HVAC CYCLE BASIC INFORMATION AND TUTORIALS

Figure 2.1 is a schematic representation of an elementary mechanical cooling cycle. While dehumidification is not an essential part of the cooling cycle, it usually occurs where the cooling medium is colder than the dew point temperature of the air. 




The cooling load QC in the conditioned space is a combination of internal and external loads (e.g., people, lights, solar) and is usually removed by circulating air through the space, with the entering air having a lower temperature and humidity than the desired space condition. 


To offset the cooling load, the temperature and humidity of the supply air are increased to equal those of the space, and then air is returned to the air-handling unit (AHU), where it is recooled and dehumidified. 


Most spaces require some (outside) ventilation air, which is mixed with the return air at the AHU, thereby imposing an additional cooling load QV. If the outside- air enthalpy is less than the space enthalpy, then QV will be negative and some ‘‘free cooling’’ will be obtained. 


Work energy is required to circulate the air—usually a fan driven by an electric motor—and this work QW becomes a part of the cooling load. 


The total load represented by QC + QV + QW must be removed by the heat rejection equipment, usually a refrigeration system. Some additional work is done here by pumping fluids and driving refrigeration machines and condenser or cooling-tower fans. 


Ultimately all this heat energy is dumped to a heat sink—sometimes water, most often atmospheric air. Note that the work portions of this cycle are parasitical. They contribute additional heat which must be removed, reducing the overall system efficiency.




Figure 2.2 shows schematically an elementary heating cycle, again using an air-handling unit. In this cycle, the ventilation load is usually negative, requiring additional heating, but the work factors contribute to the available heat. 


Thus, most heating systems are more effective overall than most cooling systems, when effectiveness is defined as the heating or cooling done, divided by the energy input. In Figs. 2.1 and 2.2, convective and radiation losses from piping, ductwork, and equipment have been neglected for simplicity. These
factors may become important, particularly in large systems.

Related post



No comments:

Post a Comment

Archive