In many applications, control of the relative humidity in the refrigerated space is important. When storing many varieties of fresh produce, the humidity should be kept high to maintain product quality. In certain other applications, such as for rooms chilling hot red meat carcasses, humidity should be kept low to avoid fog formation and to prevent moisture from condensing on surfaces and dripping on product.
To maintain high humidities, choose large coils operating with small air to refrigerant temperature differences. The air-flow rate will be high to provide the refrigeration duty with the small drop in air temperature. For low humidities, select small coils with low air-flow rates and large air-to-refrigerant temperature differences.
This section points out the limitations of coils in meeting extreme conditions and explains why humidifiers (for high humidity) or reheat (for low humidity) may be necessary. Most applications where the control of humidity is important
are in spaces operating in the temperature ranges of 0–10°C (32–50°F), so Figs. 6.32 and 6.33 provide psychrometric charts with this range of temperature enlarged.
To dramatize the challenge of selecting coils for high-humidity spaces, consider a room to be matintained at 1°C (34°F) and 95% relative humidity.
Other refrigeration load data and characteristics of the space are:
In the exchange of outdoor and room air in the infiltration process, air with a moisture content of 0.017 kg/kg (0.017 lb/lb) enters to displace room air having a humidity ratio of 0.0038 kg/kg (0.0038 lb/lb). The rate of moisture introduced is, then:
The total of the sensible and latent loads from the infiltrated air is 31.2 kW (106,600 Btu/hr) which may be checked by computing the product of the airflow rate and the differences in air enthalpy of the outdoor and interior air,
Presenting the numerical values (which, incidentally, came from an actual load calculation) shows how the designer is squeezed to maintain high humidity in this room. The slope of the coil condition curve to meet the combination of
sensible and latent loads, as shown in Fig. 6.29, will be very flat. An expanded sketch of the pertinent portion of the psychrometric chart is shown in Fig. 6.34, indicating that coil-condition-curve A intersects the saturation line at approximately 0°C (32°F).
When correlating coil condition curve A to the straight-line law, the average wetted surface temperature of the coil should be slightly lower than 0°C (32°F), and the refrigerant temperature would be perhaps, -1°C (30°F). Thus, the temperature difference—entering air to evaporating refrigerant—would be approximately 2°C (3.6°F), rather than the typical commercial Deltat mentioned in the previous section. An alternate approach is to operate with a larger airtorefrigerant temperature difference and combine with humidification. Coil condition curve B shows a steep slope resulting from operating at a higher airtorefrigerant temperature difference. Without auxiliary humidification, the
design relative humidity of 95% cannot be maintained, because the rate of moisture removal at the coil is higher than that entering with the infiltration air. Table 6.8 summarizes the two different approaches to maintaining high humidities.
The recommendation of operating with high air-flow rates to maintain a high humidity should be qualified when refrigerating floral products or leafy vegetables. Experience indicates that when these products are not covered or
protected in some way from air currents, that increased velocities actually dry out the product, even though the humidity of the air has been increased. The conclusion is that if elevation of the humidity incorporates increasing the airflow rate over the coil, it is imperative to protect the product from air currents in the refrigerated space.
An additional consideration is what happens at part load—the condition when outdoor air is cool and dry. In one sense, the moisture brought in with the high-humidity infiltrated air was a benefit because it helped maintain high humidity in the space. When the outdoor air has low humidity, this source of moisture is no longer available. The saving feature, however, is that the sensible load will also have dropped when the outdoor conditions are mild, which permits the coils to be operated with a low air-to-refrigerant temperature difference. Many designers anticipate that, when the design conditions are accommodated, the part-load operation will not require special provisions. At the opposite end of the humidity spectrum is the requirement to maintain low humidity in the space. The following example illustrates a typical difficulty and how it might be solved.
A space storing seeds is to be maintained at 5°C (41°F) and 50% relative humidity and has design refrigeration loads of 110 kW (375,300 Btu/hr) sensible and 20 kW (68,200 Btu/hr) latent. To satisfy the ratio of sensible and latent loads, the air leaving the coil must be along the load-ratio line, shown in Fig. 6.35a.
The load-ratio line is positioned by proportioning Deltahs, and Deltahl such that they match the ratio of the sensible-to-latent loads, namely, 110 kW to 20 kW. Figure 6.35a shows that the load-ratio line does not intersect the saturation line until an unreasonably low temperature of -19°C (-2°F). The remedy, as shown in Fig. 6.35b, is to perform cooling and dehumidification with a moderate refrigerant temperature, and then reheat the air back to the load-ratio line. The reheat energy need not necessarily be applied to the outlet air of the coil, but can be introduced anywhere in the space. The net result of the supplementary heat is to change the load-ratio line of the space. When the layout of the refrigeration system is appropriate, the heat for reheat can be supplied by discharge vapor from the compressor, which results in energy effectiveness of the operation.
Another low-humidity application where reheat may be beneficial is on loading docks adjacent to frozen-food storage areas. The temperature setting for the dock may be approximately 7°C (45°F) and when the outdoor temperature drops to this temperature range there may be little call for cooling by the dock coils. The relative humidity could rise in the dock area such that this high humidity air infiltrates to the low- temperature space resulting in the deposit of considerable frost on the coils. Many operators equip their docks with reheat coils supplied with compressor discharge gas to force the refrigeration coils to cool and especially dehumidify the air when the outdoor temperatures drop to near the temperature settings on the dock.