HUMIDITY CONTROL
Asking the right questions
John Barker, managing director of Humidity Solutions, tackles the importance of asking the right questions at the beginning of a job to ensure a completed and cost-eff ective project.
E
nd customers often have a clear idea of the relative humidity conditions they require, but don’t always understand what is involved. Asking the right questions is essential to providing the right solution.
It is not uncommon for us to be given a relative humidity (RH) specifi cation by a customer, based on the products, objects or processes they are seeking to safeguard through humidity control.
A typical example might be for a control tolerance of +/-2% RH with a temperature tolerance of +/-2°C. This shows a degree of misunderstanding about what can be achieved – not by us and our equipment, but by the laws of thermodynamics. For every 1°C swing in temperature, the humidity will change by 2% RH without any change in the actual moisture content of the air.
To fully understand this, it’s important to be aware of the relationship between air temperature and how this aff ects the amount of water vapour the air can hold without that moisture condensing to form liquid water – this is described by the RH. Warm air can hold more water vapour than cold air, so when air is heated its capacity to hold moisture also increases and the RH falls. In the example above, if the 1°C temperature swing is upwards, the RH will fall by 2%. Given these natural laws of thermodynamics, it is clear that a control specifi cation of +/-2% RH with a temperature tolerance of +/-2°C cannot be achieved.
There are also other practical factors that impinge on the degree of control that can realistically be achieved. These include the tolerances of the humidity sensors and the ability of the humidifi er to react for the system in which it is operating.
For example, a standard HVAC humidity sensor will typically control to +/-3% RH for a mid-range humidity of 40% RH, whilst higher specifi cation sensors are able to
22 September 2018
achieve 2% RH accuracy. In general, the better the sensor the higher the cost, but it is only worth paying this additional expense if the nature of the space or its contents demands this very precise level of control.
Whichever type of sensor is used, this will then control the humidifi er which, dependent on the type of plant and system, will have a reaction time. If a steam humidifi er is starting from cold it will take time to get to full output. Similarly, with a spray humidifi cation system the output may be delayed by factors such as starting the pump on a high-pressure system, or fi lling the tank in an ultrasonic humidifi er.
With steam humidifi ers this situation can be mitigated to some extent by using a ‘keep warm’ function so the time required to reach full temperature is reduced. Other options include using reverse osmosis water so the drain can be turned off to reduce heat-up time, or using a trickle feed of water to ensure the humidifi er is constantly boiling. However, all of these solutions add to the overall cost of the system.
In applications where very close control is required, such as in a laboratory or test room, it is vital to ensure that stable conditions are maintained over a long period of time. In these cases, all of the potential solutions described may be applied, selecting the best options for each particular project – backed by a sophisticated control philosophy. The end result will certainly deliver excellent temperature and RH control and repeatability, but at a cost.
In our experience, the very high costs associated with such solutions often lead to the project being abandoned – or the end customer bites the bullet and pays far more than is necessary. There are also many situations where the customer ends up with a standard air conditioning system because the designer had only a limited knowledge
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