HUMIDITY CONTROL HUMIDITY CONTROL

22 September 2018 www.acr-news.com

HUMIDITY CONTROL

End 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

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

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.

Asking the right questions

HUMIDITY CONTROL

HUMIDITY CONTROL

www.acr-news.com September 2018 23

Heronhill

of humidity control or the customer’s true

requirements.

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. For instance, many printers believe

they need very close RH control but, in reality,

maintaining a RH between 40% and 60% will be

perfectly adequate.

If static electricity is an issue, this can be

avoided by maintaining the RH above 55% – it

does not have to be 55% +/-1% RH.

Even in applications where the requirements

are outside the normal RH range, such as

>80% RH in a cold store, charcuterie or food

processing facility, it is possible to achieve the

desired conditions without over-engineering

the system. Simply using a sensor that is able to

control the system at these conditions with good

repeatability will usually be sufficient, and still

achieve a tolerance of +/-5% RH.

For further peace of mind, humidification

systems can now be equipped with remote

monitoring, control and diagnostics to improve

reliability and detect any issues immediately.

This approach also ensures that planned

maintenance is undertaken on the basis of hours-

run or fault indications – in other words, when it

is needed rather than simply based on time.

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 and consumables.

So, although an end customer may know the

temperature and RH they require for a particular

product or process, a lack of understanding can

lead to a request for unnecessarily tight control

that pushes the project into a price bracket 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 end

customers fully understand what their true

requirements are, and to deliver the best solution

accordingly.

For all your humidification requirements www.heronhill.co.uk

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