Dehumidification >> How To Pick A Dehumidifier

This unit will be used to illustrate all of How To Pick A Dehumidifier 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, How To Pick A Dehumidifier REHEATING Although their use in dehumidification is relatively new, 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, humid incoming air to the cold exiting air, which reduces or How To Pick A Dehumidifier eliminates the external reheat energy requirement. The air leaving the unit is warmer and dryer.

 The arrangement of the equipment room and the entering- and leaving-air duct work poses a major challenge to the side-by-side placement of the joined sections of the heat pipe or How To Pick A Dehumidifier 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 to either water circuits or electricity. • They are How To Pick A Dehumidifier not susceptible to damage from freezing conditions. • They do not require a compression tank.

 Disadvantages. Disadvantages of heat pipes are: • The coils are more expensive than are run-around coils because they are not mass-produced and each of the many refrigeration circuits in heat pipes must be individually evacuated and How To Pick A Dehumidifier 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 or capillary transfer of the condensed refrigerant from the heating coil to the precooling coil. • Multiple reheat coils are not available.

 Ã‚• If heat pipes are installed in an AHU, How To Pick A Dehumidifier 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, and charged by the manufacturer at the job site. (For small AHUs, a precharged wrap-around heat pipe is available.)

 AIR-TO-AIR FLAT-PLATE HEAT EXCHANGERS Flat-plate heat exchangers consist of a series of flat or How To Pick A Dehumidifier nearly flat heatexchange surfaces arranged in such a way that alternating air passages are connected to one air stream with remaining passages connected to the second air stream. Figure 6 illustrates the recuperative application of the air-to-air flat-plate heat exchanger. The precooling section should be sloped in the direction of air flow to a condensate drain.

 Advantages. The main advantages of flat-plate heat exchangers are: • They have no moving parts. • They have no external connection to water circuits, ethylene glycol, refrigerant, How To Pick A Dehumidifier or electricity. • They have no heat-transfer fluids that are subject to freezing.

 Disadvantages. Disadvantages of flat-plate heat exchangers are: • Control is impossible unless bypass dampers are used. • The physical arrangement of How To Pick A Dehumidifier their external components is more restrictive than that of the other recuperative-dehumidification enhancements because of the requirement of alternate air streams in cross-flow or counterflow arrangements. • ARI-certified ratings are not available. • Some leakage or cross flow may occur if the sealant at the edge of plates is not perfect.

 ROTARY-WHEEL HEAT EXCHANGERS The rotary-wheel heat exchanger usually consists of a honeycomb (fluted) aluminum rotor. An 8-in.- wide corrugated layer of aluminum sheet and an 8-in.-wide flat aluminum sheet are wound around a hub to create an 8-in.-deep rotor (depth varies by manufacturer) with diameters up to about 8 ft. The rotor usually is encased in a housing (cassette) that includes seals at the wheel periphery and How To Pick A Dehumidifier at the sectional divider between air streams. The rotor housing is ducted to the two air streams as shown in Figure 7. One half of the rotor picks up heat from the warm air stream. As the rotor rotates at about 20 rpm, this heat is transferred to the cool air stream.

 Advantages.The main advantages of rotary-wheel heat exchangers are: • There is no water, ethylene glycol, How To Pick A Dehumidifier or refrigerant charge. • Control is easy. • They have no heat-transfer fluids that are subject to freezing. 

 Disadvantages. Disadvantages of rotary-wheel heat exchangers are: • A restricted physical arrangement of inlet and outlet air streams. • The requirement of 120-v, How To Pick A Dehumidifier 60-cycle single-phase power to a fractional-hp motor. • ARI-certified ratings are not available. • The media in the rotor may be susceptible to clogging. • Some drive systems have been unreliable or have required high maintenance. • The life of the rotor may be shorter than that of other options. • Leakage between air streams reduces effective air quality and performance. The initial leakage rate may be acceptable; however, the wearing of seals, etc. can result in increased leakage. • Condensation may be detrimental to some types of rotors and media.

 REFRIGERANT LIQUID SUBCOOLING/AIR REHEATING Reheating a cooled air stream using heat from liquid refrigerant en route to a direct-expansion cooling coil is quite desirable from an efficiency standpoint, How To Pick A Dehumidifier as it requires only a single coil to accomplish what the previous arrangements do with two heat-transfer devices. Figure 8 illustrates this dehumidification enhancement. The temperature of the refrigerant liquid supplied to a direct-expansion coil often is in the 90- to 115-F range.

 The reheat coil uses this warm liquid to reheat the cold air stream while simultaneously subcooling the liquid refrigerant. The subcooled liquid refrigerant then enters the directexpansion coil and, How To Pick A Dehumidifier because it is subcooled, increases the latent and total capacity of the cooling coil. Figure 9 shows compressor-condenser capacity with and without subcooling as well as evaporator-cooling-coil capacity.

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