ASK THE EXPERT Getting humidity control right C


John Barker, Managing Director at Humidity Solutions, tackles the importance of asking the right questions about humidity control at the beginning of a job to ensure a completed and cost-effective solution.


ustomers often have a clear idea of the relative humidity conditions they require, but don’t always appreciate what’s involved. Limited


understanding on the consequences of asking for very tight control can push a project into a price bracket which is not cost effective and could result in the project not progressing. If the right questions are asked at the outset it is frequently possible to arrive at a less expensive solution that still meets the customer’s control requirements.


Give us an example of this?


It’s not uncommon for us to be given a relative humidity (RH) specification 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 there will be a change of 2% RH without any change in the actual moisture content of the air.


Can you explain Relative Humidity?


Relative Humidity is the amount of water vapour the air can hold without that moisture condensing to form liquid water. 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’s clear that a control specification of +/-2% RH with a temperature tolerance of +/-2°C cannot be achieved.


What other factors affect the degree of control that can be achieved?


Other practical factors that impinge on the degree of control that can realistically be achieved include the tolerances of the humidity sensors and the ability of the humidifier to react for the system in which they are operating.


For example, a standard HVAC humidity sensor will typically control to +/-3% RH for a mid-range humidity of 40% RH, whilst higher specification sensors are able to 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 demand this very precise level of control.


Whichever type of sensor is used, this will then control the humidifier which, dependent on the type of plant and system, will have a reaction time. If a steam humidifier is starting from cold, it will take time to get to full output. Similarly, with a spray humidification system the output may be delayed by factors such as starting the pump on a high-pressure system or filling the tank in an ultrasonic humidifier. With steam humidifiers 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 humidifier is constantly boiling.


What about when very close control is required?


If close control is required on a laboratory or test room where the conditions need to be incredibly stable over a long period of time then all the above can be activated, which along with the best sensors, very accurate temperature control and a well set up controls philosophy, will result in excellent control and repeatability. A steam humidifier served with reverse osmosis water can have the drain turned off and trickle feed water supplied so that the humidifier never loses control through the drain and fill cycle plus is virtually maintenance free – only an annual inspection required. Using a high spec sense will also ensure that a tight control tolerance can be maintained.


Equally for smaller applications, ultrasonic humidifiers offer instant reaction to control signals and can maintain close control.


What is a more typical solution?


We offer our expertise and experience to ensure that a realistic control tolerance is achieved for the process or application, whilst also taking budget, payback and running costs into consideration.


Paper in a printer for instance can be held comfortably at 40-60%rh and still maintain good condition so a control of 50%rh +/- 10%rh is readily achievable. Static can be eliminated at a relative humidity above 55%rh – not 55%rh +/-1%rh.


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Even conditions that are out of the midrange such as when a humidity required is above 80%rh in a cold store, charcuterie production or food processing, the tolerance can be considered to achieve the condition without over-engineering the equipment, by selecting a sensor able to control at these conditions with good repeatability. However the tolerance could often still be +/-5%rh.


The underlying message of this is that in the vast majority of projects, even if their requirements fall outside what might be considered as ‘standard’, the necessary tolerances can be delivered using standard off-the-shelf equipment when applied with knowledge and experience.


So, although a customer may know the temperature and RH they require for a particular product or process, a lack of understanding may lead to a request for unnecessarily tight control that pushes the project into a price bracket or running costs that is not cost-effective and might even lead to the project being abandoned. There is, therefore, a clear responsibility on those of us working in the industry to help customers fully understand what their true requirements are and to deliver the best solution accordingly.


Humidity Solutions will be exhibiting on stand D50 at HVACR Live at Excel. Or find out more about the company’s products and services at humiditysolutions.co.uk


BUILDING SERVICES & ENVIRONMENTAL ENGINEER APRIL 2023 17


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