Control of humidification systems

By Pieter Aldred of Humidair

Most humidification systems require some sort of external control interface, supplied with built-in humidity control — what do you need to know?

The correct selection and positioning of the sensor controller package are crucial to the suitable operation of any humidifier.

HUM003aOnboard controller touch panel.
Image credit: Condair

It must be remembered that a humidifier is a ‘dumb’ box — it will only do what it is instructed to do. Furthermore, if you cannot measure it, you cannot control it.

The basic principles of simple on/off control, modulated control, and how these are used in supply control and return air control systems are as follows.

Factors to consider

  1. What is the stability of the temperature control in the space?
  2. What is the humidity control tolerance required?
  3. What is the time constant for any changes in humidity?
  4. Is the control sensing point on the supply air, return air, or directly in the room?
  5. What is the absorption distance for the injected moisture?
  6. What type of control does the client actually require?
  7. What is the inertia of the area that you are trying to humidify?
  8. Is the humidifier sized correctly?

Let’s explore these factors in greater detail:

1) What is the stability of the temperature control in the space?
As relative humidity is a function of temperature, it is absolutely critical that the temperature and its control tolerance are taken into account. It is not uncommon in a controls specification to see temperature stability stated as ±2°C. If this is plotted on a psychrometric chart at, say, 20°C and 50% relative humidity (RH) it can be seen that this will give a humidity variation of ±6%; therefore, if the specification on humidity in the document called for ±2% RH, it is going to be extremely difficult to maintain this with a variance in temperature.

2) What is the humidity control tolerance required?
As with temperature, the humidity specifications often call for the unachievable. It is again not uncommon to see a specification calling for ±2%. As stated above, this has to be taken into account in relation to the temperature control. Furthermore, if control to ±2% is required, then the temperature control has to be better than ±0.2°C and the humidity measurement has to be better than ±1% RH.

3) What is the time constant for any changes in humidity?
When talking about control accuracy, it is not only the degree of control in terms of ±*% RH; it is the time constant over which these changes are allowed. In many process style applications, it is not only the degree of change, but also the speed of that change that has to be considered.

4) Is the control sensing point on the supply air, return air, or directly in the room?
It is important to know where the humidity sensor is going to be mounted, as this will have a direct influence on the selection of the humidifier and the control sensor.

If you cannot measure it (accurately), then you cannot control it.

Supply air sensing: If the sensing point is in the supply duct, there is very little time delay between injection of moisture and an increase in humidity, and conversely, a fall in humidity when the injection of moisture stops. This can make a very volatile control signal occur, which in turn can make it extremely difficult to control. Therefore, a humidifier that has no flushing cycle and that can modulate over its entire range quickly, will be required in such a case, as the reduction in steam output when a flushing cycle takes place will cause the controls to go unstable and control will be erratic. The only humidifiers suitable for this application are either a process electric steam unit or a live steam unit. Supply humidity control is extremely difficult to set up and will not always control the space into which the air is being supplied without some sort of feedback control. With all the air being supplied at set point, if there is a heat gain or a heat loss in the area, the relative humidity in the area will be different from set point; however, the absolute humidity will remain unchanged.

Return air sensing: If the sensing point is in the return air duct, there is usually a longer time interval between injection of moisture and a measured increase in humidity. This allows virtually any humidifier to be used and for reasonable ±3% RH control to be achieved. As the space being humidified acts as a buffer, small changes in output during flushing cycles do not have any major influence on the area’s humidity. This is by far the most common control used in HVAC applications.

Room-mounted control: Control from room-mounted sensors can be difficult for several reasons:

  • The sensor has to be mounted onto something, usually a wall, but the thermal mass of the object to which it is attached will affect the humidity measured.
  • It has to be mounted away from any heat sources, radiators, heat discharges, electric motors, doorways, and air-conditioning supply airstreams.
  • It has to be mounted some 100mm off any wall to escape the thermal effect of the wall itself.
  • It should not be mounted on a wall that has a solar gain.
  • Is the mounting position truly representative of the room?
  • All cabling must be away from any electrical interference.

If any or all of these are not considered, then major errors in control will occur. Control to better than ±5% will be difficult with room-mounted sensors.

HUM004 new
PI control parameter.

