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DEHUMIDIFY INDOOR SWIMMING POOL (Part-2)
THE WATER TEMPERATURE
When setting the water temperature, the following is often the case:
Private pools and hotels: 26–30 ºC Public swimming pools: 26-28 ºC Competition swimming: 24-27 ºC Therapy baths: 30-36 ºC Spa: 36-40 ºC
THE ROOM’S RELATIVE HUMIDITY (RH VALUE)
The set point should be an RH value of 50–60%.
Above 60 %RH, the higher the RH value the poorer the comfort level, and there is an an increasing risk of condensation problems and fungal attack. An RH value that is lower than 50 %RH is not appropriate, because evaporation will increase and this means greater dehumidification requirements and increased operating costs.
WATER CONTENT IN THE OUTDOOR AIR
The water content in the outdoor air (X outdoor air) varies a lot over the year, more or less all over the world. From more than 12 g water/kg air during the summer a down to 2 g water/kg air during the winter.
It is during daytime operations that you must assess the level of activity that best describes the way the pool is being used.
Winter and summer
When outdoor air is incorporated in the installation, you must use the summer load as the point of departure for the determining the humidity load. The water content in the outdoor air is highest during the summer. You must ensure that the dehumidifier is sufficiently large to deal with the humidity load with 100 % outdoor air volume during the summer. During the winter, the outdoor air is very dry and thus will increase the dehumidification, so in that situation the dehumidification capacity will be more than enough.
Volume of outdoor air
VDI 2089 states that the volume of outdoor air must be at least 15 % of the circulating air. This applies for public and commercial pools.
THE EVAPORATION FROM THE POOL The calculation of the evaporation from the pool can be carried out in accordance with different formulas. At BD Swimming Pool, we use the which is the Highly standard in Bangladesh.
In addition, in small private pools it will often be the air circulation that defines the need for dehumidification, and not how much activity there is in the pool.
The following factors are used to calculate the evaporation from the pool:
» Water temperature » Air temperature » Air humidity » Pool area » Level of activity in the pool
Set the values in the formula below.
W = e/(RD × T) × (Pb – Pl) × A
W = Evaporated water (l/h) e = Level of activity in the pool RD = Gas constant = 461.5 (J/kg K) T = Average between air temperature and water temperature (K) Pb = Saturated vapour pressure, water (Pa) Pl = Partial vapour pressure, air (Pa) A = Pool area (m2)’
Level of activity, e The level of activity in different types of pools is listed below. The values are generated automatically when the pool type is selected in DanCalcTool. For example, if you enter private pool it will automatically calculate with e = 9.5.
e = 9.5 for private pools e = 11 for hotel pools, normal activity e = 18 for hotel pools, high activity e = 14 for therapy pools e = 20 spa/Jacuzzi* e = 28 for public swimming pools, water depth more than 1.35 m e = 40 for public swimming pools, water depth less than 1.35 m e = 50 for wave pools
* assumes 30 minutes operation and 30 minutes stop.
VDI 2089 includes wet areas around the pool (tiles, etc.) and overflow gutters in the epsilon factors.
There is a large difference in the values for private and public facilities, which results in disproportionately large differences in the calculated humidity loads. In some cases, it is therefore advisable to carry out an individual assessment of the level of activity and to set “e” accordingly.
This can be done by selecting “manual water transfer coefficient” in DanCalcTool, e.g. should an indoor pool at a university be assessed individually. It is not a public pool, but it should be considered more as a hotel pool with a high level of activity. In that case, e = 18 can be chosen.
If a more precise calculation of the humidity load is desired, then a weighted level of activity can be chosen, where both daytime load and nighttime load is calculated, and then a weighted average activity load is calculated.
However in this case, it is important to note that there can be periods with peak load, where the general set point cannot hold, and so there is a need to assess whether the consequences with operating with a lighter under-dimensioned dehumidifier during a period of time is acceptable. However in general, BD Swimming Pool recommends that dimensioning is carried out in accordance with peak load situations.
