IN THIS PAGE WE HAVE INCLUDED THE
HOW-TO IN SELECTING A DEHUMIDIFIER.
1) SIZE SELECTION for SMALL
2) CALCULATE HUMIDITY LOAD by ENGINEERING
3) SELECTION OF DEHUMIDIFIER AFTER KNOWING
THE HUMIDITY LOAD
4) HOW CALCULATING HUMIDITY LOAD
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Calculate Dew Point / RH
/ T/T /DB/WB
We have chosen the usage of SI units for easy
conversion. In calculating moisture load, Absolute Humidity is
the basis used and not Relative Humidity. Please also refer to the
Psychometric Chart for various values
referenced. Absolute Humidity is measured in terms of g/kg of air at 1,2 kg/cu.
meter. That is, the weight
of water molecules in a kg of dry air at the density of 1,2 kg/cu.meter.
1) SIZE SELECTION OF SMALL
DEHUMIDIFIER BY RULE OF THUMB METHOD.
Just for the interest of non-technical users, we will provide the
rule of thumb sizing for those who are
interested mainly in the small portable units.
a) A 12 L/day unit is able to keep a 150 sq ft x 8 ft ( 14 sq meter x 2.4
m ) space at 55% RH.
b) A 16 L/day unit is able to keep a 200 sq ft x 8 ft
( 19 sq meter x 2.4 m ) space at 55% RH.
55% RH is a general minimum requirement for most low level storage. For lower RH
level like 45-50%
RH control, reduce the space by 30-35% by floor area. If one unit is
too small, go for 2 units, or use the 30-45 L/day
( Note: the above sizing is based on an unventilated room. Room with central
air-conditioning or exhaust is generally
difficult to estimate as the volume of Ventilation Air creates a very high
CALCULATING HUMIDITY LOAD BY ENGINEERING METHOD
For a more ACCURATE selection, we have the
In this exercise, we have included the 3 biggest factors that
contribute to the humidity load. There are a lot
more other factors that contribute to the moisture load but they may need some
expertise to understand and
calculate. A more stringent formula is needed for low humidity application below
40% RH or low temperature
( below 15 deg C ). This is done only by trained engineers. Total load
will be sum of all the 3 following factors.
There are 3 main factors are :
INFILTRATION is the average air that can come in thro' the walls and
cracks. It is directly proportional to the difference
between the indoor and outdoor Humidity, and the size of the space. Stringently,
it is proportional to the areas of the 4 walls
plus ceiling and floor. But to simplify the formula, total space volume is used
instead, with a additional K-factor in the attached tables .
INFILTRATION LOAD (L/hour) = (H out - H
) x 0.0012 x Space Volume x K-factor
H out is
the Absolute Humidity in g/kg of the surrounding or out door air.
H in is the
Absolute Humidity in g/kg of the space to be dried.
0.0012 is the density of air.
Space Volume is the volume
of the space , height x width x length in cubit meter.
K-factor is the factor that
converts the volume into surface area of exposure.
|80 CUBIT METER
|200 CUBIT METER
|400 CUBIT METER
|600 CUBIT METER
|1000 CUBIT METER
|2000 CUBIT METER
|3000 CUBIT METER
|4000 CUBIT METER
|5000 CUBIT METER
eg : A space of 20 x 10 x 10 meter space. Out
door 30 deg C 70% RH. Required condition is 23 deg C 50% RH.
Calculate the infiltration load.
The first step is to convert the condition to absolute
humidity in g/kg.
Out door 30 deg C 70% RH = 18.5 g/kg (Absolute Humidity)
Space 23 deg C 50% RH = 8.6 g/kg (Absolute
P-factor : is a basic offset for
differential value between the H out and H in. The higher the differential,
the greater the P-factor. It is found that the greater the difference between H
out and H in, the greater
the vapour pressure has its effect in pushing the moisture through a wall. It is
added into the formula
to improve the offset in the form of a mathematical division.
P-factor = (H out - H in
) / 11.5
INFILTRATION = (H out - H
in ) x 0.0012 x Space Volume x K-factor x P-factor
INFILTRATION = ( 18.5-8.6 ) x 0.0012 x 2000 x
0.23 x 0.86 = 4.7 L/hr
ie there is 4.7 Litre of water getting into the space every hour.
2) HUMAN LOAD -
This is a simple load based on estimation of human activities and H-factor
( human activity level )
HUMAN LOAD ( L/hr ) = Number of people x H-factor x 0.065
|SOME MEASURE OF
|HEAVY LABOUR /
VENTILATION - This is the estimate of the exhaust air or fresh air volume
entering the room. Fresh air is needed for
human being. Each person requires an estimate of 20 CMH of fresh
air. Door opening is another cause for ventilation.
Every time the door opens it adds to the fresh air intake and should be part of
VENTILATION LOAD ( L/hr)= AIR INTAKE (CMH) x
(H out - H in ) x 0.0012
where H out
- H in are similarly defined as in INFILTRATION LOAD.
DOOR OPENING LOAD / DOOR LOAD
Door opening can be considered as part of the Ventilation
Load as it introduces air into the room each time a door is
opened. It is measured as the added quantity of AIR INTAKE (CMH) in Ventilation
DOOR LOAD (CMH) = AREA OF DOOR (sq. meter ) x 3 x time(sec)
door stayed opened x No. of openings/Hr
eg. A 5 sq.meter door opens 2 times per hour, each time
stayed opened for 6 seconds is equal to = 5 x 3 x 6 x 2 = 180
CMH ventilation air.
The initial condition of the untreated space is usually just a good
indication of the "wetness level" and is NOT part
of the Moisture Load calculation Formula.
3) SELECTION OF DEHUMIDIFIER
AFTER CALCULATING THE LOAD IN Litre/Hr
Most of the Dehumidifiers catalog have some values indicated like
16L/day or 12 L/day. This values are
indicative of the capacity of the dehumidifiers. Because different manufacturers
use a different condition
to measure this value, there is always confusion in selection.
For example: Unit "A" with 15 L/day at 85% RH 33 deg C
may be smaller in capacity than unit "B" which
states a 12 L/day capacity at 70% RH26 deg C. You may ask ," That's
confusing ! " Yes, it is !
The reason is that the Dehumidifiers have different
capacity at different temperature and humidity.
To simplify the description, a specified condition like 80% RH 30 deg C is the
standard reference for most brands.
However, the better systems usually have another
reference point like 27 deg C 60% RH or 55% RH 25 deg C
etc intermediate value as mid point reference. This middle values are the real
functioning range of the Dehumidifier
/ DH as we usually want to keep the room at lower RH than 80%. Some
industrial system come with a performance
CURVE OF MODEL "X"
|A room that need to be dehumidified to
50% RH at 25 Deg C, and have a humidity load of 0,125 L/hour (3 L/day)
based on the method of calculation recommended above.
Model X has a capacity of 3,75 L/day at 50% RH at 25 Deg C is the nearest
Model X however, is advertised rightfully as having
12.5 L/day capacity at 30 deg C 80% RH.
From this exercise, we can see that the graph is
very crucial in a proper way of selection.
The above method applies to condensation type of
dehumidifier selection for between 40% to 65% RH control level.
It is a general rule of thumb method. it is not applicable to centralized
ducted air con system.
For Desiccant dehumidifier selection, various
manufacturers have different methods of selection
as desiccant dehumidifiers have a more
complicated performance curve.
Information on Humidity
Disclaimer : The information we offer above is not absolute
and fool proof. For serious engineering calculation and load estimates,
approach the trained professionals and authorities. Please consult
us for detail calculation for installation that requires a
definite humidity level.