Heating, Cooling, and Ventilation Systems

         In greenhouses, maintaining a constant and uniform temperature is essential because it has a direct impact on the feasibility of growing crops.

         Common greenhouse diseases are conveyed through condensation, and to avoid condensation, the grower can keep the temperature of leaf surfaces above the dew point so that it prevents condensation. The microclimate of the crops influences disease and pest management.Depending on the region and the seasons, a greenhouse might need to be heated, cooled or ventilated. That's what we are going to discuss in this chapter.

         Pressures for energy conservation, pollution prevention, and economically efficient production directly impact the design and operation of heating and cooling systems. To minimize the cost and negative environmental effects associated with greenhouse heating and cooling systems, a number of factors must be taken into consideration.

I. HEATING

               Cool crops such as carnation (Dianthus spp.) and snapdragon (Antirrhinum majus) were among the first to be grown in greenhouse during the winter, but the diversity of crops including tropical species (foliage plants and orchids) expanded because of the decrease in fuel cost and of a better efficient heating system available to greenhouse managers.

                Heating management has followed the efficiency and economics of heating systems since the costs involved in the purchase and the operation of heating equipment are the main components that one's should be paying attention to.

                When heating in greenhouse is considerated, the first step is to have some notion about

                      A- the heating requirement of the greenhouse,

                      B- how heat is distributed in the greenhouse, and

                      C- how to maintain this structure to keep a uniform and constant temperature.                                                      

I. A. Heating requirement of a greenhouse.

In order to install the adequate heating system, it is important to understand how the heat is transmitted through the greenhouse and to estimate the heating requirement of the structure.

         I.A. 1- Solar radiation

         Usually a limiting factor in production espeically in the winter, SOLAR RADIATION provides light and heat source. A greenhouse should be build so that it will provide maximum use of available sunlight.

         Moreover, the amount of sunlight available to plants vary with the structural frame, the covering material, the surrounding topography, and the orientation of the greenhouse. The latitude, time of year and day, as well as the sky cover will also modify the sunlight availbility. Greenhouse covers, because of their transmissivity for solar energy, will impact internal temperatures. Greenhouses have the ability to possess high absorptivities for solar energy and to convert incoming radiation into thermal energy.

The SOLAR GAIN will influence the heating and cooling requirements. For instance on sunny days, solar gain might replace some or all furnace heat required to maintain temperature constant and uniform.

Hs is the solar gain(Btu/hr)
T represents the tranmittance of the greenhouse cover to solar radiation
Is represents the intensity of solar radiation on a horizontal surface outside (Btu/hr.-ft 2)
Af represents the area pf greenhouse floor (ft2)

          I.A. 2- Modes of transfer of heat

The furnace heat is transfered according two modes:
1- CONDUCTION HEAT

2- AIR EXCHANGE HEAT

Hc is the conduction heat (Btu/hr)
U represents the transmission coefficient (Btu/hr. ° F-ft 2). R is depending on the materials and temperature.
A represents the surface area of the greenhouse
T1 represents the desired night temperature

T0 is the design temperature for the location (the average coldest temperature of a particular geographic region)

Hsa is the air exchange heat (Btu/hr)
M is the air exchange (ft3/hr)
T1 represents the desired night temperature

T0 is the design temperature for the location (the average coldest temperature of a particular geographic region)

John Kumpf says on heat loss: "In greenhouse, like in residential properties, you want to minimize the waste of energy. To do so, a good way to analyze efficiency of your heating system is to calculate heat loss. If you are buiding a new greenhouse, you need to know how much heat to put into to maintain the temperature needed for the crop you are going to grow. If you can not maintain temperature because of heat loss, you will lose money. You also need to make sure that when you design a greenhouse you talk with a greenhouse engineer about day and night temperature. Even when talking to greenhouse engineers you emphasize the need to design a system that would allow you to have uniform temperature, you still may end up with some heat loss that you have to overcome. It is important to get the right design and heating system from the beginning because fixing it later will be expensive and inefficient."

To have a better idea of the notion of heat losses, we suggest to go through the calculations to estimate heat loss in a greenhouse

I. B. Production and Distribution of Heat.

                  The most common forms of energy used for greenhouse heating are coal, oil, and gas. Many considerations including availability, cost, price volatility, pollution regulations, storage requirements, equipment requirements, boiler requirements, and maintenance requirements are taken into consideration.