When the sensitivity of the calculation must be assessed, it should be noted that the evaporation from the pool increases when:
» The water temperature increases » The room temperature decreases » The RH value decreases » The activity in the water increases
USERS AND BYSTANDERS When a public swimming pool is used for competitive swimming, the temperature of both the air and water is usually reduced.
During the summer, the outdoor air in some geo- graphical areas, primarily the Middle East and Asia, can for longer periods of time be so humid that it is necessary to dehumidify the outdoor air before it is added to the room. For example, this can be done using a cooling coil.
OTHER FACTORS
If insufficient air moves around the room, there will be areas where the dry air does not pass through and thus those areas will not be dehumidified. The room’s size is therefore crucial to how much volume of air the dehumidifier must be able to deal with. To learn more see “Air Circulation” on page 38.
Covering the pool If the pool is covered over at night when it is not used, evaporation from the pool will be minimized. Doing this can save a lot of energy.
Room insulation The worse the insulation, the lower the set point for the RH value has to be. In some cases, 50 %RH may be required to avoid condensation problems. For examples of condensation problems, see: “Understand the air” on page 6.
Height (level) In general, the formulas apply at atmospheric pressure of 1,013 mbar (sea level), but they can easily be applied to heights of up to 1,500 m. At higher heights, individual assessments should be carried out.
SELECTING THE TYPE OF DEHUMIDIFIE
CDP 40T, 50T AND 70T
CDP 40T, 50T and 70T are condense dehumidifiers for smaller, private swimming pools and spa pools. They are positioned in the adjoining technical room. The three different sizes cover a dehumidification capacity of 34 l/day to 68 l/day.
CDP 75, 125 AND 165
CDP 75, 125 and 165 are condense dehumidifiers for larger private swimming pools, fitness centres, hotel pools and smaller public swimming pools. The dehumidifier is positioned in the technical room and the air is transported through ducts to and from the pool room. The control panel can be positioned on both sides of the dehumidifier, and the supply air can be blown in horizontally or vertically. Up to 15% of the outdoor air can be added by the dehumidifier via a separate outdoor air duct. The dehumidifiers are available in a version with a water-cooled condenser, so the excess heat can be used to heat the pool water or the utility water. The three dehumidifiers for connection to a duct system are available with a dehumidification capacity of 65 l/day to 162 l/day
DANX AF
DANX AF is a condense dehumidifier, i.e. the humidity condenses on a cold evaporator coil. It is perfect for controlling the air humidity and temperature, and at the same time, it has very low operating costs. It is possible to add up to 30% outdoor air.
DANX XKS DanX XKS is a ventilation dehumidifier with a very efficient cross-flow heat exchanger. DANX XWPS
DanX XWPS is a ventilation dehumidifier with a cross-flow heat exchanger with a built-in heat pump. A temperature efficiency of up to 100 % reduces operating costs and energy consumption. The built-in mixing ensures that only the precise volume of outdoor air that is necessary for maintaining a pleasant indoor climate is added. To achieve even greater energy savings, a water-cooled condenser can be integrated in the heat pump. In this way, the excess heat can be utilized efficiently to warm the pool water or the utility water.
A certain amount of safety is included in the calculation. There is therefore no need for a safety factor on the calculated humidity load when determining the size of the dehumidifier.
Based on the entered data, the program will automatically take into consideration the difference and give proposals with both condense dehumidifiers and ventilation dehumidifiers, which can meet the requirements.
If you do not use the DanCalcTool, but read the data on a capacity diagram, it is important that you choose the dehumidifier’s size on the basis of the actual operational data.
In a condense dehumidifier, both the dehumidifier and the outdoor air contribute to the dehumidification, so the dehumidifier’s necessary capacity, W capacity dehumidifier, is found by using the formula stated below. W states the humidity load calculated in accordance with VDI 2089.
DEHUMIDIFICATION USING VENTILATION
In a ventilation dehumidifier, the dehumidifier must be able to supply the necessary capacity with a summer load, because the outdoor air in those circumstances is the most humid.
