Refrigerant side compensation for air-side maldistribution of evaporators and its effects on system performance
Evaporators found in most HVAC&R applications use multiple circuits to address the multidimensional optimization problem of cost, capacity, and air-/refrigerant side pressure drop. However, an inherent problem of having multiple refrigerant circuits in evaporators is that maldistribution can occur. This maldistribution can be caused on the refrigerant side by quality and/or mass flow maldistribution at the refrigerant distributor. On the air-side, maldistribution can be caused by uneven airflow and/or air inlet temperature; on both sides the possibility of uneven fouling exists. To reduce the performance penalties caused by air-side maldistribution, the hybrid control method was used to optimize the refrigerant distribution to individual circuits. The hybrid control uses a primary expansion valve, which provides for most of the pressure drop, and smaller secondary balancing valves, which adjust the individual circuit mass flowrates. This approach was chosen since it is expected that the balancing valves can be manufactured at much lower cost than individual expansion valves for each circuit. In order to assess how severe the influence on maldistribution is, four applications with high potential for air-side maldistribution were selected and investigated experimentally, as well as in simulation studies. Application 1 was a 3-ton R404A walk-in cooler refrigeration system for a storage room with a target temperature of 2°C (35.6°F). It was found that the increasing air-side maldistribution, caused by uneven frost built up over time can lead to severe hunting with liquid overfeed from the evaporator, when using the factory installed thermostatic expansion valve (TXV). An electronic expansion valve (EXV) was used as baseline for the performance evaluations. It was found, that in both the TXV and EXV case, significant refrigerant maldistribution occurred even if the system was tested with no enforced air-side maldistribution. After including the balancing valves, the system coefficient of performance (COP) increased by approximately 4% in COP and 6% in capacity with negligible dependence on ambient temperature. Subsequently, the influence of maldistribution, in this setup expected to be mainly caused by frost built up, was investigated. Since frost built up is difficult to reproduce, two different levels of coil blockage were chosen to produce repeatable measurements. Both blockage levels led to less COP and cooling capacity degradation than observed at the end of an 8 hour evaporator frosting test with EXV as control scheme. It was found that COP and capacity degrade with increasing coil blockage and increasing system cooling capacity. The maximum degradation, compared to the baseline, was a 30% reduction of capacity along with a 23% reduction in COP for using the EXV control scheme and the more severe coil blockage. For the hybrid control, the maximum reduction of capacity and COP both were with 8% compared to the EXV baseline much less severe. Application 2 was a 5-ton R410A domestic heat pump. In contrast to application 1, the evaporator is designed as a condenser with small fin spacing, since the unit is designed for AC mode and reversed for HP mode. As a result, even small amounts of frost built up lead to significant performance degradation - requiring frequent defrosts. It was found that with the hybrid control scheme the time period between defrosts can be prolonged by 30%, if similar COP degradation is taken as measure for the initiation of the defrost cycle. To address the issue of repeatability of frost built up, additional tests with enforced air-side maldistribution were conducted. It was found that the hybrid control leads to a 4 to 26% improvement of COP along with a 4 to 30% improvement in capacity. Application 3 was a 4-ton R410A rooftop unit. Rooftop units with economizer have a mixing chamber, in which return and make-up air from the economizer are mixed before entering the coil. Due to space constraints, the mixing chamber is small, which results in uneven air flowrates well as , temperature and humidity maldistribution. In contrast to applications 1 and 2, the degrading effects of maldistribution will even be noted during performance testing with clean evaporator coil. It was found that the achievable performance improvements for the refrigerant side COP are in the range of 1.5% to 16.6%, dependent on the ambient temperature. Application 4 was a 5-ton R410A cold climate heat pump. Cold climate heat pumps target climates that have a larger number of heating degree days close to or below freezing point than occurring in the application area of conventional heat pumps. Reduction of uneven frost built up, resulting in a reduced number of required defrosts, is essential for improving their performance and can be achieved by improved refrigerant flow distribution. For economic reasons, a reduced hybrid control scheme is employed, where 2 circuits are paired with 1 balancing valve, reducing the required number of valves and control electronics. In addition to the evaporator flow control, the same heat pump is a test vehicle for a two stage vapor injected compression using a single scroll compressor with dual injection ports. A vapor injected compressor takes flash gas generated during the expansion process of the liquid refrigerant and injects it into the ongoing compression process. To address the issue of cost, numerous low cost balancing valve concepts for the hybrid control were developed as part of the CEC project. These valve concepts can be divided in passive concepts which used thermal expansion or saturated vapor pressure, and active concepts which used thermoelectric or electromagnetic principles. While the passive approaches have the benefit of not using any additional energy, the active concepts lead to more flexibility for control algorithms and can be used as virtual sensors. The valve characteristics and dimensions were estimated and the results were used for preselecting possible candidates. The 2-step electromechanical valve looked most promising. Therefore a simplified valve prototype was designed and built by Emerson. Subsequently, the dimensions of the plunger were determined for the usage in application 4, using a dedicated valve test stand. An entirely passive concept could be a favorable alternative to actively controlling the flow distribution among circuits, particularly in regard to possible reliability issues with valves. An interleaved circuitry evaporator was therefore investigated as possible alternative through simulation studies.
Braun, Purdue University.
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