Humidification & dehumidification
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owering the humidity in a room can be a complicated task. The drying of air can have a dramatic impact on its temperature and, depending on the type of dehumidifier being used, there can be various installation obstacles to overcome. So what should a contractor consider before setting out on a drying project? Firstly, which technology to use? There are two main types – condensing and desiccant. A condensing system collects moisture by condensing it on a cold surface and then sending it to drain. A desiccant literally adsorbs moisture by passing air through a desiccant rotor. Due to the nature of condensing systems,
they work better in warmer conditions, as more condensation occurs. This technology is ideal in rooms of 20°C or more and for projects that need to lower humidity down to around 45-50%RH. As the room’s temperature has no impact on the ability of the desiccant to adsorb moisture, it can be used in any temperature, making it a more flexible solution and ideal for cold environments. This technology is also able to lower humidity below 45%RH. Installing a condensing system is potentially simpler as it only requires connections for power and drain. A desiccant dehumidifier doesn’t need a drain but does need a duct to vent hot wet air away from the area being dried. To remove moisture from the desiccant rotor, a secondary hot airstream is passed through the rotor, which carries the moisture away to outside. Pressure management must be considered
whenever a dehumidifier has ducting. Each model has a pressure rating and inadequate or excessive airflow through the system may reduce efficiency or cause wetting issues. As there are two airflows through a desiccant system, care must also be taken to avoid crossflow through the rotor. Installing short ducts on both intakes will allow for dampers to be fitted, which can help manage internal pressure and avoid cross flow issues. Alongside pressure, temperature management must also be reviewed by the contractor. The thermodynamic principle of drying produces heat at around 680W per kg of moisture extracted. In addition to this, the mechanical elements incorporated into both technologies produce heat. Without assessing and controlling this, the area being dried can experience significant heat gain. All too frequently, the heat gain from
18 January 2023
Dehumidifiers – a contractors’ guide
Dave Marshall-George, sales director at Condair, looks at what a HVAC contractor should know to successfully deliver dehumidifier project
be located outside. This is similar to a split air con unit and allows the heat in the refrigerant loop to be remotely exhausted and the dry air delivered to the room at a precise temperature and humidity. Heat gain in desiccant systems will be greater
Left: Dave Marshall- George, sales director at Condair
dehumidification is countered by venting an area. But opening windows or proactively bringing in outside air, can reintroduce moisture and impact humidity control. For condensing systems, if the heat gain is too great, some units, such as the Condair DC-N, have a secondary condenser that can
than condensing, as the “regeneration” portion of the desiccant wheel is heated to remove the moisture. This heat transfers to the process airflow as the rotor revolves, so adds significant heat to the dry airflow. To counter this, a contractor can install a cooling coil before or after the dehumidifier. Dehumidifier location is another important decision to be made early in the project. If the unit is placed inside the area to be dehumidified, installation can be simpler with less ducting needed to direct airflows. Air can be taken from the area
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