Data Sheet

Technical System Catalogue/Climate control
Cooling units
3 - 17
Condensation and dehumidification of enclosure air
when using cooling units
One unavoidable side-effect of using cooling units is the
dehumidification of the enclosure’s interior air. As it cools
down, part of the humidity contained in the air condenses
on the evaporator coil. This condensate must be reliably dis-
charged from the enclosure. The amount of condensate
occurring depends on relative humidity, the air temperature
inside the enclosure and on the evaporator coil, and the air
volume present in the enclosure. The Mollier h-x diagram
shows the water content of air depending on its temperature
and relative air humidity.
Mollier h-x diagram
for calculating the water content of air.
Practical tips
In all situations where optimum operating temperatures are
required inside an enclosure, even at high external tempera-
tures, a Rittal enclosure cooling unit can provide the right
solution. It is even possible to cool the interior temperature of
the enclosure to well below the ambient temperature.
The favourable aerodynamic arrangement of the air inlet and
outlet openings in the internal and external circuits ensures
optimum air circulation inside the enclosure. This sample
calculation will show you a quick, time-saving method of
selecting a cooling unit.
Example:
A cooling unit with a cooling output of 1500 watts begins
operation with a temperature setting of T
i
= 35 °C.
The relative ambient air humidity is 70%. If air at 35 °C is
passed over the evaporator coil, the surface temperature of
the evaporator coil (evaporation temperature of the refriger-
ant) is approximately 18 °C.
At the boundary layer adhering to the surface of the evapo-
rator coil, water is deposited at the dew point. The difference
Δx=x
1
– x
2
indicates how much condenstion is produced per
kg of air with complete dehumidification. The leak-tightness
of the enclosure has a decisive effect on the quantity of con-
densation.
The quantity of condensation is calculated from the following
equation:
Enclosure door closed:
Only the enclosure volume is dehumidified.
Poorly sealed cable entries, damaged door seals and the
fitting of display media to enclosure surfaces lead to
increased rates of leakage in the enclosure. Hence, with
a leakage rate of, say, 5 m
3
/h, a continuous condensate
volume of up to 80 ml/h may occur.
Summary:
Enclosure cooling units should only operate with the door
closed.
 Seal the enclosure on all sides.
 Use a door limit switch.
 Use TÜV-tested equipment.
 Only set the enclosure internal temperature as low as is
actually needed.
Pd
T
x
50
45
40
35
30
25
20
15
10
5
0
–5
–10
–15
–20
0
5
0
10
15 20 25
x2
x1
30 35
40
6030 45155 10 2025 3540 5055
10%
20%
30%
40%
50%
60%
70%
80%
100%
➀
90%
P
d
= Water vapour partial pressure (mbar)
T = Air temperature (°C)
x = Water content (g/kg dry air)
➀ = Relative humidity
W = Water quantity in g
V = Volume of the enclosure in m
3
r = Density of air kg/m
3
Dx =
Difference of water content in g/kg dry air
(from the Mollier h-x diagram)
V=W  H  D = 0.6 m  2 m  0.5 m
V = 0.6 m
3
W=V  r  Dx
= 0.6 m
3
 1.2 kg/m
3
 11 g/kg
W = 7.92 g 8 ml
Project planning
Calculate your required cooling output:
Q
E
= Q
V
– k  A  ΔT
.
.
W = V  ρ  Δx