Cooling and Dehumidifying

The process of sensible cooling is represented on the psychrometric chart by a horizontal line extended to the saturation line. If air is cooled sensibly, it changes in its db and wb temperature, RH, and total heat, but does not change in its moisture content, dewpoint temperature, or vapor pressure. For example, to determine how much heat must be removed to sensibly cool air having a db temperature of 80°F (27°C) and 50% RH to 50°F (10°C), identify the humidity ratio and enthalpy at 80°F (27°C) and 50% RH. This is 26 grains and 23.4 Btu (0.0037 kg/kg and 54.4 kJ), respectively, per lbm (kg) of dry air. Proceed horizontally at a constant humidity ratio to 50°F (27°C), at which point enthalpy is 16.1 Btu/lbm (37.4 kJ/kg) of dry air. The difference, or sensible heat removal requirement, is 7.3 Btu/lbm (17.0 kJ/kg).

Until the air temperature reaches its dewpoint, all of the cooling is sensible. For example, an air stream at 70°F (21°C) and 50% RH can be sensibly cooled to 51°F (11°C) before any moisture is removed. At 51°F (11°C), it is saturated (100% RH). If it is cooled further, its moisture will begin to condense out of the air.

The db temperature and absolute humidity of the air stream and the cooling coil surface temperature determine sensible and latent cooling. If the cooling surface temperature is below the initial dewpoint temperature, this

Fig. 34-12 Cooling/Dehumidification Process. Source: Munters Cargocaire and Mason Grant Company process can be portrayed on the chart as a straight line extending from the initial condition to the surface temperature on the saturation curve. During this process, the db and wb temperature, moisture content, vapor pressure of the moisture, and total heat all decrease. The amount of moisture removed depends on how cold the air can be chilled. The lower the temperature, the drier the air.

The cooling/dehumidification process is illustrated diagramatically and the process air path is drawn on a psy-chrometric chart in Figure 34-12. In this example, air is cooled from 70 to 45°F (21 to 7°C) and moisture level is reduced from 56 to 44 grains/lbm (0.008 to 0.006 kg/kg).

To meet the specified temperature and humidity set points in a given space, the sensible and latent removal capacity of the refrigeration system must be equal to the corresponding fractions of the cooling load. In situations in which the LHF is much greater than the SHF, or where a low dewpoint temperature is desired, excess sensible cooling capacity must be designed into the system.

In order to satisfy the latent cooling requirement, the air stream must be cooled below the dewpoint temperature, or excess air must be introduced and cooled. When a system is designed to produce low humidity levels, the air stream must be reheated before being discharged to the space.

The required coil temperature depends on the humidity level desired. If latent load is high, design options to consider are:

• Use of lower temperature to dry part of the air and then mix.

• Use of some other special design.

An inherent inefficiency in conventional AC systems is the need to overcool in order to achieve a low dewpoint. For each degree of dewpoint temperature that the air stream must be cooled beyond the point it satisfies the sensible cooling requirement, the system must be over-designed, driving up system cost and driving down system performance. This means that there is more total cooling capacity requirement than there is total cooling load. This additional energy use is further increased by the energy required for reheating. For each required Btu (kJ or kWh) of sensible overcooling, there is a corresponding Btu (kJ or kWh) required for reheating.

At lower temperatures, moisture removal by cooling is less efficient. Also, the rate of sensible overcooling continually increases with each declining degree of dewpoint temperature. The corresponding rate of reheat require

• Maximum moisture content is proportional to air temperature

• Cooling the air removes moisture o


Sensible Cooling

• Cooling the air removes moisture

Sensible Cooling

Process Air

ment increases proportionally with the rate of sensible overcooling.

When cooling coil surfaces are held below the freezing point, frost will develop on the coil. This places a limit on how low conventional AC systems can reduce dewpoint temperature. The frost insulates the coil and it becomes physically clogged, reducing airflow and heat transfer efficiency. Defrost equipment must be designed into the system in order to eliminate ice build-up, further adding to the energy inefficiency. Thus, designing a system to achieve low humidity levels results in increased capital and operating costs for excess cooling capacity, reheat, and defrosting equipment.

While beyond the scope of this discussion, it is important to note that for refrigeration freezing processes, the psy-chrometric process extends to the product freezing range. Product temperature is brought down to the freezing range (sensible heat removal above freezing) and then water in the product changes to ice while the temperature remains constant (latent heat removal). Product temperature is then lowered below the freezing point to the ultimate storage temperature (sensible heat removal below freezing).

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