For a ventilation unit, the dehumidifier must be installed in accordance with the volume of air, Q, which is replaced in the room. It is determined by the following formula:
Q = 1000 × W capacity dehumidifier/ (ρ × (X indoor Air-X outdoor air)
The volume of air, Q, can be found in the catalogue.
AIR CIRCULATION
The air circulation is defined as the total number of times which the volume of air circulates in the room in an hour.
Air circulation in a pool room is important, because air that moves does not condense as easily as air that is stationary. In other words, the air circulation must not be too small, since this will give insufficient flow across surfaces. Very cold surfaces can cause condensation problems. Poorly insulated buildings lead to cold surfaces and thus have a lot of influence on how much air circulation is required.
On the other hand, excessively high air circulation is irritating in the form of noise and draughts, so you cannot just circulate endless large volumes of air.
AIR DISTRIBUTION
It is important because otherwise overpressure will occur in the room, which can lead to condensation in the walls and chlorine odor in the adjacent areas. It can be done by dehumidifying the pool room with one system, while the adjacent areas are dehumidified using a separate ventilation system.
To ensure negative pressure the volume of exhaust air + extract air should be approx. 10 % higher than the volume of supply air. A solution with a condense dehumidifier and addition of outdoor air, must therefore have an exhaust fan installed.
To ensure negative pressure in the pool room, the control of the exhaust air should relate to the control of the outdoor air.
The control of outdoor air and exhaust air can among other things, be based on the following signals:
» Sensor for relative humidity » Manual switch, e.g. cover. When the pool cover is rolled back, the dehumidifier starts automatically. » CO₂ sensor, alternatively CO₂ equivalent sensor » Time setting in relation to daytime and nighttime operation
If the room’s relative humidity is used as an input signal for the dehumidifier, it is important that the sensor is positioned in the extract duct or appropriately in the room. If it is placed in sunlight or in a position where it is hit by splashed water, there will be a risk of erroneous measurements.
Avoid air being blown on the water surface, as this will increase the evaporation. The air above the pool should be as still as possible and with a maximal airspeed across the water of 0.15 m/s, Of course due regard must be shown for swimmers, so that they receive sufficient outdoor air.
The return air, which is led back to the dehumidifier, should be extracted from the opposite end of the pool room, so that “dead zones” are avoided where the air is not replaced sufficiently.
It is normally sufficient to position a return air fan, which should be positioned high up in the room, 3–5 m above the floor. In this way, short-circuiting of the supply air and the extract air is avoided.
There are two different ways to do this:
Below grades ducting
The warm dry air is sent up towards the window panes. This is the optimal way to distribute air, because it utilizes the fact that warm air naturally rises up. In line with the dry air rising up, it draws in the humidity, and the humid air is extracted back to the dehumidifier via a return grate. The grate’s positioning is ideally 3–5 m high on the wall.
Ceiling ducting
Ceiling ducting can be used if the supply duct extends to the ceiling, because if required, dry air shall be blown on a skylight. Approx. 80 % of the air shall be blown on the windows, while the rest of the air is blown to the ceiling to avoid still areas with cold humid air.
A skylight should add 10–20 % extra air because of the risk of condensation.
REGULATING THE VOLUMES OF AIR14
Nominal volumes of air are stated on the basis of an external pressure loss. If the external pressure loss is exceeded, the result will be a reduced volume of air, which will increase the risk of ice forming on the evaporator coil. The evaporation temperature falls when the volume
of air falls. If necessary, this can be solved using a de-icing kit (accessories). In this way the dehumidifier can have an active deicer, which means that warm refrigerant is drawn through the evaporator, when this is about to ice up.
If the volume of air is too high, e.g. because the dehumidifier works against a small duct pressure loss, this leads to problems with lower capacity.
Ventilation dehumidifiers in a ventilation dehumidifier, it is the volume of air at 100 % outdoor air, which is the basis for regulating the volume of air and dimensioning of the ducts.