Keeping Things Cool Through Evaporation
Venting in cool air is only one way to lower the temperature of a greenhouse or growroom. Here, Dr. Lynette Morgan shows us how to dissipate heat using a breeze and water.
Heat and humidity: plants need certain levels of these for optimum growth, but too much of them means plants get roasted or steamed in an overly hot environment. Then, if these conditions are combined with a lack of air flow and cause plants to stagnate, physiological disorders become common place and fatalities can occur.
A good rate of fresh air flow can rejuvenate suffocating crops, but heat can still be an issue if it’s stiflingly hot and dry outside—in this case, the air vented into the growroom has little cooling effect. Luckily, indoor and greenhouse growers have long known how to take advantage of what is termed the “latent heat of evaporation,” meaning that combining water with sufficient air flow can give some fairly impressive cooling, provided that a few other conditions are met. Calculating just how much ventilation and evaporation are required to sufficiently cool down a small growing area for maximum growth does require some number crunching; however, once the basics are understood, this process is not as difficult as many growers think.
Ventilation and Cooling Methods
The simplest way of using ventilation to cool a growing area is to bring in fresh, cooler air from outside while venting out the stale, moist and warmer air at the correct rate. Using ventilation this way can reduce the inside temperature to about that of the outside air; however, the rate at which air must be exchanged can be surprisingly high, particularly in small, enclosed spaces where unvented high-intensity lights can be adding to the heat load.
In warm climate greenhouses with mature crops, the rate of ventilation required to simply remove the heat from incoming solar radiation, supply fresh CO2 for photosynthesis and lower humidity from crop transpiration is often as high as one complete air exchange per minute. Still, a high rate of ventilation might not be enough in warm climates. If the outside air is already super heated on a hot summer’s day, the incoming air can seem like a blast from the furnace rather than a cooling breeze. Further cooling methods need to be incorporated into the ventilation system in this situation, and one of the most effective and commonly used is evaporative cooling.
How Evaporative Cooling Works in a Growroom
As water evaporates into the air, energy is lost and causes the air temperature to fall—this is the same as the cooling we feel when wet skin is exposed to moving air. In a growing area, this evaporation of water for cooling can take advantage of plants’ requirements for fresh air. The latent heat of evaporation of water is 970 BTU/lb, meaning that 970 BTU of energy is removed from the environment for each pound of water that is evaporated.
Ventilation is an essential component of evaporative cooling as the evaporation of water causes humidity levels to rise. Since high levels of humidity can create problems for plant growth by slowing transpiration and increasing the occurrence of disease, this humid air needs to be vented out at a sufficient rate to keep relative humidity below 85 to 89%.
Air pulled into a greenhouse or indoor garden can be cooled by fan and pad cooling systems, which usually consist of exhaust fans at one end of the indoor garden or greenhouse pulling air in through vents on the opposite side. The vents contain porous pads that have water circulating over them. Water evaporates as air is pulled through the wetted pads, thus removing energy from the air and lowering the temperature in the process. The cooled air is then circulated through and over the crop, cooling the growing environment, and vented out the other side.
For fan and pad cooling to work at maximum efficiency, it is important to keep the growing area as air tight as possible. This way the air being drawn in from outside is forced over the wet pads and does not come in through other openings where it won’t be cooled. Also, greenhouses should have air-inlet vents positioned to take maximum advantage of prevailing winds.
This way, the system takes advantage of natural air flows rather than working against them. Some smaller growers in the past have made their own fan and pad system using capillary matting and other similar materials; however, pads can rot over time if they are continually in use and, therefore, wet for most of the day.
Commercially designed evaporative cooling pads are manufactured from material that might incorporate wetting agents and other compounds to resist rot. An algaecide might also be used in the water to prevent the build up of algae on surfaces, which lowers the efficiency of the cooling pad.
Another method of evaporative cooling is using misters or high-pressure fog systems. These are more suited to larger greenhouse environments where uniform cooling of a wider area is required. Fog or mist systems use nozzles to form fine water droplets under high pressure.
Fog contains droplets of 0.05 to 50 microns, whereas mist is larger droplets of 50 to100 microns with the smaller droplets absorbing heat from the environment faster as they vaporize.
Fog is a better option for most situations, as it tends to fully evaporate before any droplets can fall on the plants below—wet plant surfaces encourage the development of many fungal and bacterial pathogens and should be avoided with any evaporative cooling system in use. Just as with fan and pad cooling, it is vital with fog and mist systems that there is sufficient air ventilation and movement to both circulate the cooled air and draw it out once humidity levels have been increased.
