Considerations when controlling humidity

​The following interview was conducted with Reg Quiring. Reg was Conviron’s Chief Designer for 30 years and led many engineering developments. Reg is recognized by many as one of the most experienced controlled environment engineers in the world, with many of today’s plant growth chambers and rooms being the outcome of his design. Reg was an active participant and contributor to the industry’s largest information exchange seminars and is a past Chair of the Committee on Controlled Environment Technology and Use. Other sources include the Plant Growth Chamber Handbook, Edited by R.W. Langhans and T.W. Tibbitts, North Central Regional Research Publication No. 340, Iowa Agriculture and Home Economics Experiment Station Special Report No. 99.

Background

Humidity is the water vapor or moisture content of the atmosphere. The maximum amount of water vapor in the air (saturation) varies greatly with temperature. Relative humidity (RH) is expressed as a percentage of the maximum possible moisture content. It is important to understand that simply stating an RH percentage range without a coincident dry bulb temperature leaves the actual moisture content undefined. Figure 1 below shows how starkly different the absolute air moisture content is for a RH range of 40% to 85% at 10°C, 25°C and 40°C.

Humidity level affects transpiration and other gas exchanges of plants of all varieties both in highly varied growth conditions in the field and under tightly monitored and controlled conditions such as in a growth chamber or room. The balance of water loss by transpiration and gain by root absorption determine plant water status at any given time. Since relative humidity (RH) levels influence transpiration and thus plant growth, measurement and control of RH in controlled environments can be important to ensure consistent plant response and subsequent interpretation of experimental results.

Is it important to control humidity in a growth chamber?
Temperature and the duration and/or intensity of light are variables that we commonly provide by default as controlled parameters in a growth chamber or room. Both have a significant impact on photosynthetic rate and plant morphology. Relative humidity less so, but it is one of the most commonly misunderstood parameters in controlled environment research. Most plants grow well in a temperate range and can adapt to a wide range of relative humidity values providing they are irrigated accordingly. However, if humidity is not controlled it can have an impact on outcomes as it becomes a hidden variable that is not accounted for.

How could lack of humidity control be a hidden variable?
Even though growth chambers are controlled environments they are not entirely closed systems. Plant growth chambers have fresh air intake and exhaust ports for air exchange/ventilation. This fresh air supply is required to compensate for the CO2 used during photosynthesis and to prevent the buildup of volatiles.

Through no fault of the chamber, it may draw in air with a very different moisture content during summer months compared to winter, it may be consistently high if your facility is in a tropical or other humid zone, or simply vary dramatically from day to day depending on if it is a warm, cool or rainy day. The variability of the chamber ventilation air supply depends on how tightly controlled your facility’s conditioned air is. As a consequence, the RH inside the chamber becomes a resultant value, not a controlled or repeatable one. Therefore it is a hidden variable that is not accounted for.

Plant growth chamber manufacturers should present information to clients on the case for having humidity control as a standard business practice - either through the company’s disbursement of information materials or during consultations. Researchers and other users of controlled environment equipment should be cognizant that plant growth chambers and rooms that are not equipped to control relative humidity levels can be impacted by the humidity profile of the ambient air in one’s facility.

What are the cost implications of humidity control?
Additive humidity is a relatively low cost option. It is easier to control reliably and it requires less mechanical equipment compared to active dehumidification. For additive humidity there are many methods; spray nozzle humidification, centrifugal atomizing humidification and ultrasonic. Dehumidification (whether by separate cold coil, modulated air bypass of the main cooling coil, electric re-heater, chemical desiccant dryer or other methods) requires larger mechanical equipment and electrical supplies. Control design nuances increase and also add more to overall cost.

What are some common misconceptions?
Some clients incorrectly assume that just because they have opted for additive humidification to achieve higher RH values that they automatically have dehumidification. Clients might wonder why they are not achieving a lower humidity level after events like watering or on a humid day, despite having selected a control setpoint that is lower than the chamber’s actual sensor readings. Without an active dehumidification system installed on the chamber, there is no way for excess moisture to be removed in a deliberate manner. Fresh air exchange can reduce RH inside a growth chamber, but only when the source of air has a lower absolute moisture content.

In one case a client assumed that having ordered a plant growth chamber with the additive humidity option, they would be able to control dehumidification as well. Ultimately they found themselves not able to achieve the low RH values they wanted and faced a decision whether or not to obtain a field retrofit to add active dehumidification capability. While there may be options in such cases, a retrofit for active dehumidification is challenging to install cost effectively once a chamber has left the factory. Consider if one were to endeavor to retrofit a chemical desiccant dryer as a means of active dehumidification. Since a chemical dryer process adds heat to the chamber there would be an increased load on the refrigeration system and the client may be faced with having to upgrade their chamber’s cooling system as well.