A heating system is usually composed of 1- heat source, 2- heat exchanger, 3- distribution devices (circulating pump), and 4- control devices (thermostat).

The selection of heating equipment depends on the size and the type of greenhouse operation, structures, and availability and cost of fuel system components.

I.B. 1.Heat Source

                   Depending on the topographic location of the greenhouse, the requierement for heat source varies. For instance in Florida or in Texas, because of the high external temperature, boilers and traditional hot water/steam heating systems are not used extensively there is a variation of these systems that is becoming increasingly popular.

Heat sources can be on one hand centralized, (one or more boilers or hot water heaters located in a single position --service building usually) and then the steam or hot water is piped throughout the greenhouse range. On the other hand, if heat sources can be localized, many heaters, usually forced hot air, are located in the area that it heats.

1- Central Heating: The principle is that the heat produced by the combustion of fuel (wood chips, coal, natural gas, liquide propane, or heavier oil) in a boiler producing hot water or steam.

Advantages and disadvantages of hot water:

     * In case of boiler failure, the hot water in the pipes could act as a heat reservoir for a short period of time

     * Usually large volumes of water are required.

     * It involves complicated plumbing and circulating pumps

Advantages and disadvantage of steam:

     * Less plumbing than with hot water

     * no circulating pumps .

     * More Btu can be provided by steam than hot water

     * steam provides more rapid temperature adjustments but do not provide heat reservoirs .

Pressurized hot water is used in Europe and in some places in US because it allows the water to be delivered at a higher temperature. The advantage of creating a high temperature is that the volume of water required is reduced decreasing the boiler size and plumbing required.

To measure the efficiency of a boiler one should observe the temperature of the flue gas as it leaves the boiler keeping in mind that the goal is to transfer as much heat as possible from the gas (from the heat source) to the steam or the water. It has been shown that the best efficiency of a boiler is reached when the stack temperature os the flue gas is about 150F (65C) above the temperature of the water in the boiler. Burners should be adequately adjusted so that the level of oxygen is between 1-2% and levels of carbon dioxide, and carbon monoxide are maintained constant depending on the type of burners (Langhans, 1982).

John Kumpf says on Boiler and steam versus hot water: "Always a back-up boiler. For instance, at Cornell University, the central heating plant has six boilers using coal, gas, and oil which are capable of producing a 1/2 million pounds of steam an hour. Generally hot water will lead to a more uniform greenhouse temperature, but steam is a more efficient means to move large quantities of Btus. Therefore, most large scale greenhouses use steam systems"

2- Localized Heating

1- Unit Heater (forced air heaters) is composed of three components:

•  the fuel box where the fuel ( kerosene, liquid propane, or natural gas) is burned producing and hot exhaust

•  The hot exhaust passes through thin-walled metal called heat exchangers . The heat is transferred to the metal and the exhaust is removed from the greenhouse through an exhaust stack .

•  Placed behind the unit, the fan is used to send the greenhouse air to the hot metal tubes whereas the heat is transferred from the metal tubes to the greenhouse air.

The hot greenhouse air can either be released directly into the greenhouse or it can also be forced through a polyethylene tube, also called jet tube, stretched for the length of the greenhouse.

Two main problems can occur with that kind of heating systems:

•  If the greenhouse is too tight, then the flame can go out. But since the unit heater requires oxygen to burn efficiently, this situation will lead to a loss of heating and to an increase in carbon monoxide in the greenhouse, which can cause sickness and eventually death of people working in the greenhouse

•  Also, a unit heater that is malfunctioning may release ethylene, which is hurtful to plants.

 

The disadvantage of unit heaters is that the distribution and uniformity of heat in the greenhouse are not as good as conventional radiant pipe systems because those units produce horizontal discharge of heat. The hat distribution can however be improved when convection tubes are associated with unit heaters because the heat discharge becomes more uniform within the greenhouse.

2- Radiant Heater

Radiant heaters possess two components: an aluminum tube and a reflector. The fuel is combusted in the aluminum tube until reaching a temperature around 900° F At this temperature, the tube emits infrared radiation, an electromagnetic energy which is converted to heat when radiations strike any objects (plants, benches, soil, etc.). These surfaces deliver then some heat to the greenhouse atmosphere. Less energy is wasted heating the entire air volume of the greenhouse reducing heating costs by 30 - 50%. However, the initial cost can be expensive, and radiant heating units must be placed in such a way that cold spots do not occur in the greenhouse.

