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IN THIS PAGE WE HAVE INCLUDED THE HOW-TO IN SELECTING A DEHUMIDIFIER.

1) SIZE SELECTION for SMALL DEHUMIDIFIER

2) CALCULATE HUMIDITY LOAD by ENGINEERING METHOD

3) SELECTION OF DEHUMIDIFIER AFTER KNOWING THE HUMIDITY LOAD

4) HOW CALCULATING HUMIDITY LOAD

Click here to link to on site 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  35L/day unit.   
( 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 humidity load. 

2) CALCULATING HUMIDITY LOAD BY ENGINEERING METHOD

For a more ACCURATE selection, we have the following:
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 : 

1) 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 .

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.

K-factor table

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 Humidity)

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 )

H-Factor Table

3)  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 this formula.

  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 Load.

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.
 

Note: 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 chart.

Example:

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.

Related Topics:
Introduction to Dehumidification ,
Recommended RH, 
Low Humidity Application,
Conversion Table,