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WHY SHOULD YOU DEHUMIDIFY AN INDOOR SWIMMING POOL?
Large volumes of water constantly evaporate from the surface of a pool, and from the damp tiles around the pool. Several thousand litres of water can evaporate from a swimming pool in a single day.
The aim of the guide is to provide engineers and pool fitters with a tool for choosing the correct pool dehumidifier – regardless of whether it is for smaller pools, hotel swimming pools or public swimming pools.
The dimensioning of a pool dehumidifier is not an exact science, and there are many mutually dependent factors which have an impact. Making a change in one area, often affects other areas. Dehumidification requires careful thought and above all, good dialogue between the architect and consulting engineer.
A good dehumidification solution is an overall solution, and therefore this guide relates to both dehumidification, temperature regulation and ventilation.
This Selection Guide is based on BD Swimming Pool extensive expertise and many years of experience in dehumidifying thousands of indoor swimming pools.
This is why we can say with confidence that you are on safe ground if you follow the directions in this guide.
UNDERSTAND THE AIR
To understand the processes that happen in the air, you need to think of the air as a mixture of dry air and water vapor.
We call this mixture, humid air. Air expands when it is warmed and contracts when it is cooled. When air is warmed up, it can hold more water vapor and when it cools down, it can hold less water vapor. When the air cools down a lot, it reaches the dew point, which means the air is saturated, and the water vapor condenses as water.
The highest relative humidity that can be allowed with a given room temperature is defined by the insulation grade of the building, the water content of the air, the outdoor air temperature and the air circulation in the room.
We use ρ = 1.175 kg/m3 air as the average value of the air’s density indoors and outdoors. Air temperature (t) Dry bulb (t) The vertical pink axis on the left with corresponding lightly sloping lines. The temperature we measure in the air using an standard thermometer. The temperature is given in °C. Enthalpy (h) The purple diagonal lines.
EXAMPLE: HOW TO USE THE MOLLIER DIAGRAM
The Mollier diagram allows you to read the dew point temperature and water content in the air, if you know the relative humidity of the air and the air’s temperature. The point is plotted in the diagram and the water content of the air is found on the X-axis. The dew point temperature is found by moving vertically, until you intersect the 100 %RH curve, after which you read the dew point temperature on the Y-axis.
At 28 °C; 60 %RH, the water content is approx. 14 g water per kg air, and the dew point temperature is approx. 19.5 °C. This means that if there are surfaces in the room that have a temperature that is less than 19.5 °C, condensation will form on these surfaces.
If the building is poorly insulated, it may be necessary to lower the relative humidity to 50 %RH, the water content will change to 12 g water/kg air and the dew point temperature will change to 16.5 °C.
ENSURE THE CHEMISTRY OF THE WATER IS CORRECT
The correct combination of chemicals in an indoor swimming pool is crucial, both for the health of users and for the inventory inside the pool room and the swimming pool’s technical room.
Insufficiently treated water results in poor hygiene, while water that has been excessively treated results in gases in the air that contain chlorine, which can irritate the eyes and cause breathing difficulties.
At the same time, the incorrect composition of chemical ingredients in the water can destroy all of the inventory in a very short space of time, including the dehumidifier and other equipment that have been installed to process the air.
WHEN ADDING CHEMICALS
Free chlorine 1.0 – 2.0
Combined chlorine Max 1/3 of the free chlorine content
pH 7.2 – 7.6
Total alkalinity 80 – 150
Calcium hardness 250 – 450
Total dissolved solids < 2,000 Sulphates < 360 WITH OWN PRODUCTION OF CHLORINE Ingredients PPM Salt (NaCl) < 30,000 Total dissolved solids < 5,500 pH 7.2 - 7.6 Total alkalinity 80 - 150 Calcium hardness 250 - 450 Sulphates < 360 CHOOSE THE CORRECT MATERIALS The environment in an indoor swimming pool is aggressive, which means it is very important that the materials that are used in a dehumidifier are corrosion-protected, because the lifetime of the products will otherwise be very short. The basic material that is used in the dehumidifier for heat exchangers, fans and surfaces, can have different corrosion classes, depending on the required lifetime. It is recommended that the basic material is aluminium or hot-dip gal vanised steel that has been powder coated. The heat exchangers should be made of aluminium that is epoxy-protected and coated. The evaporators and condensers in particular need to have good protection, first with a primer using epoxy coating and then with top coating. TWO WAYS TO CONTROL HUMIDITY In the past, the standard practice was to replace the air in a swimming pool with new air. Outdoor air is good for comfort but warming the new air is expensive in terms of energy consumption. Increasing energy prices and increased awareness of energy consumption has led to greater focus on saving energy – and this includes dehumidification. Therefore today there is focus on recirculating as much air as possible or recovering heat in heat exchangers. TERMINOLOGY Outdoor air: Air that is drawn in from outside the building and used as replacement air. Exhaust air: The air that is blown outside the building. Supply air: The dry, dehumidified air, which is blown into the pool room. Extract air: The humid air from the pool room that is drawn into the dehumidifier. The cooling unit consists of an evaporator, a condenser and a compressor. How does a condense dehumidifier work? In a condense dehumidifier, the humid air is led over a cold coil (evaporator in the cooling unit), and the water vapour in the air condenses into water. The cooling unit functions by letting a refrigerant evaporate at low pressure and low temperature in the evaporator and then condensing it at high pressure and high temperature in the condenser on the other side of the compressor. In some types of condense dehumidifiers, there is also the option of taking some of the dry outdoor air in – up to 30 % – if there is a need to take air quality and comfort into consideration or if there is a requirement in