NEXT GENERATION COMMERCIAL HEAT PUMPWATER HEATER USING CARBON DIOXIDE USING DIFFERENT IMPROVEMENT APPROACHES Page: 3 of 8
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Paper No. 181
Displaying this potential was a secondary aim of this work. To achieve this, the R744 system was assembled
in a frame with the same footprint as the baseline R134a unit, and the volume reduction was demonstrated in
the reduced height of the unit. This reduction was primarily achieved through the reduction of heat
exchanger size. A comparison of the dimensions for both the evaporators and condenser/gas cooler can be
found in Figure 2. The reduction of the evaporator volume was achieved by using an evaporator coil
originally designed for a 17 kW R134a system. This resulted in a 40 % reduction in face area, primarily in
height, and a 55 % reduction in evaporator volume. The R744 gas cooler was a commercially available
model with a much narrower design compared to the R134a condenser. The reduction of the gas cooler
volume was approximately 50 %. An R744 compressor that would provide similar capacity at the rating
condition for the baseline R134a was chosen. This compressor was of a semi-hermetic reciprocating design.
It has been demonstrated several times before (Bullard, 2004 and Elbel and Hrnjak, 2008) that the
performance of transcritical R744 system can be optimized using the high side pressure. In order to allow for
the ease of this optimization at each test condition, an electronic expansion device was installed that allowed
the high side pressure to be varied during testing. The same model blower was used to move air over the
evaporator in both systems, at a volumetric flow rate of 1800 1/s. For the R134a system, this resulted in an
evaporator face velocity of 1.56 m/s. The face velocity in the R744 system was 2.60 m/s.
Evaporator Dimensions Condenser/Gas Cooler Dimensions
R134a R-744
R-134a
R744
2
22m 9m 109mm m
127 mm m - 203 m m
Figure 2. Heat Exchanger Dimensions
In addition to the two systems described above, an R744 system with internal heat exchange was tested. This
system was the same in all respects as the original R744 except for the addition of an internal heat exchanger
between the liquid line and the suction line. This internal heat exchanger (IHX) was designed to have an
effectiveness of approximately 70 %. A schematic of the instrumentation installed in the baseline R134a and
R744 units and the test facility is shown in Figure 3. The heat pump system was instrumented in such a way
as to achieve five separate energy balances, two on the cooling side of the cycle, and three on the heating
side of the cycle. On the cooling side of the cycle, the two balances are achieved on the air stream and the
refrigerant stream, respectively. Determination of the cooling capacity on the air stream is determined using
a separate wind tunnel directly connected to the evaporator air discharge of the heat pump unit. This wind
tunnel was built and instrumented according to ASHRAE Standard 37-2005. While the heat pump unit is
equipped "off the shelf' with a blower to provide air flow over the evaporator, this blower is not strong
enough to provide the pressure head required to overcome the pressure drop caused by the flow nozzles used
to determine air flow rate. For this reason, a "helper" blower has been installed at the exit of the wind tunnel
to provide the additional pressure lift required to maintain the flow rate provided by the blower integrated
into the heat pump water heater unit. The second determination of the cooling capacity of the system is
through measurements obtained on the refrigerant flow stream.
On the heating side of the system, three different energy determination methods were employed. Using the
instrumentation on the refrigerant cycle described above, the heating capacity can be determined from the
temperatures, pressures, and mass flow values in the condenser/gas cooler. The second heating capacity
determination was made using temperature and mass flow measurements on the water stream. This is in
accordance with the testing of Type IV heat pump water heaters specified in ASHRAE Standard 118.1.10"h IR Gustav Lorentzen Conference on Natural Refrigerants, Delft, The Netherlands, 2012
3
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Bowers, Chad; Petersen, Michael; Elbel, Stefan & Hrnjak, Pega. NEXT GENERATION COMMERCIAL HEAT PUMPWATER HEATER USING CARBON DIOXIDE USING DIFFERENT IMPROVEMENT APPROACHES, article, April 1, 2012; United States. (https://digital.library.unt.edu/ark:/67531/metadc828356/m1/3/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.