5) What is the absorption distance for the injected moisture?
When positioning a humidity sensor, it is important to know the absorption distance of the moisture being injected; this distance is often referred to as Bn. Bn is a function of the type of moisture injection, the air temperature into which the moisture is being injected, together with the velocity of the air into which it is being injected. This distance should be calculated for each application and no humidity sensor or humidistat should be positioned less than 5 × Bn downstream of the injection point. It is very common on AHUs to see either the high limit stat or even the control sensor mounted 1–2m down the duct from the injection point. Mounting in this position will cause the humidifier to work in an on/off control mode, irrespective of the complexity of the controls package or the precision of the humidity sensor used.

6) What type of control does the client actually require?
Fundamentally, there are two types of control: on/off and modulated. On/off control is the lowest cost and has the lowest accuracy — ±5% at best would be typical. Whilst this is suitable for many applications, if the control accuracy is tighter than ±5%, it will be difficult to achieve with on/off control.

Modulated control can, and will, with correct set-up, give control stability better than ±5%, down to ±2%, using standard HVAC style sensors.

7) What is the inertia of the area that you are trying to humidify?
As mentioned earlier, when designing a controls package, it is important to know what the inertia of the system being controlled is. The time loop for an area is based on the time taken for moisture to travel around the system and reach the sensing point. Obviously, with supply humidity control, this loop time is very low and the set-up of the controls should reflect this.

8) Is the humidifier sized correctly?
Humidification systems are often oversized; particularly steam humidifiers, to allow for ‘safety factors’ in respect of the design conditions. The provision of an output far greater than that needed under certain operating conditions cannot only downgrade controller performance, but it can also cause condensation formation on the duct walls. This is particularly true for electrode humidifiers that have initial 20% on/off control. The element type units are less affected, as the control is 0–100%. Required capacities for winter and summer selections are often quite different. Modern humidifiers all offer a capacity limiting feature, still maintaining full control over the reduced range.

Installation effects on controls: Steam humidification is one of the most difficult to control when installations of the steam pipework are incorrect. The most common disturbance factors are sagging steam hoses between the humidifier and the distribution pipes, and sagging distribution heads within the duct.

The effects of these are different: with sagging steam hoses, the steam delivery may be reduced due to back pressure on the boiling vessel, or condensate may be blown into the duct as liquid water, causing puddles to form. As outlined above, these can cause a control disturbance. Obviously, major restrictions in steam delivery pipework can reduce the amount of moisture injected so the system never reaches set point. The sagging steam distribution pipes within the AHU can also cause back pressure and puddle formation; however, because distribution across the entire air path may not now be possible, excess steam can be injected into a smaller than designed air volume and this will extend the saturation distance, Bn. This could cause wetting of the humidity sensor, with consequential failure.

These effects cannot be rectified by changes in controls alone; the installation has to be correct before any system will work correctly. Whilst the above has been based around steam humidification, the same really is true for any humidification system, be it in-duct or direct in the air. Poor installation will result in poor control of the sensor.

HUM002PI control parameter.

Available control options

The permissible control tolerances play a major role in the selection of the control device. Control tolerances give the maximum variation allowable from the set point, and the choice of controls has to match this control tolerance. The choice is fundamentally on/off, multi-step, or continuous modulating control.

On/off control: The choice of on/off control as described earlier will not give close control due to the inherent nature of the control system; there are only two modes: on or off. Whilst it is possible to change the time duration of these on/off periods by adjusting the deviation from set point for the operation of the switch, the overall effect is still poor control.

Continuous proportional control: As with on/off control, there are many different systems offering continuous control — these can be built-in or stand-alone controllers. Most modern humidifiers have controllers onboard to allow independent control. There are two options commonly used in humidification control: P and PI. P controllers are proportional control only, whereas PI controllers are proportional and integral controllers. The fundamental principle is that the control signal sent out by the controller to the humidifier increases the further the set point is from the measured value. With continuous control, the oscillation of the control signal, as seen in on /off control, does not occur, provided that the controls are correctly set up. However, as modulated control has to take into account a number of factors, they can be more complicated to set up and if the proportional band is set too small, the overall effect can almost be the same as on/off control.

Care has to be taken when advising on the required proportional bands. Some controllers work with a direct relationship, so a 15% proportional band means 15% RH deviation from set point, over which a full output is scaled.

