Dehumidification >> Calculate Dehumidification For A Flooded Building

Run-around coils for enhanced dehumidification systems are not a new technology. They are well-described in texts from as long ago as 1939.1, 2, 3, 4 The psychrometric-chart analysis of run-around coils for precooling and reheating is identical to that for ordinary reheat (see sidebar) with one Calculate Dehumidification For A Flooded Building exception.

The precooling coil cools the entering air from 84- to 72-F dry bulb (DB); therefore, the air entering the main cooling coil is cooler, Calculate Dehumidification For A Flooded Building which means that the main cooling coil does not have to remove as much heat. The reheat is provided not by an external source of expensive energy, but by the heat picked up by the circulating fluid in the precoil. 

The cycle and savings are illustrated in Figure 2. Figure 3 is a piping diagram showing that run-around coils require a small closed-loop piping system. When freezing is a possibility, Calculate Dehumidification For A Flooded Building the heat-transfer fluid is dilute ethylene glycol; at other times, it is water. The system requires a means of fluid makeup. A diaphragm compression tank normally is installed for fluid expansion. 

If water (not glycol) is used as the circulating fluid and chilled-water or heating-hot water lines are in the vicinity, Calculate Dehumidification For A Flooded Building it is possible to achieve fill and fluid expansion with a 3 ⁄8-in. connection to the chilled-water or heating-hot-water closed loop. The expansion of the fluid in the small run-around system then would be provided by the fluid-expansion equipment in the larger system. 

With only a single connection between them, Calculate Dehumidification For A Flooded Building there can be no flow between the two systems except for that induced by density forces of water at different temperatures. The system requires a fractional-hp pump, electrical service, and controls. The recommended pump for the circuit is a 120/60/1 hermetically sealed pump that does not require a shaft seal. 

If a pump with a seal is used, the seal should be mechanical rather than packed to minimize or eliminate leakage. The closed-loop piping system is close-coupled and confined to the immediate vicinity of the two coils. Once the circulating fluid is charged into the system, there is little possibility of leakage; therefore, Calculate Dehumidification For A Flooded Building the fluid-makeup system can be a hose connection or a funnel fill valve. 

Coils and piping must include manual air vents at high points to vent air at the time of initial filling. The piping connections Calculate Dehumidification For A Flooded Building to each coil must be arranged for "counterflow.” The lines between the two coils (and the coil headers if exposed to ambient air) must be insulated to prevent external heat transfer. The pump should be located in the warm-fluid line. 

All of the air passing through the air-handling unit must pass through both coils. Any deviations from these guiding principles will cause a loss in performance. Advantages. Advantages of runaround coils are: They can be widely separated and arranged in any position. If desired, Calculate Dehumidification For A Flooded Building a single precooling coil can be used in combination with multiple reheat coils. 

They carry the certification of the Air-Conditioning and Refrigeration Institute (ARI) and are available from at least 10 manufacturers. Because they are mass-produced using machinery, Calculate Dehumidification For A Flooded Building labor contributes little to their cost. They are, for all practical purposes, commodity items and, therefore, relatively inexpensive. 

Their selection can be made by the manufacturer or by the manufacturer's coil-selection program using software on a personal computer. The precooling Calculate Dehumidification For A Flooded Building and reheat function can be controlled by pump on-off cycling or valve control. The preheat coil can be piped to a third coil in the exhaust-air stream to capture heat for preheating outside air during the winter. 

Disadvantages of run-around-coil systems are: Possible freezing if water is the circulating fluid (this can be overcome by using a dilute ethylene glycol solution Calculate Dehumidification For A Flooded Building selected to provide burst protection at the coldest anticipated entering-air temperature). A 120-v, 60-cycle single-phase electrical circuit is required for the circulating pump. 

Air trapped in the coils, pump, and piping must be vented upon initial startup to ensure effective fluid flow and heat transfer. Figure 4 shows run-around coils at the entering Calculate Dehumidification For A Flooded Building and leaving sides of a specially configured unit. This unit will be used to illustrate all of the recuperative precooling and reheating concepts. 

It will graphically show that they perform the same function—even though some require specific physical arrangements. HEAT-PIPE RUN-AROUND PRECOOLING, REHEATING Although their use in dehumidification is relatively new, Calculate Dehumidification For A Flooded Building heat pipes have been around for decades. A heat pipe is a heat-transfer assembly that includes a heating section connected to and located slightly above a cooling section. 

In Figure 5, the lower cooling section is upstream from the main cooling coil, while the upper heating section is downstream. This allows for a transfer of sensible heat from the warm, Calculate Dehumidification For A Flooded Building humid incoming air to the cold exiting air, which reduces or eliminates the external reheat energy requirement. The air leaving the unit is warmer and dryer. 

The arrangement of the equipment room Calculate Dehumidification For A Flooded Building and the entering- and leaving-air ductwork poses a major challenge to the side-by-side placement of the joined sections of the heat pipe or an alternate arrangement with the cooling section below the heating section. The lines between the two coils (and the coil headers if exposed to ambient air) must be insulated to prevent external heat transfer. 

Advantages. The main advantages of heat pipes are: They have no moving parts. They have no external connections Calculate Dehumidification For A Flooded Building to either water circuits or electricity. They are not susceptible to damage from freezing conditions. They do not require a compression tank. 

Disadvantages of heat pipes are: The coils are more expensive than are run-around coils because they are not mass-produced Calculate Dehumidification For A Flooded Building and each of the many refrigeration circuits in heat pipes must be individually evacuated and charged with a critical amount of a volatile refrigerant. ARI-certified ratings are not available. 

Selection is made only by the manufacturer. Control is difficult and expensive (however, in many applications, control of the run-around coil or heatpipe system is not required). The physical arrangement of the coils must enable gravity drainage Calculate Dehumidification For A Flooded Building or capillary transfer of the condensed refrigerant from the heating coil to the precooling coil. 

Multiple reheat coils are not available. If heat pipes are installed in an AHU, then the AHU's coil section must be shipped to the heat-pipe manufacturer or the two heat-pipe sections must be joined by field-installed piping, with each circuit tested, evacuated, Calculate Dehumidification For A Flooded Building and charged by the manufacturer at the job site. (For small AHUs, a precharged wrap-around heat pipe is available.)

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