Technical Bulletin 1
Refrigerant Dehumidification
Technology
I
NTRODUCTION
enters the evaporator coil. (Point B Figures 1 and 3) Heat
transferred from the air and energy from water vapor
condensing on the evaporator coil convert the refrigerant
completely to vapor. Desert Aire's TXV has a variable orifice
that regulates the amount of pressure drop across the valve.
This maintains temperature of vapor at 12°F (6°C) higher at
outlet of the evaporator than at the inlet of the evaporator
coil. This temperature difference is called superheat. (Line
CD, Figure 1)
This technical bulletin will describe the refrigeration process
used in a mechanical dehumidifier. In addition, it will describe
key design features of the major components.
R
EFRIGERANT PROCESS
The best way to understand what is happening in a refrigerant-based dehumidifier is to keep in mind the impact pressure
has on the boiling point of a fluid, and that energy will migrate
from hot to cold.
This superheated vapor now goes to the compressor, which
acts as the other separating device between the high and
low-pressure sides. (Point D, Figures 1 and 3) As its name
implies, it compresses the refrigerant, thereby increasing the
pressure to between 200 psi and 270 psi. The refrigerant also
acts as the compressor's cooling device, so the refrigerant
picks up additional heat during this step. The refrigerant is
now a hot gas, typically above 130°F (55°C). (Point E,
Figures 1 and 3)
It is common knowledge that it takes longer to boil an egg at
5,000 feet (1,600 m) than it does at sea level. At higher
altitudes there is less pressure, which lowers the point at which
water boils. At 5,000 feet (1,600 m), water boils at 202°F,
(94°C) while at sea level it boils at 212°F (100°C). Refrigerant
behaves just like water in that the boiling point changes at
different pressures. Refer to Table 1 for examples of R22's
various boiling points.
Pressure Temp. Pressure
(PSI)
(PSI)
(°F)
As the hot gas enters the condenser it is cooled by air or
water until it reaches its condensation point and is converted
to a liquid. It is at a high relative pressure, but the change of
state (gas to liquid) gives up significant energy, thereby
cooling to a hot liquid near 90°F. The liquid refrigerant then
flows through a receiver until it is needed to repeat the cycle.
(Point F, Figures 1 and 3)
Temp. Pressure Temp.
(°F)
(°F)
(PSI)
50
26
140
78
240
114
60
34
160
87
260
120
80
48
180
94
275
124
100
59
200
101
290
128
120
69
220
108
320
136
Liquid Only
Table 1 - R22 Pressure/Temperature Relationships
Condenser
Pressure
We will choose the starting point of the refrigeration cycle at
the inlet of the thermal expansion valve (TXV). The TXV
creates a pressure drop across its internal orifice, which
separates the dehumidifier's refrigerant system into two halves:
high side and low side. (Point A, Figures 1 and 3)
Vapor Only
Liquid/Vapor
Supercooling
A
F
E
TXV
Compressor
Evaporator
C
B
D
Superheat
As the refrigerant leaves the TXV, it starts to boil because the
pressure is reduced to approximately 50 to 90 psi. The refrigerant is a mixture of cold saturated vapor and liquid as it
Enthalpy
Figure 1 - Refrigeration Process
D EHUMIDIFICATION COMPONENTS
Evaporator Coil Design
The optimization of dehumidification in the refrigeration
process requires careful design and selection of the system’s
components. The design becomes more complicated when a
wide range of inlet conditions occur and multiple condensers
are used to allow year-round control.
An evaporator must be designed to transfer the required energy.
Common variables used to design the evaporator include air
volume, number of rows of tubes, and type and amount of
fins per inch.
Compressor
Desert Aire uses scroll technology in all of its units (except
<2hp ) to provide the most favorable energy output for every
electrical input. In addition, its design reduces the number of
parts and provides long life.