Proportional control works by comparing the deviation from set point and continuously adjusting the output signal to the humidifier, dependent on the proportional band set. With a set point of 50% RH and a true proportional band of, say, 15%, the output from the controller will increase as the measured humidity deviates below the set point. With these settings, maximum control output would not be achieved until the humidity is down to 35%. As the humidity recovers towards set point, the control signal output decreases proportionally.

One difficult area for proportional control is in what is known as the partial load mode for a humidifier. Resistive units and live steam units do not have a partial load mode, as they can modulate over the entire range; however, electrode boiler humidifiers only modulate between 20 and 100%; below 20% of steam output, they work in an on/off mode. This can make smooth control difficult as the system switches between on/off control and modulated control at 20% demand. This can make for unstable control when there is low demand, yet reasonably smooth control above 20% demand.

Another point that must be remembered when looking at control problems, is that all humidifiers, with the exception of live steam and process units, have some time to flush water to reduce mineral build up within the boiling vessel. During this time, the moisture output will reduce and as the supply of moisture reduces, the demand will increase as the humidity falls. If the proportional band is too small, this can make the control very volatile, to the extent that the system becomes totally uncontrollable. This is particularly apparent on supply humidity control systems.

All proportional controlled systems will always fall slightly short of the actual set point, as the controller always needs some output to work — it will never completely settle at stable output.

Sensing

I again make the statement that if you cannot measure it (accurately), then you cannot control it. The most common sensor used, measures relative humidity. This is generally an acceptable form of control, but there are instances where much greater accuracy and stability can be achieved by using dew point control. Dew point control is normally used on supply air control systems.

As an example we describe the following scenario:

Assume that the set point is 25°C and 50% RH and that the temperature control is, say, ±2.5 C.

If you run with humidity (RH%) control: As the temperature rises by +2.5°C, the humidity falls to about 42% RH. The humidifier will ramp up its output to try and maintain the set point of 50% RH, so will re-establish the RH level at a higher absolute humidity, 10g/kg to 12.2g/kg. Conversely, if the temperature falls by 2.5°C, the humidifier will back off as the sensor is seeing a humidity above the set point. This would allow the absolute humidity to fall from the 10g/kg down to about 8.4g/kg.

Integration with modern building management systems via Modbus, LonWorks, and BACnet is fast becoming a standard feature on humidifiers. This allows the humidification system to be accurately and efficiently controlled and monitored.

However, in both cases, the humidity is being maintained at 50% RH. Basically, a conventional humidity control sensor does not consider the temperature; it just uses the single parameter of humidity.

If you run with dew point control: The dew point of the set point 25°C and 50% RH is 14°C Td. If you have a sensor that calculates the dew point of the air that it is seeing, it looks to maintain the absolute humidity at 8g/kg, irrespective of the temperature it is seeing. Basically, it does not care what the actual humidity is between 1 and 100%. It is looking to maintain the absolute humidity level at 8g/kg and effectively constantly recalculating the humidity set point it needs to achieve this.

So, if you look at the same shift in temperature, as the temperature rises from 25°C to 27.5°C, the RH% set point reschedules itself to 42%. Conversely, if the temperature falls to 22.5°C, the new set point becomes 60%. In both cases, the absolute RH is maintained at 8g/kg. Consequently, the humidifier output remains constant.

If you run with combination RH and dew point control: This convenient option is most practical in low-temperature applications, for example 2°C and 95% RH control. If the temperature reduces by 1°C, then we reach dew point temperature on the sensor. A wet sensor does not dry off quickly under those conditions, so control ceases until it dries out.

Sensors are supplied with small heating pads that activate above a pre-set RH level. A separate sensor probe, not affected by the heating pad, reads the actual temperature. The relative humidity value is calculated based on the dew point value obtained from the humidity probe and the actual temperature obtained from the additional temperature probe.

HUM001Duct sensor.

Building management integration

Integration with modern building management systems via Modbus, LonWorks, and BACnet is fast becoming a standard feature on humidifiers. This allows the humidification system to be accurately and efficiently controlled and monitored. Typically used in large projects with extensive mechanical, HVAC, and electrical systems, it creates a platform in which two-way communication is possible. Limiting the chance of failures and emergencies, economising a humidifier’s energy use dramatically, and improving the unit’s reliability and life.


Click here to read the October 2018 issue of RACA Journal


 

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