Conversely, if a client requests a retrofit for additive humidity, it typically does not impact the refrigeration or electrical systems because it only involves evaporation of water droplets which is a net cooling process.

Users should understand before they buy what the limitations of the plant growth chambers or rooms are and what humidity targets they actually need to achieve across their intended temperature range.

Do I need active dehumidification?
If actively controlling more parameters of the plant environment enhances the value of your research or crop then you may want to consider active dehumidification. Note that dehumidification systems are typically not used to stress the plant (unless that is the objective of the research). Rather they are generally designed to stabilize RH, eliminate a variable, and to prevent relative humidity levels from drifting too high due to plant loading, watering, lack of air exchange, or high ambient conditions in one’s facility (a wider factor than just controlling another variable). The ability to avoid prolonged high humidity exposure in a chamber can prevent unwanted outcomes (e.g. disease) that could result in lost time or plant resources.

The relationship between the temperature and humidity should be looked at from a psychrometric standpoint as illustrated in figure 1 above. The relationships between RH, the coincident temperature and the air’s moisture content are often misunderstood. This can present a particular challenge if multiple users participate in setting the specifications for their chamber and desire humidity targets at dramatically different temperature ranges. For example, one user may define a very narrow range of temperature that they want to achieve such as 20-25°C with a 40-85% relative humidity range. While fulfilling that particular request may not pose a particular challenge, another user of the same chamber may want to conduct cold temperature work and request that the chamber reach 10°C assuming there is no additional challenge hitting the same 40-85% humidity range. This becomes problematic because 40% at 25°C actually represents a higher moisture content condition, and is easier to achieve than 85% at 10°C. It may seem counterintuitive that an 85% RH setpoint would need dehumidification, but at 10°C the absolute moisture content is quite low.

In considering whether to opt for active dehumidification, carefully consider reasons for wanting to hit a particular low humidity target. If a user wants to achieve especially low humidity levels at cooler temperatures, it may take a dehumidification system twice the capacity to achieve the same RH than it would at a higher temperature. Compounding this, the lower one drops in temperature, the less effective both a refrigeration compressor and any dehumidification system become. This compounding penalty on efficiency is a nuance that needs to be understood when setting dehumidification targets because doing so at the edge of the chamber’s performance envelope can add unexpected cost. That cost can be reduced if the target humidity is flexible.

What humidity control do you need?
In one case a researcher required a controlled environment to rear sea life in aquariums and requested that the room be held at 50% RH. With water evaporating off the top of each of the aquariums, our engineering team’s calculations showed that the researcher would require a 600 CFM dryer to remove that moisture load and it would have added 20-30% cost to the project. As it turns out the target of 50% relative humidity was an arbitrary target that did not in any way impact the environment inside the aquariums. Once pointed out, and the researcher agreed to raise the 50% humidity target, it enabled them to reduce the budget for the overall project.

In another case a client was utilizing a greenhouse to study maize. The client desired tighter environmental control than was possible in her greenhouse in the UK in the winter and was looking to procure a large plant growth room. When she examined the proposal she remarked that the room met all of her requirements but was outside of her budget. When asked to modify the proposal to meet her budget, our technical team observed that a significant amount of the cost was attributed to the lamp loft system. The loft design is such that it has the high intensity lighting enclosed behind clear barriers and is cooled separately from the growth area. The technical reason for the lamp loft system is to reduce the heat imposed on the growth area cooling system which means the cooling heat exchanger coils can operate at a net warmer temperature. This in turn enables the higher humidity levels of 85% to 90% that the standard specification offered. The client realized she did not require the growth room to achieve 85% or 90% RH. In fact, the client clarified she preferred not to go above 65% RH. With this new understanding, Conviron could eliminate the lamp lofts. With that, a significant amount of the physical and mechanical complexity for the growth room was removed. Our team could install open light fixtures from the ceiling without the lamp loft system at a much lower cost. In the end, the client received the growth room engineered and installed within their budget. They now have a growth room they can utilize year-round with tight control that was hitherto not possible in the greenhouse.

Conclusion
In summary, there are many reasons why it would be desirable to have both additive humidity and active dehumidification on a plant growth chamber or room. There is added cost associated with each feature but it is better to weigh the pros and cons ahead of time prior to installation of your controlled environment rather than to opt for a potentially expensive retrofit at some point in the future - which is especially the case with active dehumidification.