This type of heating system is best appropriate for crops with uniform canopy such as bedding plants

Radiant heaters are usually designed to burn natural gas (propane or butane) or manufactured fuels such as oil #2.

Those heatesr are advantageous because they do not require storage tanks, because they burn fuel clean, and the are not demanding in term of labor for maintenance.

 

3- Solar Heating

Sometimes the requirement in terms of heat in the greenhouse can be met by the incoming solar radiation during the day. However, this type of heating is barely used in commercial greenhouses because it is an expensive method which requires a high degree of control, and reliability is not constant. Solar heating can be efficiently used in some hobby greenhouses.

3- Distribution of Heat:

2. Pipe : a network of pipes distribute hot water or steam. Pipes may be made of cast iron, aluminum, or copper.
Using steam offers two advantages over hot water including 1- the use of smaller diameter pumps because there is less resistance in moving steam, and 2- the number of pipes needed is reduced because steam provides more Btu than hot water per surface of pipe.

The arrangement of pipes is very important to minimize heat loss and maximize heating efficiency.
The easiest and most convenient place to put the pipe is on the perimeter of the greenhouse, which gives the most uniform temperature. Up to 20% energy saving can be realized when the pipes, placed nearer or below the ground, heat the plants directly and lead to less radiation through the transparent greenhouse cover . When the pipes are placed closed to the ground, air movement is stimulated through the canopy, warm air rising through the canopy removes moisture and creates uniform microclimate sround the plants.

3. Fin Radiation : Fin radiation is a more efficient system at heat transfer than smooth pipes their surface area is increased. Fin radiations release more heat in a small area.

 

4. Bench heat: in new greenhouses, pipes can be placed directly under or on the benches to conduct heat to the root zone of the plants and improve air movement through the canopy. This system is similar to floor systems, except pipes that are elevated need to be supported. This system is expensive and the response to modification in temperature is not spontaneous

5- Electric resistance heaters: [the drawing doesn't look like an electrical resistance heater to me]mats or grids can ne used for bottom heat with pots or flats directly on them. These systems are suitable for small greenhouse operation because they are expensive to run and to buy. Bigger greenhouse would use a much efficient system. The heat source is an electric heater controlled by a thermostat placed under the bench. The bottom of the bench is enclosed with black plastic, which contained the heating consequently heating the bottom of the bench. The heat moves through the bottom of the bench warming the bottom of the pot so better root development is noticed.

6- When unit heaters are used to produce heat in the greenhouse, a temperature gradient can be created. To avoid this phenomenon, it is better to place unit heaters at opposite ends.
For long greenhouses, unit heaters can be assisted with Horizontal Air Flow Fans (HAF) which distribute not only the warm air from the heater, but also the incoming cold air during winter cooling, as well as the interior greenhouse air when no heating or no cooling are on.
Sometimes, the unit heater can be mounted in the gable of the greenhouse and linked to a polyethylene jet tube. When filled with warm air, this tube will release warm air through hole along its side. Jet tube can also be associated with HAF.
The HAF and the jet tube can be used sometimes to cool the greenhouse when the boiler is turned off.

I.C. Heat Conservation

As mentioned earlier, the heat loss from a greenhouse is five to ten times greater than of residential houses. Therefore, greenhouse workers determined means to insulate greenhouse at night to reduce heat losses. These includes the use of night curtain, extra plastic covering, north wall insulation (Langhans, 1982).

The methods of heat conservation include:

1- Greenhouse design: by minimizing the exposed surface area (using gutter-connected designs), heat loss can be reduced.

2- Glazing methods: when it comes the time to choose a glazing material, then it is recommanded to use a glazing with low thermal conductance values so that heat loss can be reduced.

3- Wall Insulation:

        End and sidewall insulation: Air infiltration at the end and side walls can be reduced using a tight film plastic cover, a microfoam layer, a double layer of film plastic, or air cap which is a film plastic material with wetted and applied to the wall air bubbles. Aircap is claimed to reduce heat loss by about one half.

