Way Technovation
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The
Humidity
Control Specialist |
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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
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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 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 .
|
INFILTRATION LOAD (L/hour) = (H out - H in
) 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.
K-factor table
| SPACE LESS THAN |
K-Factor |
| 80 CUBIT METER |
0,5 |
| 200 CUBIT METER |
0,4 |
| 400 CUBIT METER |
0,35 |
| 600 CUBIT METER |
0,3 |
| 1000 CUBIT METER |
0,27 |
| 2000 CUBIT METER |
0,23 |
| 3000 CUBIT METER |
0,21 |
| 4000 CUBIT METER |
0,19 |
| 5000 CUBIT METER |
0,18 |
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 )
HUMAN LOAD ( L/hr ) = Number of people x H-factor x
0.065 |
H-Factor Table
| PASSIVE, OFFICE WORK |
2,0 |
| SOME MEASURE OF MOVEMENT |
2.5 |
| HEAVY LABOUR / EXERCISE |
3,0 |
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.
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 Load.
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.
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:
'CAPACITY 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 choice.
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.
Related Topics:
Introduction to Dehumidification ,
Recommended RH,
Low Humidity Application,
Conversion Table,
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