Dehumidification load

Dehumidification load The mass of water vapor to be removed from a space or a process in order to meet design conditions. 

Dehumidification. Dehumidification may be accompHshed in several ways (see Drying). Moderate changes in humidity can be made by exposing the air stream to a surface whose temperature is below the dew point of the air. The air is cooled and releases a portion of its moisture. Closed cycle air conditioning systems normally effect dehumidification also. The cooled air may require reheating to attain the desired dry-bulb temperature if there is insufficient sensible load in the space. 

Example 5 Cooling and Dehumidification Find the cooling load per pound of dry air resulting from infiltration of room air at 80 F dry-bulb temperature and 67 F wet-bulb temperature into a cooler maintained at. 30 F dry-bulb and 28 F wet-bulb temperature, where moisture freezes on the coil, which is maintained at 20 F. 

This is the exchanger where heat flows from the room return or mixed air to cold refrigerant or to chilled water. It is an arrangement of finned tubes normally of aluminum fins on copper tubes, but copper fins can be specified for corrosive atmosphere. Performance characteristics are controlled by fin and tube spacing. If the room rh is high, dehumidification may be brought into use by operating the coil or one of a number of parallel coils at a low temperature. If the room s sensible heat load is low reheat must be allowed to operate at the same time. 

Dehumidification can be achieved by partially bypassing the coil such that the remaining air travels through the coil at low velocity. This can also be inherent in the full-load design operation of the coil. 

This is used when (1) the room needs heating instead of cooling or (2) for reheat as described above. It is vital in close control systems that its capacity is sufficient to maintain room temperature under these conditions, otherwise the system may fall into a loop, with the controls continuing to see high rh due to temperature. Using only part of the cooling coil for dehumidification will alleviate this situation. A heater capacity of the sensible heat extracted during dehumidification plus half the peak winter fabric loss is recommended where the room load could be nil in winter such as a start-up situation. 

In conclusion, the demonstration runs have shown that the membrane contactor dehumidification system is effective when the total loads of the refrigerated cell do not require an evaporative temperature of the cooling coil below —10 °C. In these conditions it has been shown that energy, operating costs and C02 emissions can be decreased up to 20% when optimized systems are built. The optimization is related mostly to the selection of adequate pumps, fans, heat exchangers. 

In assessing the ability of an HVAC system to maintain acceptable humidity levels in the building, the more important performance parameter is the sensible heat ratio (SHR) of the equipment. The SHR is defined as the ratio of the sensible cooling load to the total cooling load. Improved latent performance is characterized by a lower SHR. Defining the relationships between SHR and other dehumidification indices is also convenient. In particular, the latent heat ratio (LHR) can be defined as the latent cooling load fraction, and the latent to sensible ratio (LSR) can be defined as the ratio of latent load to sensible load. These ratios appear as... 

Brandemuehll evaluated the ability of unitary equipment to maintain adequate space humidity levels. The evaluation involved the dehumidification requirements in different buildings and at different climates. He analyzed the building load characteristics to identify and describe key factors that affect humidity control in commercial building applications. The results indicated that ventilation loads dominated the dehumidification characteristics of most commercial buildings and dramatically affect the SHR of the system loads. He concluded that, in most cases, the dehumidification requirements were beyond the capability of conventional unitary HVAC equipment. ... 

Water Removal Heat Load. In addition to the amount of water to be removed, an important parameter in the design of a dehumidification unit is the associated heat load. That quantity can be computed from the relevant enthalpies read from the humidity chart. We have for the case cited... 

The heat transfer coefficient is a function of vapor and liquid flow rates. The volumetric heat transfer coefficient value from Equation 6-23 is based on a vapor capacity factor (Fg) of 1,27 Ib /ft s and a water rate of 3,000 Ib/ft h. Even though theory suggests that the heat transfer coefficient varies as the 0.83 power of the gas rate, Nemunaitis and Eckert report that the coefficient varies as the first power of Fg for the dehumidification of air [10]. The same authors also state that the coefficient varies as the 0,68 power of the liquid rate. Thus, if the liquid-to-gas ratio is held constant, the heat transfer coefficient increases much faster than the hydraulic loading. 

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