        North wall insulation: Research in Canada and North of the US has shown that replacing translucent north walls with an insulated wall is highly beneficial. A well insulated stud will greatly reduce heat loss (over 90%). However, this will lead to a reduction in heat gain in summer. Two ways can be followed to reach this obkective: 1- either the reconstruction of a new wall, 2- either cover the galss with aluminium foil.

4- Thermal screens: screens made of polyester, cloth, or polyethylene can be pulled at night to reduce heat loss through the roof panels of the greenhouse

5- Windbreaks: Wind can be the cause of heat loss, therefore the use of windbreaks (high walls or trees)can reduce the effect of wind on heat loss. However, the use of windbreaks has to be realized judiciously because it might also affect negatively the quantity of light entering the greenhouse.

6- Close air leaks: Openings in the greenhouse structure such as broken panels, loose panels, poorly sealed doors, have for consequence to increase the mass air flow (infiltration and exfiltration) leading to an increase heat loss.

I.D. Maintenance

Proper maintenance of the entire system is critical because it will maximize efficiency of the heating system and it will protect the heating system against a malfunction that can result in the release of ethylene and/or carbon monoxide into the greenhouse.

Proper maintenance include appropriate cleaning, checks of the air intake, checks of the exhaust system, checks of the fuel line, checks of fans, checks of the burner system and the heat exchanger, calibration of the thermostat, and any other maintenance items prescribed by the manufacturer.

1- Maintenance of the central heaters:
Boilers and heaters should be checked every year if they are cleaned and adjusted by a boiler specialist.
It would pay for every greenhouse grower to buy a furnace efficiency test kit. Thus when the problem is noticed a boiler expert could be called right away.

2- Maintenance of the heat distribution system
The heat distribution system should also be checked and cleaned at least yearly.
Sometimes, for aesthetic reasons especially aluminium paint is used on pipe. With time, aluminum paint can also cause a problem when on the pipe, leading to a loss of heating efficiency of 15-20 %. Therefore I would recommand to use any kind of latex or good oil paint over the existing aluminium paint because it's the surface that matters but not what is underneath.

3- Thermostat

The critical center of the temperature control system is the thermostat or sensor which should be in the plant growing area so that it can sense the same temperature as the plants. This device should also be in an aspirated box to protect it from being warmed by the sun. The back should be white. Plans for an aspirated thermostat/sensor box can be found in the Cornell Recommends available from your NY Cooperative Extension agent. An aspirated box means a fan draws greenhouse air past the sensor to measure the air temperature acurately. Remember the fan should not be blowing the air past but drawing it past the thermostat because the heat from the fan motor would be blowing past the thermostat and produce an error in the reading.

John Kumpf says on thermostat: "Don't try to save money on a thermostat. A heat thermostat or sensor which is off by a couple of degrees, wastes energy, and will affect plant growth."

However, thermostats are notoriously inaccurate. They need frequent calibration (once a year) .

A good way to calibrate a thermostat is to put the thermometer next to the thermostat and see if they agree on the air temperature. If not, adjust or replace the thermostat. Don't assume that a thermometer is correct however. It can be checked by putting it into a dish of cracked ice and water and it reads 32F. Several thermometers can be put together to see if they agree.

4- Renovations to reduce heat loss

Now let's look at ways to reduce the loss of heat through the greenhouse cover. We'll assume that you've done everything reasonable to fix all the cracks and leaks and are now ready for some renovation.

A- Installation of insulation

- Foam
Renovation may include the use of insulation system such as foam installed on the outside of the wall so that the wall does not freeze. It's recommended that when the wall is to be insulated, the insulation go down into the ground at least 24 inches. One or two inches of foam insulation should be used and should be of the closed cell type to avoid absorbing water and ruining the insulation's value. Foam must be protected from the affects of sunlight. Asbestos board, stucco, and paint will work. However, if you do use paint, test it first to be sure that the paint does not dissolve the foam.

- Polyethylene layers
If the structure is ready to be renovated, it is recommanded to cover glass houses with two layers of polyethylene and inflate the two layers to create an air space similar to the air space in a double poly house. The installation of poly over glass was shown to save more than 50% of the heating. However, this can lead to a reduction of in solar ardiation acquisition and its transmitivity.

