Design Criteria and Software Development for Finned Tube Evaporator Using R744
Friterm R&D Department has evaluated CO2 evaporators and gas coolers used both in subcritical and transcritical refrigeration systems under the title of environmentally friendly technologies, which took place in the scope of The Scientific and Technological Research Council of Turkey (TUBITAK) Funding Program. As part of this evaluation, not only a laboratory has been constructed but also new software has been developed for finned-tube R744 (CO2) heat exchangers. The software data have been compared to the test results so as to confirm compatibility between these two methods. The information in the following parts is aimed to address the data collected from this evaluation.
Finned-tube evaporators can be employed both in subcritical and transcritical refrigeration systems.The key designing data are classified into three fundamental groups.
- Air side design data
- CO2 side design data
- Heat exchanger design data
Air Side Design Data
- Atmospheric pressure - the atmospheric pressure must be known so that the physical properties of air relative to the pressure can be defined. The term of “altitude” is preferred instead of “atmospheric pressure”.
- Air flow - the amount of air flow should be known to meet required capacity.
- Inlet temperature - the inlet temperature of the ambient in where the cooler run should be known.
4. Relative humidity - the relative humidity of the ambient in where the cooler run should also be known. Additionally, wet bulb temperature can substitute for relative humidity.
CO2 Side Design Data
- Refrigerant flow - If the refrigerant flow is known, it becomes easier to calculate the outlet conditions of the given evaporator. The superheat can be calculated according to refrigerant flow. If unknown, the following properties along with dryness fraction should be given.
- Evaporation temperature – the temperature difference between ambient temperature and evaporation temperature should be determined.
- Dryness fraction - the dryness fraction of the refrigerant entering the evaporator in transcritical R744 cycle must be known in order to define the inlet conditions of the refrigerant. This value is dependent on the gas cooler’s operation pressure and outlet temperature. In Subcritical R744 cycle, the condensation temperature and subcooling degree should be known.
Figure 1. CO2 evaporator testing.
- Superheat - the dryness fraction should be known to define evaporator outlet conditions.
- Subcooling - it is the difference between condensation temperature and the temperature before expansion valve described in subcritical R744 refrigeration cycle.
Heat Exchanger Design Data
The finned-tube heat exchanger’s design data are given as follows:
In light of the data given above, the CO2 evaporators were designed and after that they were tested in the “Calorimeter Room”.
In order to test CO2 evaporators, the given conditions are shown in Table 1.
The results collected from the tests have been analyzed in detail by means of FRTCOILS and are shown in Table 2.
Test results were in comply with the calculation of software. In other terms, the software calculations have been supported by the test results. Consequently, a new software has been built which enables the designing of finned-tube CO2 evaporators and gas coolers.
So far it has been seen that a system choice which is in line with the designing conditions is crucial. The needed software evaluation has been carried out for the evap-
Figure 2. CO2 refrigerant preparation unit.
Table 1. Evaporator test conditions
|Table 2. CO2 Evaporator testing results.
orators and gas coolers -the most important components of the system. In this brief paper, only the studies carried out for the evaporators are considered. The software efforts have been supported by the test results of the prototypes. It is concluded that test results are in comply with the calculations of the software. In conclusion, FRTCOILS has been developed for the finned-tube CO2 heat exchangers.