Compressors put out a variable amount of energy depending
on the pressure differential of the high and low sides. The
greater the differential, the harder the compressor must work
to overcome the difference and the less it can output.
Therefore, the designer must know the dynamics of the
system before compressor performance can be determined.
Refer to Figure 2 for an example of the significant variance in
compressor capacity at typical conditions.
55%
100%
40%
75%
Outside Ambient
COLD
HOT
An air conditioner will generally use three (3) or four (4) rows
with 400 cfm per ton (12,000 BTU's) to be removed. This
combination provides a high sensible-to-latent ratio, which is key
to the system achieving a high EER (energy efficiency ratio).
A dehumidifier will use six (6) to eight (8) rows with a reduced
air volume of 200 cfm per ton. This provides a high MRE
(moisture removal efficiency) value. This becomes important in
high humidity applications such as pool facilities, industrial
plants and treatment of 100% outside air.
The second major impact is that the air conditioner type coil
creates an approach temperature (the difference between
leaving air temperature across the coil and the actual refrigerant
temperature in the coil) of 12°F to 15°F (6°C to 7°C), while
the dehumidifier design creates an 8°F to 10°F (4°C to 5°C)
approach. This is important in part-load situations where the
leaving air temperature limits are 47°F for the A/C design and
40°F (5°C) for the dehumidifier before the coil starts to reach
32°F (0°C) and hot gas bypass would be required.
A
EXPANSION VALVE
LOW
Internal Load
HIGH
B
RECEIVER TANK
Figure 2 - Relative Compressor Capacity
A compressor is selected for the system to produce a given
output at one design point. For example, to cool and dehumidify
1000 cfm of air from 95°F db / 78°F wb to 55°F dewpoint, you
need 82 MBH of energy. The compressor must provide this
amount with the high and low pressures balanced by the
evaporator and condenser coils.
F
EVAPORATOR
COIL
COMPRESSOR
CONDENSER
COIL
C
D
Figure 3 - Refrigerant flow
E
TECHNICAL BULLETIN 1
Refrigerant Dehumidification Technology
Location
Pressure Temp.
(PSI)
(°F)
Description
TXV discharge
50 to 90
26-54
Cold, saturated liquid + vapor
Evap. discharge
50 to 90
38-66
Superheated vapor
>130
Hot gas
90
Hot liquid
Compr. discharge 200 to 270
Cond. discharge
200 to 270
Table 2- R22 Typical Refrigerant Conditions
Hot Gas Reheat Condenser Design
All dehumidifiers contain a hot gas reheat (HGR) condenser
to avoid overcooling the space while dehumidifying. If the
HGR condenser is to reject the total heat of rejection, then
extra air must be bypassed around the evaporator coil. This
increases the air volume and makes the leaving air temperature
100°F to 110°F (38°C to 43°C). In pool facilities, this air can
help maintain the 82°F to 86°F (28°C to 30°C) interior
temperature.
Condenser Design
The condenser or combination of condensers must be sized
for the total heat of rejection (THR) of the system. There are
three (3) types used: reheat, remote air-cooled and watercooled. Remote Air-cooled condensers are placed outside the
conditioned space to reject excess energy to the outdoors. A
water-cooled condenser would use pool water, cooling-tower
water or chilled water as the medium to reject the excess
energy. These can be cost-effectively sized to optimize the
condensing pressure/temperature for minimum energy usage.
Air-cooled condensers must balance their physical size and
air volume with a maximum condensing temperature.
Generally, a maximum of 120°F (49°C) condensing temperature
is used to balance first cost with operating efficiency.
Most systems are sized to achieve a 25°F (-3.9°C) differential
which would require a maximum outside air design of 95°F
(35°C). When air temperatures go higher than 95°F, the
condensing coil must be designed with more surface area
because the difference between ambient and 120°F condensing
temperature is smaller, yet the amount of energy to be rejected
has not changed.
