Dehumidification >> When Is Dehumidification Required?

The main cooling coil without being re-evaporated by radiant or conducted heat. The main cooling coil must be When Is Dehumidification Required? selected at a coil face velocity at least 150 fpm below the maximum cataloged face velocity to ensure that moisture does not carry over onto a reheat coil (liquid water carryover onto an active reheat coil results in additional energy consumption and humidification).

AHU fans and motors (if the motor is in the air stream) should be located downstream from the recuperative reheat system for optimum effectiveness. If a supplementary reheat coil is When Is Dehumidification Required? used and its leaving-air temperature is above 100 F, then the fan motor must be selected for reliable operation at the actual temperature conditions at the motor. 

It may be necessary to derate the motor and use Class F or H high temperature motor-winding insulation. Special motors are not required for recuperative-reheat systems not equipped with supplementary reheat coils. 

Because of the possibility of moisture condensing on precooling heat-transfer surfaces during part-load operating conditions, provisions for When Is Dehumidification Required? collecting and draining condensate must be made. Heat-exchange surfaces will accumulate particulate even with well maintained filters. Surfaces should be accessible and cleanable without difficulty or high cost.

The fifth recuperative-dehumidification enhancement is superior to the other four in many respects because it achieves almost the same result with a single coil located downstream from the main cooling coil. The single coil reduces first cost, When Is Dehumidification Required? halves the air-side friction of the recuperative elements, and requires no space or modifications upstream from the main cooling coil. 

With the fifth enhancement, which is applicable only to systems using refrigerant coils served by compressors, the amount of reheat is limited. If a significant temperature rise is required, When Is Dehumidification Required? this enhancement may need to be combined with one of the other four. Details on each of the five recuperative-dehumidification enhancements follow. 

Run-around coils for enhanced dehumidification systems are not a new technology. They are well-described in texts from as long ago as 1939.
The psychrometric-chart analysis of run-around coils for precooling and reheating is identical to that for ordinary reheat (see sidebar) with one 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,When Is Dehumidification Required? 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, 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-hotwater lines are in the vicinity, When Is Dehumidification Required? 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, 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, When Is Dehumidification Required? 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, When Is Dehumidification Required? 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 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 of runaround coils are:  They can be widely separated and arranged in any position. If desired, When Is Dehumidification Required? 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, labor contributes little to their cost. They are, for all practical purposes, commodity items and,When Is Dehumidification Required? 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 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. Disadvantages of run-around-coil systems are: Possible freezing if water is the When Is Dehumidification Required? circulating fluid (this can be overcome by using a dilute ethylene glycol solution 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, When Is Dehumidification Required? 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 and leaving sides of a specially configured unit.

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