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4.4.2 Supplementary heating options Below a certain outdoor temperature (the balance point), with the compressor displacement at it maximum, the capacity of the system cannot be increased to meet the requirements of the load and some sort of supplementary heating is required. Three possible air-side comfort control options are illustrated in Figure 4.13. For Option A, the air temperature would be raised below the comfort constraint in the gas cooler, then increased to the comfort constraint with the supplementary heat. With Option B, the air temperature is raised to the comfort constraint in the gas cooler, then above the comfort constraint by the supplementary heat. For Option C, the airflow can be split and the temperature of each stream raised such that when they are recombined the comfort constraint is met. Since, theoretically, any division of the airflow in Option C is possible, it is the least constrained case, making a meaningful analysis difficult. As a result, only Options A and B are compared, as shown in Figure 4.14 assuming a balance point temperature of –10.5oC. With Option A, the gas cooler pressure is reduced sharply as the evaporating temperature decreases. Because the low efficiency supplementary heating dominates the overall cycle efficiency, however, the advantage of the fixed supply air temperature case is not as high as might be expected. At an evaporating temperature of –20oC there is only a 3% improvement in delivering air at 60oC by fixing the supply air temperature (Option A) as compared to delivering air at 93oC by letting the air temperature vary (Option B). Since the hotter air would require less blower power, Option B could be the most efficient if adequate room air circulation could be maintained. Since the hotter air would require less blower power, Option B could be the most efficient if adequate room air circulation could be maintained. A Heat B C Exch. Suppl. Heat Comfort Const. C B AA Heat Exch. Suppl. Heat Parallel Heating Option C A Heat B Exch. C Suppl. Heat C B Comfort Const. Fixed Supply Air Temperature Option A Figure 4.13 Supplemental heating options Variable Supply Air Temperature Option B 33 Te m p e r a t u r e Te m p e r a t u r ePDF Image | Comparison of R744 and R410A
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