consideration of the dehumidification needs. A condense dehumidifier generates energy because of the condensation process and the compressor and fan power input. In a ventilation solution, the air is completely or partially replaced. It may be preferable to let some of the airflow recirculate, so that anything between 0 % to 100 % of the air is circulated, depending on the requirements. The heat pump will warm the air during winter and can also dehumidify the air during the night, if there is a need for this. WHICH TYPE OF DEHUMIDIFIER SHOULD YOU CHOOSE? Shown below is a list of simple characteristics, which can help you to decide which type of dehumidifier is the right choice. CONDENSE DEHUMIDIFIER Advantages Easy to install and use Low investment Disadvantages Generates heat Can only take a limited volume of outdoor air VENTILATION DEHUMIDIFIER Advantages Optimal regulation of humidity and temperature Up to 100 % outdoor air Free cooling Active cooling option Disadvantages Cannot be used when the outdoor air is more humid than the indoor air. Higher investment than the condense dehumidifier Requires more space than the condense dehumidifier TEMPERATURE MANAGEMENT In addition to dehumidification, the dehumidifier can also be used to regulate temperature of the air and pool water. However, the pool water can only be warmed by a unit that has a built-in cooling circuit. HEATING/COOLING THE AIR The thermal requirements for a swimming pool are determined by several factors: » Ventilation loss, both mechanical and through infiltration loss from leaks in the building. » The building’s transmission loss, i.e. the thermal loss through walls, ceilings, windows, etc. » Solar gains eg sunlight entering through windows. Depending on which dehumidifier has been chosen, there are different options available for temperature regulation. Heating coil A heating coil is the most popular way to control the temperature. Depending on the unit, the heating coil is run by an ON/OFF (small unit) or 0–10 V signal (large unit). There are two different types of heating coil: A hot water coil for connecting the central heating system and an electrical heating coil. Cooling with reversible heat pump (only with dehumidification using ventilation) In some countries, free cooling is not sufficient, and in those cases it is possible to build a heat pump in a reversible cooling unit using a 4-way valve. Reversible means that the heat pump reverses the flow of the refrigerant, so that the section that was originally an evaporator in relation to the exhaust air now becomes a condenser, and the section that was originally the condenser in relation to the outdoor air, now becomes an evaporator. In small pools and spas there is rarely a desire for large volumes of outdoor air. In these cases, people will often choose a condense dehumidifier, since the investment is small and it is simple to use. Depending on the unit, the cooling coil is run by an ON/ OFF (small unit) or 0–10 V signal (large unit). There are two different types of cooling coil: A water cooling coil for a central cooling unit or a DX cooling coil for direct expansion to a separate compressor unit. Both types can normally be supplied in different sizes, depending on cooling requirements. In countries with very hot summers, the absolute humidity of the outdoor air can be too high to use outdoor air to dehumidify with. In this way, it is possible to warm up the air again to avoid too cold air being blown into the pool room. Cooling using an external condenser (condense dehumidifier only) A condense dehumidifier will always produce some excess heat. This is due to the electrical energy that is used in the compressor, and that every single liter of condensed water produces a thermal output of around 0.7 kW. Often a water-cooled condenser is built into this unit. This makes it possible to transfer the excess heat to the pool water or to the utility water. However, in countries with a hot climate during the summer, the excess heat from the unit is often greater than the water’s thermal loss, so the excess heat must be dealt with in some other way. In this situation, it is possible to use an external air-cooled condenser. This means the excess heat can be transferred fully to the outdoor air, and it is even possible to transfer more than that. Thus, an external condenser can be used for active cooling of the pool room without having an extra built-in cooling coil. External air cooled condensers are not normally supplied by BD Swimming Pool, but BD Swimming Pool makes all of the necessary data available for the calculation of these. Condense dehumidifiers are supplied ready to be connected to the external condenser. The control of the external condenser is fully-automatic, done through the unit’s control unit. POOL WATER HEATING The heating requirements of the pool are mainly determined by two factors: » Evaporation from the pool water. » The pool’s transmission loss, i.e. the thermal loss through the pool’s bottom and sides. With dehumidifiers with built-in cooling units or heat pumps, it is possible to have the dehumidifier delivered with a built-in water-cooled condenser as a supplement to the air-cooled condenser. In a condense dehumidifier, warm air is always added when the compressor is operating, and in some cases – especially when the temperature outside increases and the heating requirement in the pool room falls – the excess heat is undesirable. In such situations, a water-cooled condenser can be used and the excess heat utilized to warm the pool water or utility water. In other words, with a water-cooled condenser more comfort is achieved than with a condense dehumidifier. Furthermore, the running costs will decrease. A water-cooled condenser in this type of dehumidifier does not provide more comfort, but it will be able to extend the operating time of the compressor and thus utilize the excess heat more efficiently. HOW TO DETERMINE THE OPERATING CONDITIONS VDI 2089 Based on the physical laws there are exact formulas to calculate the humidity load from an indoor swimming pool. These formulas are however complicated to use in practice. This has lead the German institute VDI to make a guideline which comprises recommendations for which values to set for internal condition, outside condition and amount of outdoor air. In general, it is recommended that the room temperature is set to 2 ºC higher than the water temperature. Only in therapy baths and spas should the air temperature be allowed to be lower than the water temperature.