October 2007

Grower 101: Greenhouse Cooling Options By Gene Parsons

If you decided to build a greenhouse or currently own a greenhouse, it's likely that the question of what methods are available for cooling the inside of it will arise. One main purpose for building a greenhouse structure is to create a controlled environment for optimum growing conditions compared to growing outside in a non-controlled environment. As a grower, you have many options in the design of the greenhouse structure and on how much control you want or need for the crop you are growing.

A greenhouse structure by its design and nature can become too warm, thus greatly affecting production and quality of the crop. Cooling is a critical part of your controlled environment. Basic cooling designs and systems incorporate the following designs/principles.

Natural Ventilation

As its title implies, "natural ventilation" allows the greenhouse structure to ventilate and cool by natural air movement within and outside the structure. Engineers will design a greenhouse structure with a vent or multiple vents, which allows air to enter and exit the greenhouse. Combinations of vents help maximize the natural flow of the air and enhance the airflow within the structure. As an example, an engineer can design a ridge vent in combination with sidewall vents. This design allows heat in the peak to leave the house while helping to bring in fresh air through the side vents. The adjustment of these vents by an environmental controller can increase and decrease the airflow inside the structure based on the direction of the outside winds.

The goal of natural ventilation is to maintain the same temperature inside the greenhouse as it is outside the greenhouse. This can be hard to accomplish because of influences by the solar gain through the covering, the type of covering used on the structure and directional placement of the structure on the land in relation to the prevailing winds. In greenhouses with natural ventilation, internal and external shade systems can control the heat generated by the solar gain. Shade systems also help control the intensity of the light in the greenhouse. Based on the design of the naturally ventilated greenhouse, you can expect to see ambient to 10 degrees or more temperature increase.

Mechanical Ventilation

If your greenhouse structure is naturally ventilated and you want to reduce heat, the addition of mechanical ventilation will improve the airflow and extract the warm air out of the house. Designing for one air exchange per minute for the greenhouse will make ambient temperatures possible inside the greenhouse.

In a typical mechanically ventilated greenhouse design, the engineering parameters used for selecting the required number of exhaust fans are calculating the volume of the greenhouse, identifying what the static pressure in the house will be and selecting the placement of the fans in regards to the location of the vents. With the parameters identified, fan selection is based on the cubic feet per minute (CFM) rating of the fan, the static pressure ratings, the actual physical size of the fan and the horsepower rating. With the above fan ratings, one can select the correct fan that produces the highest CFM per watt for energy-efficient operation.

Now that the greenhouse has mechanical ventilation, you can use your environmental computer or mechanical thermostats to control the fans. Staging of fans to come on in groups will help save energy costs. There are limitations on the length or distance that you can effectively pull air through the greenhouse without having large temperature differentials from the inlets to the exhaust fans.

Evaporative Cooling

Evaporative cooling for greenhouses relates to the evaporation of water, either by re-circulating evaporative pad cooling systems or by high-pressure fog systems, or a combination of both.

Evaporative pads. Cooling by evaporative pads requires the presence of a defined airflow, usually by mechanical fans that exhaust or pull the air through the wet pad medium. The ambient air as it passes through the wet pads cools down close to wet bulb temperature.

The following will serve as an example for designing a pad cooling system for a gutter-connected greenhouse. Say that your greenhouse has a length of 100 ft. and a width of 240 ft. (six 40-ft. bays) with a gutter height of 12 ft. A simple equation for calculating the cubic volume of the greenhouse from the ground to the gutter would be length x width x gutter height. Calculating 100 x 240 x 12 equals a cubic volume of 288,000 cu.ft. of air.

Evaporative pad manufacturers and some universities publish guidelines for how many cubic feet of air are required per square foot of pad material. Figures of 200-350 cu.ft. of air per square foot of cellulose pad material are common for greenhouses based on their system design and pad thicknesses. Cellulose pad material is available in thickness of 4 and 6 inches, with 6 inches now becoming the standard for companies manufacturing pad systems and for customers demanding better cooling performance. For this example, the grower has installed 12 50-inch exhaust fans, which extract 25,000 CFM each at a static pressure of 0.10, for a total exhaust rate of 300,000 CFM. Even though the airflow is above the 288,000 CFM, the design will be fine. Having a greater airflow is better than having a slower ventilation rate.

For this design, we are using 300 cu.ft. of air per square foot of 6-inch thick pad material. Dividing the ventilation rate of 300,000 CFM by the design guideline of 300 cu.ft. of air per square foot of pad material, we need 1,000 sq.ft. of pad material. On one 240-ft. gable end, there will be 12 exhaust fans, two per 40 ft. bay, which will pull the air through the pad system. On the opposite gable end, the design is to have 240 ft. of evaporative pad cooling. In order to define the height of the pad, we divide the 1,000 sq.ft. of pad material by the available length of 240 ft. to find that a pad height of about 4 ft. is required. The pad system can be controlled from mechanical thermostats or by an environmental computer. When the pad system operates, water pumps from the systems sump up to the top of the pad where it flows down over and through the pad medium. Water not evaporated goes back into the sump, where it is recycled back to the top of the pads to again flow down through the pads. A well-designed and engineered pad cooling system provides up to 85 percent of wet bulb temperature.

High-pressure fogging. Cooling with high-pressure fog is successful in both natural and mechanically ventilated greenhouses. Design engineers use psychrometric calculations, which take into consideration the ambient weather conditions of the greenhouse location, the ventilation rate of the greenhouse and any other design features of the structure for properly designing the system. High-pressure fogging is a very efficient way to cool and control your environment. It uses less water than the pad system, and it not only cools but also corrects the vapor pressure deficit (VPD) in the greenhouse and can be installed with little or no structural modifications.

Fog systems tend to be less expensive than a re-circulating pad cooling system. Many growers feel that fog systems may not work well in high-humidity conditions, believing that only pad cooling systems can cool in those humid conditions. However, a well-designed and engineered fog system will work equally well or better. Fog systems generating drops in the 10- to 20-micron range provide better cooling in high humidity conditions by 3-5 degrees over pad systems. This is primarily due to the evaporation efficiency and the ability of the micron size drops to trap and evaporate a greater amount of latent heat in the air. An important rule to remember is for every 20¼ rise in temperature, the water holding capacity of the air doubles.

Modern high-pressure fog systems have emerged as the chosen system for enhancing the environment in greenhouses. Advances in environmental computers and the way they control the fog system have drastically improved the function and operation of the system. High-pressure fog systems can quickly change the environmental conditions, thus demanding very fast and responsive sensors and programs that act and control by psychrometrics and VPD. The efficiency of the modern fog system coupled with advancements in control logic and incorporation of HAF and vertical flow fans has led some growers to produce in closed greenhouse environments.

In summary, if your greenhouse requires cooling, you have a variety of options from which to choose. It is important to seek out qualified companies with qualified engineers who can direct and assist you in the decision process of all cooling methods, from natural or mechanical ventilation to pad or fog cooling. Logging on to Web sites like www.weatherunderground.com will allow you to see real-time weather conditions for the climatic region where you live.