ASHRAE Journal - June 2021 - 43

COLUMN ENGINEER'S NOTEBOOK
FIGURE 1 Hot gas reheat. Reheating the air allows the cooling coil to stay active
and continue to dehumidify without overcooling the space.
Variable Speed
Compressor to
Minimize Cycling
Reheat Coil
Condenser Coil
Air Conditions After Evaporator Coil
55°F DB/54°F WB; 94% Relative Humidity
Can Be Lower Temperature
TXV
Evaporator Coil
Condensate Drained to Sump
and Pumped to Cooling Tower
Pump to Cooling Tower Fill Piping
FIGURE 3 Dual wheel system. This system operates on similar principles to the
wrap-around heat pipe.
FIGURE 2 Wrap-around heat pipe. This is an effective way of removing more
moisture than a standard system without using any extra energy or moving parts .
Methods to Improve Dehumidifi cation Capabilities
The cheapest method to improve dehumidifi cation
for DX systems is to add an electric reheat coil that is
controlled based on space relative humidity (RH) or dew
point (DP). This forces the compressor to run. However,
it is not energy effi cient. (Think of it as a car speed control
system in which you run the engine fully and modulate
the brakes to control speed.) This method is not recommended
unless you are in a climate in which dehumidifi
cation is very rarely needed. Another method,
hot gas bypass, is similarly ineffi cient and is restricted
or prohibited in many situations by energy codes such
as ASHRAE Standard 90.1 and the International Energy
Conservation Code (IECC).
On the other hand, one option we often use is hot
gas reheat (Figure 1). This option diverts some or all hot
refrigerant vapor from the condensing unit to a reheat
coil. Hot gas reheat can be modulating or nonmodulating.
Reheating the air allows the cooling coil to stay
active and continue to dehumidify without overcooling
the space.
Another option is a wrap-around heat pipe (Figure 2).
One coil is located upstream of the cooling coil and
another downstream. The coils are connected by refrigerant
tubing. When hot air fl ows over the upstream heat
pipe coil, the refrigerant inside the coil evaporates and
fl ows to the downstream heat pipe coil. The upstream
coil precools the entering air, reducing the load on the
cooling coil. When the cooled and dehumidifi ed air exits
the cooling coil, the downstream heat pipe coil reheats
the air with the warm refrigerant from the upstream
coil. The refrigerant inside the heat pipe condenses and
fl ows back to the upstream coil by gravity fl ow. Normally,
no pump is needed. This is an effective way of removing
more moisture than a standard system without using
any extra energy or moving parts. A variation of this is to
add an outdoor air enthalpy recovery device.
A third option is a dual wheel system (Figure 3), which
operates on similar principles to the wrap-around heat
pipe. It consists of a sensible heat wheel that exchanges
heat between the return or exhaust air and the supply
air. This provides reheat to the dehumidifi ed supply air
and precools the return air. It also has an enthalpy wheel
that uses the cooled return or exhaust air to precool the
outdoor air, which allows the cooling coil to save energy
when cooling the supply air.
When Is it Appropriate to Use One of These Methods?
Many resources describe these methods and others
in more detail, but how do you know if you should use
one of the part-load capacity control systems for your
J U N E 2 0 2 1 ashrae .o rg ASHRAE JOURNAL
43
COURTESY GREENHECK AND BRIJESH PANCHAL (IMEG)
Precooling Coil
Cooling Coil
Reheat Coil
IMAGE COURTESY OF FLÄKTGROUP SEMCO
IMAGE COURTESY OF HEATPIPE.COM, ARMIN HAUER (EBM-PAPST)
AND BRIJESH PANCHAL (IMEG CORP.)
http://www.HEATPIPE.COM https://www.ashrae.org/
ASHRAE Journal - June 2021

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