John Kumpf says on the use of polyethylene insulation: "If you choose to use double poly over glass, I suggest that you take it off in the spring and use new poly in the fall. There is still a lot of controversy about double poly over glass but there are still some strong advocates of using it to save energy. But if you are considering using it be very careful and do a good analysis of the benefits and the cost."

An alternative to using double layer poly over glass is to use a single layer. The only problem with a single layer is how to hold it in place when the wind blows and how to create an air space between it and the glass. One way to install a single layer is to use a length of unpreforated ventilation tube is run the length of each side of the roof and halfway up from the eaves to the ridge. The poly is stretched over the roof and the tube underneath is inflated with air. The inflated tube acts like a spring to hold the Poly off the glass. And it also gives as the wind blows and as the Poly stretches and shrinks with temperature changes. For more details you can contact your local cooperative extension agent. The single layer of Poly saves as much energy as does the double Polly over glass.

- Thermal curtains
Thermal curtains can be installed very easily when heat loss is observed in the greenhouse.

John Kumpf says on the use of thermal curtains: "Ease of handling is important: you should be able to open and close the curtain with minimal effort and time. Ideally the curtain will be mechanized and operated automatically with a photocell controller. Durability and greenhouse humidity are important considerations. On our own curtains here at Cornell, we found it better for the curtain to tear first if it got hung up rather then having the entire pulling mechanism destroy itself. Longevity is critical: thermal curtains are expensive and they would need to last more than a year or two to be profitable. Flammability is also important because it will affect your insurance premiums."

Many thermal curtain materials are opaque and can be used for photoperiod control. Other materials are translucent and can be used for summer shading or heat retention.

Traditional thermal curtains have been designed as accordion fold systems. They use a single layer of material stretched from eave to eave

II. Cooling equipment

A 30 degree Farenheit difference between the temperature outside and inside the greenhouse is generally noted all year long. Therefore, there is a need for summer and winter cooling. Cooling systems include vents, fans, shading, evaporative pad systems, fogging systems.

II. A. Summer cooling

1- Shading system
The shade, especially during the summer, can reduce the irradiance, leading to a decrease in heat load. However, because light is a limiting factor for plant growth, when you use shading to decrease temperature in summer time, you have to consider the amount of shading desirable. When the weather is cloudy and rainy for few days, shading becomes a problem. However, shading is still a common means to control temperature during the summer season. Shading methods include paint, lath, cloth.
Shading compounds should be choosen so that it sticks well to the glass, it should be easy to apply, should not come off with rain, and should be easy to remove in the fall when temperature becomes cooler and light intensities are decreasing.
Lath made of wood or aluminium can also be used as shade. They are used when laid on the greenhouse structure or rolled up and down the roof. This method can be adjusted. It is the more expensive method of shading.
Cloth made of different material cheese or polypropylene cloth are set up in the greenhouse and are semipermanent installation that are difficult to adjust for varying weather conditions. Saran cloth is the most common and is sold in various densities so that several levels of shade are available.

2- Fan and pad systems
Fans and pad evaporative cooling systems are working in association to keep the greenhouse temperature constant. This system takes advantage of the latent heat of evaporation. When liquid water evaporates, it absorbs energy from the surrounding air, decreasing the temperature of the surrounding air.
Fan and pad systems are composed of exhaust fans at one end of the greenhouse and a water circulating pump that allows water contained in a tank (reservoir) to circulate through and over a porous pad installed at the opposite end of the greenhouse. This will create a vacuum inside the greenhouse causing an entrance of air in the greenhouse through the pads at the opposite end of the greenhouse.
The water is first supplied to a feed line running the length of the pads. Holes in the top of the feed line allow The water can be forced out of the line because the feed line is riddled with holes. The water is then forced upward striking a cover plate which make the water trickle down to the pads and then the water is collected in a catch basin and recycled back to the reservoir. Because water also evaporates when it passes through the pads (about 1 gallon per minute can be lost through 100 ft2 of pad on a hot dry day), water must be continuously resupplied to the reservoir which should be controlled by a floater. The reservoir should have a capacity large enough to hold enough water to fill all pipes and saturate the pads. The water supply system should operate so that the entire pad is kept wet. For better wetting of the pad, a cover can be placed on the top of the pad

Some problems are associated with cooling evaporative pads if they are not properly maintained such as clogging, algae, and decay. When the pad is clogged very little or no air passes through that portion. If you observe decay, the only alternative would be to install a new pad.
Salt accumulation can also be a problem because it will physically block air movement through the pads and alter uniform wetting. Blended water can be then used in case of high concentration of salts in water.
Certain biocide such as calcium hypochlroride or sodium hypochloride can be added to the water (at a rate less than 1%) to prevent algae accumulation. Bleach will have the inconvenient to increase the pH of the water which may also damage the pad. Hence, biocides are preferred.

