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9.1 E The greenhouse climate control (edited)
Greenhouse Technology (GHT100S)
Cape Peninsula University of Technology
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The greenhouse climate control
a) Fundamental principles
The greenhouse presents a real challenge to the thermodynamic application of climate control systems. It is one of the most difficult buildings to heat in winter and cool in summer and it is subjected to wide extremes in temperature, yet must maintain a good growing climate for plants.
Some knowledge of the principles of thermodynamics is necessary before one can design a satisfactory system of climate control and establish the proper operating procedures.
There is almost always a transfer of heat into or out of a greenhouse and the prevailing house temperature is the result of a balance between these two. To maintain a desired uniform and steady house temperature, we must regulate the heat supplied or removed to maintain a heat balance.
Solar heat input In the outer space the sun’s solar radiation intensity on a surface perpendicular to the sun’s rays is about 445 BTU’s per square foot per hour. By the time the rays penetrate the earth’s atmosphere and reach ground level in a clear air region, they are reduced to about 277 BTU’s in the summer time. In coastal or Industrial areas where smoke and more water vapour exist in the air, the solar intensity is reduced to about 200 BTU’s.
This reduction in solar intensity affects the sun’s light as well as its heat and explains why growers in industrial or coastal areas often need little or no shading. Up to 85 percent of the solar heat in the form of electromagnetic and light rays may enter the greenhouse, but due to absorption and radiation at a different wave length, most of this heat becomes trapped inside and greatly increases the greenhouse temperature. Since temperature control is a problem in a greenhouse the climate control system requires a design that can remove this heat, to maintain a heat balance during hot weather.
Heating system input The greenhouse heating system must have a larger capacity than needed for any other comparable sized structure and must be engineered for uniform distribution, close temperature control and quick response to changes in heating requirements.
Since the plants in the greenhouse are largely heated by the air, the heating system whether steam, hot water or gas fired types, is mainly used to provide a uniform temperature of the air within the greenhouse.
Supply heat at source of heat losses It is important to supply the heat within the house in a manner to best offset the heat losses form the house to avoid having areas of reduced temperatures.
In most localities, this requires a perimeter heat system for the wall losses and another heating system for roof losses. The fact that warm air rises and cold air falls makes a perimeter system particularly important in cold weather climate, to prevent cold floors since cold air immediately adjacent to the walls flows downward to the floor.
(i) Psychrometric science
Humidity control requires a moisture balance. A greenhouse must maintain a proper relative humidity for optimum plant growth and health conditions. This is achieved by means of a moisture balance within the house. All the water piped into the house must eventually leave. Since much of it becomes water vapour by the transpiration of moisture into the air by the plants and evaporation from the soil, most of it must be removed by ventilation. This is readily accomplished during cold weather since outside air at that time contains less moisture than the air within the house. By bringing in this drier outside air, mixing it with the humid house air and then expelling the mixture, the water vapour is removed from the house and the relative humidity is reduced.
Many older greenhouses with more leakage did much of this automatically by infiltration losses, but newer, tighter houses require sufficient ventilation to properly control the moisture balance.
On houses constructed of plastic, the moisture removal problem is further increased because the houses are not only tighter but most plastic does not readily wet so the condensed moisture drips back into the house instead of being drained away. Air is a mixture of water vapour, nitrogen, oxygen, carbon dioxide and traces of other gases. Although its water vapour content is often less than 1 percent of the total, it is a major factor in determining the condition of the air mixture. This is due not only to the necessity of water in the lifecycle but also to its great energy content when in vapour form. The latent heat in water vapour (the energy in the form of heat required to change water from liquid to vapour) is the largest of any common liquid. As a result the small amount of water vapour in the air mixture often contains the major part of the total heat energy of the mixture.
Psychrometric chart A psychrometric chart is a graphical representation of the thermal properties of moist air and is used by engineers to analyze and solve thermodynamic problems of air. It is important that growers understand some of its concepts to enable them to do a better job of controlling the climate in their greenhouses. The following paragraphs briefly explain a psychrometric chart and show how this information can be useful to growers.
(i) Dry bulb temperature
Look at the psychrometric chart and observe the vertical lines on the chart having numbers along the bottom. These are the “dry bulb” temperatures
Follow the 60 ̊F dry bulb line down to the 50 percent RH line. The horizontal water vapour content line shows that air at this latter point contains only half as much or approximately 38 grains of water vapour. The air is only 50 percent full and thus has a RH of 50 percent. RH therefore, is a fraction in terms of percent that states the amount of water vapour in air compared to the amount it actually could hold at that temperature.
