High performance humidity control has a vital role in a vast range of applications and industrial processes. Terry Stevens-Smith, Director - National Accounts at Watkins Hire, highlights the main approaches and how the technology works.
The range of applications requiring effective humidity control is vast, extending from the construction industry and IT sector, to pharmaceuticals, waste treatment, process manufacturing and the offshore oil and gas industry.
Humidity is something of a wild card in terms of air treatment. While the gaseous composition of air is practically the same everywhere, the amount of water vapour it contains varies greatly depending on geographical location, current weather conditions and local factors in the immediate vicinity.
Airborne moisture is highly active chemically, biologically and thermodynamically. It also plays an important role in determining the electrostatic properties of air. As a result, moisture levels have a direct influence on metal corrosion, mould growth, the behaviour of powders, the quality of surface treatments, and the stability of hygroscopic products such as drugs and confectionery.
As a result, for many industrial applications the level of humidity is a critical factor influencing the resultant quality of a product or industrial process, and significant investment may depend on its effective control.
Fortunately, humidity levels in air can be controlled in a number of ways. The particular technology of choice will depend on the precise temperature and humidity conditions required in the application, and the scale and rate of moisture removal involved.
The simplest method of dehumidification is condensation through mechanical cooling. This involves drawing moist air over a refrigeration coil, in the process cooling it to below the dew point. Condensed moisture is collected in a drain pan or holding tank and disposed of or reclaimed for a secondary use.
Cooling-based dehumidification is suited to relatively small scale applications, where ambient temperatures are relatively high. For larger scale applications, and where ambients and relative humidity are low, other more powerful approaches are required.
Desiccant dehumidification systems are designed for use in such higher volume, higher intensity applications. A desiccant dehumidifier operates on a different principle to a refrigerant-based unit, harnessing the properties of hygroscopic chemicals to adsorb moisture from the air, and then give it up in a regenerative process.
The main benefit of desiccant dehumidifiers is that they are highly effective in cooler conditions, or when a low dew point is required. Since liquid water is not produced by the process, it can be used at sub-zero temperatures.
At the heart of a desiccant system is a rotating wheel made from alternating layers of flat and corrugated sheets, impregnated with a highly hygroscopic chemical. This presents an array of parallel air channels through which the humid air stream is drawn. As it passes through, water vapour is transferred to the desiccant, in the process releasing energy.
Rotors are often compartmentalised into two separate zones, each dedicated to either moisture removal or regeneration. As the rotor is slowly rotated by a motor, air is drawn into the active adsorbing section. Energy generated by the moisture adsorption process is transferred to the air, and it leaves the system dry and warm. See attached picture.
At the same time, on the regeneration side of the rotor a heated air stream is pulled through the rotor’s reactivation zone, driving out moisture adsorbed from the process air. The air, now warm and wet, is normally exhausted to outside.
These parallel processes proceed in a continuous cycle, enabling highly effective and sustained dehumidification. The moisture removal capacity of the system can be controlled by varying the degree of heating to the regeneration side of the rotor.
Energy efficiency can be significantly increased by using a heat recovery system on the regeneration side, harnessing the heat generated within the system itself to augment electrical inputs.
This can involve the use of a third zone within the rotor, in which incoming reactivation air is pre-warmed by adiabatically generated heat from the rotor, before moving on to the regeneration heater.
In liquid desiccant systems, the air stream is passed through a spray of desiccant solution (often lithium chloride), in the process removing moisture. Varying the concentration of lithium-chloride and airstream speed determines the rate of moisture removal.
An additional benefit of this approach is that the desiccant solution disinfects the airstream, which may be desirable for certain applications, such as in food manufacture and pharmaceuticals.
The desiccant solution is regenerated by heating, whereupon it gives up its moisture and is reintroduced as spray into the air stream, and the hot, moist waste air is exhausted from the unit.
As mentioned, the best system for a particular application will depend on the precise conditions, and the scale and rate of moisture removal required. Watkins offers both refrigerant-based and desiccant dehumidifiers, and following recent investment operates one of the largest fleets of specialist dehumidifiers in the UK.
Our specialist humidity control team provides comprehensive support for building owners and industrial end users, with access to humidity control equipment for hire from our network across the UK and Ireland.
Equipment is capable of treating between100m3 to 8000m3 of air per hour, and removing between 29ltr/24hr (based on 60% RH at 20°C) and 71 litres/hr (based upon 80% RH at 20°C).
As part of a strategic growth plan in this important and growing sector of environmental control, we recently invested in a new fleet of state-of-the-art desiccant dehumidifiers, based on the KwikDRY system.
Equipment is deployed across the country at manufacturing sites, ship yards, sports halls, commercial buildings, restoration projects following flood or water leakage, storage facilities, food processing, and floor laying on construction sites.