II. B. Winter cooling

1- passive venting
If the temperature outside the greenhouse is lower than the temperature inside the greenhouse, one thechnique would be to let the cool air to come in because the warm air will be passively exhausted through roof vents.

( is it more summer or winter cooling?) Evaporative pads are usually corrugated cellulose that has been impregnated with wetting agents and insoluble salts so that they resist rot. Eventhough they are expensive to purchase, they are very efficient in cooling air and if they are properly maintained, they will be utilisable for about 10 years. Other kind of pads are available such as aspen pads, which are very susceptible to algae infestation. You might have to add fungicide to the water to reduce pest infestation. Pads made of aluminium and plastic fibers are not as common because they are also expensive and did not show better efficiency that those made of corrugated cellulose.
When it comes to choose which pad will be more suitable for a greenhouse, it is essential to compare costs, life expectancy claims, cooling efficiencies, eventual maintenance problems to expect.
Usually, pads are continuous along the entire wall or mounted on "artic" cooler.

Ventilators are usually continuous along both sides of the ridge and on the side below the eaves. Two mechanisms can be used to open the vents including a rack and pinion mechanism and an arm and rod mechanism. The rack and pinion mechanism can handle more vent.

Three kinds of ventilation can be used:

1- Natural of ventilation: most of the greenhouses are constructed with top and side ventilators

2- Tube ventilation: simple and inexpensive, it allows the outside and the inside air to be exchanged and mixed quickly and uniformly

3- Fan-jet ventilation: allow for uniform ingress of cool air and maintian a steady strirring of the air reducing stratification.

Evaporative cooling reduces the temperature of air by the evaporation of water into the airstream. As water is evaporated, energy is lost from the air reducing its temperature. Fan and pad cooling systems are then used to evaporate water, move cooled air through the greenhouse, and exhaust warm air from the greenhouse.Lately, high pressure fog systems have been used in greenhouses. They maintain more uniform temperatures and humidities in greenhouses than fan and pad systems. Fog systems are more expensive than fan and pad systems but they are more efficient and reliable when uniform temperatures and high humidity levels are required.

 

Mist systems can also be used: they provide short bursts of mist to both cool up to 20 degrees and humidify. It is a great supplement to ventilation, which merely cools the outside temperature.

John Kumpf says on the cooling systems: "Make sure all vents close properly and are well-sealed when they are closed. Be sure the vent arms are not bent and that they toggle freely in the guides. Roof vents are harder to check but it is very important to check them carefully every year. To save energy some or all of the roof vents may be sealed with plastic during the winter to stop air leaks through them. The warm air in the greenhouse will rise by the action of bouancy and it will escape around all the small cracks around the roof vents."

II. C. Maintenance

Fans can be weatherized in two ways. Polystyrene b-board fitted inside to insulate part of the fan's cabinet. The b-board should be sealed well to the fan cabinet to limit condensation between the two.
Condensation will damage the fan and it is also an indication that the insulation is not doing the job. If the fan will be need during the winter, a better way to insulate it would be on the outside with the insulation strapped on tightly and sealed to the cabinet. If the fan will not be needed for the winter it should be sealed off completely. A sheet of plastic taped tightly to the cabinet will do a good job of sealing.

II. D. Calculating greenhouse cooling requierements

I would like to add an example of how to calculte greenhouse colling requirement.

 

Green House Management Growing Media Benches Watering Crop Nutrition Heating and Ventilation Lighting Systems Pest Control Learning Objectives Laboratory Exercises Glossary Commercial Applications Literature Cited Site Map Links

Green House Management Growing Media Benches Watering Crop Nutrition Heating and Ventilation Lighting Systems Pest Control Learning Objectives Laboratory Exercises Glossary Commercial Applications Literature Cited Site Map Links