Because dehumidifiers operate in many condensing modes,
there needs to be a method of controlling the pressure of
these elements. Desert Aire uses flooding control, which
allows liquid refrigerant to back up into the condenser, thereby
reducing the effective surface area of the coil.
The colder the air or water entering a condenser, the less coil
surface is required; conversely, the hotter the air or water, the
more surface is required to maintain the desired pressure.
This flooding control allows a dehumidifier to reject energy to
an outdoor condenser in a range from 0°F to 95°F (-18°C to
35°C) and water temperature from 45°F to 105°F (7°C to 40°C).
For other applications no bypass air is allowed. A typical
example is a 100% outside air unit where the entire air
volume must be treated. In these types of units, the internal
HGR coil must be partially sized and combined in series with
another condenser to reject all of the energy.
There are two (2) basic designs when doing simultaneous
rejection: The first is to send the hot gas to the alternate
condenser and then after it condenses, send the hot liquid to the
reheat coil. This is known as liquid sub-cooling. The main
advantage of this technique is its ability to provide more
stable leaving air temperatures in a range from 58°F to 85°F
(14°C to 29°C). At given entering air conditions, the output
will balance at some temperature in the range and remain stable
and not be impacted by the variability in the alternate
condenser. The major drawback is that the hot liquid only has
enough energy to reheat the air at part-load situations to
between 50°F and 65°F (10°C and 18°C).
The second design is to send the hot gas to the reheat coil
first and then to the alternate condenser. Essentially, you are
trying to get two coils to act as one. Since the system is
using hot gas, it has significantly more energy to do more
re-heating even at part-load days. In its basic form, this system
creates sharp drops in leaving air temperatures because of
the changes caused by the flooding valve used to maintain
system pressures. Like a toilet bowl, the condenser is slow
to fill, but fast to drain. An enhancement to this is to use a
modulating valve to control the amount of hot gas to the HGR
condenser. Through a temperature sensor downstream, the
valve can maintain precise leaving air temperature.
Receivers
H
A receiver is just a storage vessel for liquid refrigerant. Since
most dehumidifiers use several condensers, each with a
variable requirement for refrigerant, there must be a place to
store the liquid not in circulation. In addition, many dehumidifiers
use outdoor condensers that may need to operate over a wide
range of ambient temperatures, causing a significant change
in the volume of liquid refrigerant required to operate.
A dehumidifier without a receiver must be limited in its
operational range or else experience many problems in
controlling moisture removal.
Hot gas defrost is used to intentionally freeze the evaporator
coil and then defrost the ice and remove it from the system as
water. Such a system measures the internal refrigerant
temperatures and only defrosts on demand for a short
duration. The drawback is that this technique does not
dehumidify during the defrost cycle, and can therefore only be
applied on re-circulation systems such as ice rinks.
H
OT GAS BYPASS
Hot gas bypass is a technique used to prevent a coil from
freezing up at low load conditions. As a system reaches its
minimum approach temperature, the refrigerant temperature
drops below 32°F (0°C) and any water vapor in the air passing
over the coil will freeze.
C
OT GAS DEFROST
ONCLUSION
Dehumidifiers operate in many modes of operation in order to
insure continuous moisture removal. To achieve this, the
system’s design incorporates many different critically sized
components that must function seamlessly. Each mode can
be explained by understanding how each refrigerant process
moves along the pressure-enthalpy diagram of R22 shown in
Figure 1.
By installing a small feeder tube from the discharge side of
the compressor to the coil's inlet header, a small amount of
hot gas can be metered into the coil to raise the refrigerant's
temperature above the freezing point. This creates a false
load on the system and reduces the efficiency of the system
since part of the electrical energy used to run the compressor
is short-circuited to create the load. However, this small electrical cost can save a major compressor failure should the coil
continue to freeze up and starve the compressor into failure.
8300 West Sleske Court
Milwaukee, WI 53223
(414) 357-7400
FAX: (414) 357-8501
www.desert-aire.com
101 10/02