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Heat Exchanger Design And System Balance Of Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/TA1997/78675/V001T13A006/4460073/v001t13a006-97-aa-030.pdf by guest on 01 October 2021 An Air Source Heat Pump/Chiller Using Brazed Plate Heat Exchangers. III I 1 111.11111 II 111111
By Changiz Tolouee (M.Sc), Member of International Institute of Refrigeration, Member of Australian Institute of Refrigeration, Air Conditioning and Heating.
ABSTRACT
Air Source heat pump/chiller is used to provide chilled water for cooling and hot water for heating purposes. This is one investment for both applications with no requirement for boiler and fuel with the advantage of heat pump efficiency. In this paper we are going to analyse both air side and water side heat exchangers used in air source heat pump/chiller with special attention and emphasis on brazed plate heat exchanger which is used in refrigerant to water side of this unit in order to achieve optimum performance in both the heat pump and chiller operations. Due to compactness of brazed plate heat exchangers it is very important to balance system volume in both operating conditions which will also be examined in this paper.
NOMENCLATURE
Symbol: Definition: Unit: 2 A Effective heat transfer area m lire Convection heat transfer co-efficient of refrigerant kW/m2K h„, Convection heat transfer co-efficient of water kW/m2K LMTD Logarithmic mean temperature difference °C U Overall heat transfer co-efficient kW/m2K
BRAZED PLATE HEAT EXCHANGERS
Brazed plate heat exchanger is made by stacking several corrugated brazed plates in a particular manner. The result of such a plate arrangement is a large surface area. Due to the herring-bone of the plate, in small flow rates, the flow is more turbulent compared with flow in a copper tube of the shell and tube heat exchanger. It is this turbulence intensity which enhances the heat transfer. For a given condition and same performance, a brazed plate heat exchanger is about five times (on average) smaller in size compared with conventional shell and tube heat exchangers.p) Since BPHX's are compact and superior to shell and tube heat exchangers in terms of heat transfer capability, they have great versatility in positioning the components of units due to such small volume required to be occupied with BPHX's. Moreover, their heat transfer capability can easily be adjusted by changing number of plates used that is smaller or larger heat transfer area in order to required system performance and balance to be achieved.
Presented at the ASME ASIA '97 Congress & Exhibition Singapore - September 30-October 2,1997
HEAT TRANSFER ANALYSIS OF BRAZED PLATE HEAT EXCHANGER
Due to turbulent flow in water side even in low flow rate (low Reynolds number) surface resistance (1/h) is considerably low. So the refrigerant side heat transfer co-efficient (h ref) is the determination factor in over all heat transfer co-efficient (U) in evaporator and condenser applications. As an evaporator there is an optimum flow rate of boiling refrigerant in order to obtain optimum heat transfer co-efficient (bn) with acceptable refrigerant pressure drop, hence, reduce efficiency (performance) of compressor and lower flow rate resultant in a lower heat transfer co-efficient (N ei).
In our heat pump design hwf 1500 W/m2k and It, = 15000 W/m2k which is approximately ten times Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/TA1997/78675/V001T13A006/4460073/v001t13a006-97-aa-030.pdf by guest on 01 October 2021 higher than href. As a condenser due its film condensation variation of hal as a function of flow rate is minimum and it tends to perform better as surface number of plates increased (h oe A (AT)) to some extend. In a heat pump/chiller this heat exchanger is used as an evaporator and also condenser. Considering the fact that by increasing the number of plates, it improves Condenser performance, however on the other hand, it may reduce even distribution of the refrigerants in the channels and also reduce refrigerant flow rate per channel, hence, reduce (14, e) in evaporator mode (see figure 1). We designed an expansion and distribution system and installed it in our brazed plate heat exchanger. This apparatus provides more even distribution of the two phase flow (boiling gas) among Channels in evaporator mode (as you can see in figure 2) and can sequentially increase efficiency of the evaporator. There is a compromise point with emphasis on evaporator function due to its affect on performance and reliability (protection from freeze- up) (see figure 3).
