Dehumidification of
waterworks

Page_28Humidity conditions at a waterworks can be quite extreme. Here dehumidification is a question of protection and preservation of the water pipes, pumps and other equipment as well as the building itself.

If the relative humidity is too high you will get a large amount of condensation on all metal surfaces. Paint will peel off the water pipes and serious attacks of corrosion will set in. This increases maintenance costs and reduces the lifetime of the installations and the building.

The humid environment also accelerates the growth of fungus and mold. Mosquitos thrive in the humid atmosphere and deposit their eggs in the open reservoir making it altogether very difficult to meet the hygienic requriements.

In most cases the water temperature is 6-9°C. This means that the surface temperature of the pipes is roughly the same. To avoid condensation the dew point temperature has to be lower than the surface temperature of the pipes.

Normally you should maintain a temperature inside the waterworks that is at least 2°C higher than the water temperature. At the same time you must keep the RH-value at a relatively low level, and to do so you need dehumidification. Usually ventilation is applied at waterworks. An air change between 0.1 - 0.3 times per hour is recommended.

In general the temperature inside a waterworks will rarely raise to more than 16-18°C due to the cold water pipes and because the building is normally underground. This means that a RH-value below 45% will suffice to avoid condensation all year round. The table below shows the max. RH-value at different temperatures to avoid condensation.

Room temperature (°C)101214161820
Max. RH-value,
water = 7°C   (% RH)
80  70  61   54 48  42 

 

The total dehumidification load is determined by:

W(total) = W(water reservoir) + W(ventilation)

 W(water reservoir) = c * A * (xsa - x1)

  

W = g water/hour
c = constant empiric value 6,25 when the air temperature is min. 2°C higher than the water temperature
A  = water surface area (m2
xsa = water content in the saturated air at water temperature (g water/kg air) at 100% RH. 
x1 water content in the air at the desiredRH value and temperature (g water/kg/air) 

 

W(ventilation) = ρ * V * n * (x1 -x2)

In this example we want to determine the dehumidification load needed in a waterworks with an air temperature of 15°C and a desired RH value 50%. The size of the waterworks is 300 m3, the water surface is 40 m2 and the water temperature is 8°C. 

Page_30

The data:

Volume of waterworks

 300 m3

Air change rate: 0.3 pr. hour
Water surface: 40 m2
Water temperature: t = 8°C (and 100% RH)
Water content in the air at water temperature xsa = 7 g water/kg air (see hx-diagram)
Desired condition t = 15°C and 50% RH > x1 = 5 g water/kg air (see hx-diagram)

 

The calculation:

W(water reservoir) = 6.25 * 40 * (7-5) = 500 g water/hour
W(ventilation) = ρ * V * n * (x1 -x2) = 1.2 * 300 * 0.3 * (10-5) = 540 g water/hour
W(total) = 500 + 540 = 1.04 ltr/hour

 

The dew point temperature at 15°C and 50% RH is approximately 5°C according to the hx-diagram. This means that the surface temperature of the water pipes must drop below 5°C before condensation occurs on the pipes. If the water temperature is 8°C there will be NO condensation of water on the pipes as the actual water temperature is higher than the dew point temperature.  

Page_31Recommendation: Two CDT 60 mobile dehumidifiers. Capacity: 0,6 litre/hour per unit at 15°C/50% RH. As we have seen it is extremely important to have full controll of the relation between temperature and RH value in this type of situation. To do so we recommend that you equip each CDT 60 mobile dehumidifier with a hygrostat set to 55% RH. This will automatically control that temperature and humidity conditions are always kept at a level ensuring that condensation is avoided. 

Selecting the right size dehumidifier can be challenging. There are several factors to look for when selecting the right dehumidifier.

In the box to the right you can get a free copy of Dantherm's Guide how to select the right size of dehumidifier.

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