A disadvantage with fog or misting cooling, however, can be blockages of the high-pressure nozzles; so, in some cases, the water supply could need ultra fine filtration to prevent this occurring.
Air Temperature and Humidity
The amount of cooling that can be achieved with evaporative methods is not unlimited. It is linked to temperature and humidity of the incoming air. To work out how much cooling is possible in a well-ventilated growing area, two variables are important: dry bulb and wet bulb temperatures.
The normal air temperature that we measure with a standard thermometer placed in the ventilation stream of the growing area is the dry bulb temperature. The web bulb temperature is a little more complex. It is the lowest temperature that can be achieved by evaporation of water only, and it is linked to the moisture content or humidity of the incoming air. In fact, the web bulb temperature tells us how much cooling is possible given the temperature of the incoming air and its relative humidity level.
The higher the relative humidity of the air, the lower the amount of evaporation can occur; thus, less cooling is possible.
Wet bulb temperatures can be easily measured on-site with use of certain electronic humidity meters. Growers who don’t have meters with the ability to measure web bulb temperatures can still work out the cooling potential of evaporation by the slightly more old fashioned way of using psychometric charts.
These charts allow wet bulb temperature to be calculated based on dry bulb or normal air temperature and the relative humidity, which can be measured with a standard RH meter placed in the air stream. The best time to measure or calculate wet bulb temperature is midafternoon when the outside air, which is to be vented into the growing area, is at its warmest.
This should give the greatest difference between wet and dry bulb temperatures and thus the maximum cooling effect of evaporation can be determined. As a general rule, the air in the growing area or greenhouse can be cooled and held at wet bulb temperature plus three degrees Fahrenheit.
When evaluating how much cooling can be achieved with current temperature and humidity levels, it also important to remember that the cooled air will gather heat as it circulates in the growing area and is eventually vented out. Heat accumulation will depend on factors like how far the cooled air travels inside the growing area, heat output of lamps and solar radiation from direct sunlight.
It is possible for the temperature buildup inside the indoor garden or greenhouse to reheat the circulating air to greater than that of the outside environment, so correct calculations of both the amount of evaporative cooling and amount of air to be vented in and out need to be carried out correctly.
Calculating the Cooling Potential of Evaporation
The following equation can be used to work out how much water needs to be evaporated into a known area and how much air movement or ventilation is required in order to reduce the temperature inside a growing structure to the required levels. Note that most of the equation is calculated using metric figures, but the end result is imperial.
Equation for Fog Cooling of Greenhouses
T1 = Outside Temperature °F x 5/9 - 32
T2 = Required Inside Temperature °F x 5/9 - 32
Ra = Outside Relative Humidity (% / 100)
Rb = Required Inside Relative Humidity (% / 100)
Calculating X—the saturation temperature—and H—the specific enthalpy (the energy contained in 1 kg of air):
Xout = (Ra x 0.0212T12 + 0.2435T1 + 4.278) / 1000
Xin = (Rb x 0.0212T22 + 0.2435T2 + 4.278) / 1000
Hout = (1.006T1 = Xout x 1.84T1 + 2502) x 1000
Hin = (1.006T2 = Xin x 1.84T2 + 2502) x 1000
D = The air density (altitude (ft / 3.28) x 0.001) + 0.8167
I = Incoming solar radiation (moles.m-2.sec-1)
T = Radiation transmission coefficient of greenhouse (values from 0.1 to 1.5. The average for a polythene-covered structure is 0.75.)
a = Inside evapotranspiration coefficient (values from 0.3 to 0.7. This is an indication of the amount of the floor area that is occupied by transpiring plants.)
U = Heat transmission coefficient of greenhouse cladding (values from 4 to 10 W.m-2.oC-1. Note that this is a coefficient, so the units will not matter.)
Calculating the required air flow rate (ventilation rate) out of the greenhouse:
AFR = ITa - U(T2 - T1) / (Hin - Hout)
Converting air flow rate to cubic meters per minute
AFR = AFR x 3600 x D / 60
AFR = (AFR x 3600 x D / 60) x 35.3 cu.ft.min-1
Calculating the water flow rate for fogging:
EFR = AFR x (Xin - Xout)
Converting EFR to gram of water per m2 per hour (1 g of water = 1 ml):
EFR = EFR x 3.6 x 106
Divide the number obtained in the last step by 46546 to get your final answer in gallons per ft2 per hour.
Now you know how to use breeze and water to dissipate from your greenhouse or growroom. With your new knowledge and these handy calculations in hand, you can really give your crops a fresh air experience.