Evaporator Hx (RC180-57) Standard
- R22 In
a. - R22 Out
4 2 .7 .„ i .. 0) V) N r 0) CI r— to est Cs4 r tr) C.3 • sr tcy to to Plate Number
Figure 1. Evaporator Hx (RC 180 - 57)
Evaporator Hx (RC180-57) with Distribution Apparatus Fitted
10 ' 7 - PT-Z 17-
8
ture 6 - R22 In era 4 • - R22 Out
Temp 2 • r , 0 cn en cn cn n e at en n • —• N N NC, Cr) 'it tV tO Plate Number
2 Figure 2. Evaporator Hx (RC 180-57) with Distribution Apparatus Fitted. Evaporator Capacity vs No of Plates for a Given Compressor Capacity Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/TA1997/78675/V001T13A006/4460073/v001t13a006-97-aa-030.pdf by guest on 01 October 2021
51 57 63 Number of plates
Figure 3. Evaporator Capacity vs No of Plates for a Given Compressor
REFRIGERANT TO AIR HEAT EXCHANGER (AIR COIL)
Use Of Air Coil With Heat Puma/Chillers
Coils used with heat pump/chillers in order to source heat from or reject heat to the air in the respective modes. To design an air coil as a condenser, refrigerant surface area should be increased. In order to achieve this, more numbers of tubes with small diameter required. This also contributes to lower air side pressure drop. For the medium size capacity heat pump/chiller applications 3/8" tubes, are practical. This proved to perform efficient as a condenser, but as an evaporator I/2" tubes perform better than 3/8" with the reduced number of circuits. PI Due to maldistribution of two phase refrigerant flow through a distributor in high number of capillary outlets, the performance of the evaporator is decreased. Using bigger diameter tubes help to reduce number of distributor outlets so, improve the distribution at the same time as condenser performance reduced. In the commercial world in order to justify a design, performance is not the only factor, cost, inventory, etc. are also important. Gain in performance in evaporator mode using 1/2" tubes and less number of distributor outlets is not , considerable, furthermore less than loss in condenser performance. After evaluation of the design, air coil with 3/1" tubes and 12 circuits fitted with a distributor has been selected. As shown in Figure 4, heat transfer performance is slightly low in evaporator mode compared to condenser mode.
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PERFORMANCE OF AIR COIL AS CONDENSERS and EVAPORATORS
9 u. 8 IL 0 7 o 6 z < Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/TA1997/78675/V001T13A006/4460073/v001t13a006-97-aa-030.pdf by guest on 01 October 2021 < wig 5 ei r „; 4 tf, FL 3 • UA (kW/Kyas CONDENSER • UA (kW/K)-as EVAPORATOR I— 2 _s-a
Figure 4. Performance of Air Coil as Condensers and
SYSTEM BALANCE
Refrigeration system of a heat pump/chiller should be balanced to operate a higher performance and efficiency also more important in stable condition both in heat pump and chiller modes. To design the right heat exchanger for optimum performance in both modes in the first stage, means to get required capacity in optimum subcooling in condenser and superheat in evaporator. Brazed plate heat exchangers are compact by their nature so they hold very small volume of refrigerant or water. On the other hand air coils hold considerably larger volume of refrigerant inside tubes. Amount of refrigerant in the cycle is one of the critical factors in stable operation of the systems and refrigerant charge for each mode can only be estimated and final figure can be drawn from comprehensive tests. System refrigerant charge should be determined in the mode which condenser holds bigger refrigerant volume in this case is air coil and tests carried out to determine accurate figure. Then a make up receiver to be designed to hold excessive refrigerant outside cycle in smaller volume condenser mode in this case is brazed plate heat exchanger. In order to draw excessive refrigerant from the cycle or feed it back to the cycle in charge over from chiller mode to heat pump mode and vice versa in a very short time (matter of seconds) the necessary arrangements should be implemented in the design. Failure to do so, may cause extreme high pressure or extreme low pressure in the system and consequently damage it. This has been considered in our exclusive expansion system design (see figure 5).
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COMPONENTS DESCRIPTION 1. Compressor 10. Filter-dryer 2. Outdoor coil 11. Sight glass 3. Plate heat exchanger 12. Valve TX 4. Disperser 13. Solenoid valve 5. Reversing valve 14. Liquid receiver 6. TX valve 15. Fan 7. Check valve 16. LP-HP Dual pressure switch 8. Distributor 17. HP pressure switch 9. Solenoid valve 18. Defroster thermostat
Figure 5. Refrigeration Schematic of the Heat Pump/Chiller.
5 CONCLUSION
To design an efficient and reliable air source heat pump/chiller using air to refrigerant and water to refrigerant heat exchangers we need to follow certain stages. Sometimes it is not possible to achieve maximum performance of heat exchangers in both condensing and evaporator modes, so the best way is to achieve optimum design. This means to evaluate all the variables effecting performance in both modes Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/TA1997/78675/V001T13A006/4460073/v001t13a006-97-aa-030.pdf by guest on 01 October 2021 and also considering system balance factors and conclude with an optimised design system (see figure 3). With this unique modular system we need to design only one size module system so we could focus on and carry out comprehensive tests and achieved optimum design which covers a large capacity range by adding up modules to achieve the required capacity.
REFERENCES
I. American Society of Heating, Refrigeration and Air Conditioning Engineers, ASHRAE Handbook Fundamentals 1993, SI Edition 2. Xu, D.H.T. Gotham, M.W. Collins, J.E.R. Coney, C.G.W. Sheppard and S. Merdjani, A numerical and experimental study of turbulent flow through the evaporator coil in an air conditioning unit, International Journal of Refrigeration, Volume 19, November 1996. 3. Hessami Mir-Akbar, Zhou X, Australian Refrigeration Air Conditioning and Heating, AIRAH Journal, Volume 49, No. 8, August 1995, Article: Brazed Plate Heat Exchangers in Water Chillers.
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