the benefit over the cost, increasin the wor* will decrease the DP. Hence the actual DP will be less than the DP obtained for the ideal system. 0n the ideal system, the followin e"uation will be true,
with the assumptions made above. However, in the actual scenario, the e"uality does not hold due irreversibilityFs. A method to "uantitatively compare the actual and ideal cycles would be with the thermal efficiency. )imilar to the DP, thermal efficiency will also be less for the actual then the ideal cycle. A simple comparison calculation would be the percentae decrease in thermal efficiency of the actual versus the ideal cycle.
13
Error
7urin the ac"uisition of temperatures, thermocouple errors may have influence the ma!ority of errors in this investiation. The (eroth law of thermodynamics allows one to relate the enery transfer from one ob!ect to another and "uantify any chanes in transfer to another body. Gsin this method has its limitations as temperature, which is a measure of averae *inetic enery of molecules can never be exact. -or an accurate temperature measurement one would re"uire a sensor or thermometer which is calibrated and consistent within the ranes of operation. Althouh sufficient time was allowed for the
system to reach thermal e"uilibrium with the sensors, electronic errors such as switch contacts and increased resistance durin loner operatin times may have played a role in the accumulation of errors. Pressure inconsistency, vibrations and minor lea*aes may have influenced incorrect pressure readins at the tappin points. The pressure readins were measured in aue pressure however absolute pressure is re"uired in calculations, which was assumed to be 1 /ar but is actually 1.<1%3= /ar at sea level in international standards. The difference here is minor but this must be considered for refrieration plants which operate at alternatin altitudes and whether conditions such as aircraft air conditionin units. -urthermore, calculations involvin the power input may be incorrect as a flashin indicator liht is used to calculate the power input this re"uires circuitry to conclude unit intervals of enery delivery. This method could also be validated with an electromanetic dis* rotatin in a manetic field caused by the flow of current flowin into the motor. Alternatively, by introducin a Hall Effect sensor on the power cord to later wor* out the power input with current and other *nown electrical properties. The amount of time allocated for the system to reach steady state was approximately 3<#3= minutes which is sufficient in order to pass throuh bubbles of the refrierant. However, a collection of li"uefied refrierant may have developed in the lower sections of some plumbin wor* or bubbles within the top sections within the tan*s and other areas and thus providin a false indication of the actual flow rate. The flow rate could be observed aain towards the end of the experiment to confirm that the flow rate is steady at the value previously recorded.
1%
Conclusion
Any heat pump has an ideal coefficient of performance DP2. /ut in reality, the coefficient of performance is less than the ideal value, and this is because of multiple reasons that cause inaccuracy or losses in the system, such as mechanical or electrical losses in the fan or the compressor, or errors in measurin devices, especially when ac"uirin the wet bulb temperature W/T2. 0n the above experiment, the thermodynamic cycle of vapor 6 compression heat pump and refrierator were studied. The heat pump wor*ed on a vapor#compression cycle in which the refrierant flew throuh four processes+ a2 Evaporation at low pressure and temperature. b2 ompression to hih pressure. c2 ondensation at hih pressure and temperature. d2 Expansion by throttlin from hih pressure to low pressure. /y *nowin two variables it was possible to ob tain the value of the third un*nown and for this purpose the pressure#enthalpy diaram was used. The experiment was made upon three assumptions+ 1#0t was assumed that the refrierant flowrate and the property v alues were steady, so )-EE was valid. 3# $easonably it was assumed that 'E and PE were neliible and e"ual to (ero. %# Assumed that heat transfer in compressor was (ero and neliible. Re(erences
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