(iv) Effect of temperature on RH
The temperature of air greatly affects its capacity to hold water vapour. In the previous example, a pound of air at 60 ̊F dry bulb and 100 percent RH contains 77 grains of water vapour. If this air is warmed up 20 ̊ to a temperature of 80 ̊F the RH is now about 50 percent. Air at 80 ̊F could hold 156 grains of water vapour when saturated, but it actually contains 77 grains, therefore is only half full. If this air is warmed another 20 ̊ up to 100 ̊F, its RH will drop to about 25 percent. These examples show that for every 20 ̊F rise in dry bulb temperature, within ranges encountered in greenhouses, the capacity of air to hold water vapour approximately doubles and conversely its RH is reduced by one half. This also demonstrates a typical daily fluctuation in the outdoor relative humidity. Assume a night time air temperature of 65 ̊F with a 90 percent RH. As the sun warms up the air to 85 ̊F the following day, its RH will drop to 45 percent and as night again approaches the cycle is reversed to produce a high RH even though the actual water content in the air may have remained unchanged at 84 grains per pound of air. A further study of the affect of temperature on air shows that as air becomes colder its capacity to hold water vapour is reduced. Assume a greenhouse condition of 65 ̊F dry bulb temperature and 75 percent RH. If due to poor heat distribution, random air currents or wind affects, a cold spot developed in a certain area that dropped the temperature only 9 ̊F down to 56 ̊F dry bulb the relative humidity would reach 100 percent at this spot. This would immediately become a potential disease propagation area. This therefore shows the need to avoid wide variations in the temperature within a house because of its direct affect on the relative humidity. It was shown in earlier examples that a pound of air at 100 percent RH and 60 ̊F contained 77 grains of water vapour. If it is cooled down to 40 ̊F it can now contain only 38 grains. The other 39 grains of water vapour it previously contained, had to condense out as liquid drops of water.
(v) Reducing the RH in a greenhouse
If cold air, having a low water vapour content is brought into a greenhouse and warmed up, its capacity to hold water vapour increases and therefore has a great drying ability. This is demonstrated in the following example.
Assume a rainy weather condition of 40 ̊F, obviously at about 100 percent RH and a greenhouse having an inside temperature of 65 ̊F at 90 percent RH. If the inside condition is considered too humid for plant health, it is readily possible, even in this situation to lower the relative humidity in the house to a satisfactory level. The psychrometric chart shows that each pound of air removed with an exhaust fan, 83 grains are carried out while only 37 grains enter the house with the make-up air. This results in a net removal of 46 grains of water vapour per pound of air exchanged. An exhaust fan operating only a few minutes to remove one half of the humid greenhouse air would lower the inside RH to about 65 percent. It would be lowered even more if this ventilating process was continued for a longer period. Obviously, the colder incoming air would have to be warmed up 25 ̊F by the heating system to maintain the desired house temperature of 65 ̊F. Using normal fuel costs, 20 000 cubic feet of the 40 ̊F outside air could be heated to 65 ̊F for approximately R0,05. By understanding and properly applying these principles of climate control, an excessively humid greenhouse condition can be lowered to a more desirable level for only a fe cents cost, even under adverse weather conditions.
(vi) Sensible heat
Psychrometric chart also indicates the energy content of the air-wet vapour mixture. The energy or heat content of dry air is completely determined by, and is proportional to, its dry bulb temperature. It is called “sensible heat” because on can feel it.
(vii) Latent heat
The energy content of water vapour is due to its latent heat of vaporization and it is proportional to the amount of water vapour in the moist air mixture. Latent heat is the energy, in the form of heat, that is required to convert water from liquid to vapour without any change in temperature. It requires approximately 1060 BTU’s of heat to convert one pound of water from a liquid to a vapour. This energy cannot be felt because there is no change in temperature, only a change in its form, therefore it is called “latent heat”.
(viii) Total heat
The sum of these two energies is called “total heat” and is proportional to the wet bulb temperature of air. The wet bulb temperatures are designated by the diagonal lines that slope from the upper left to the lower right of the chart. The wet bulb temperature of air is measured with a thermometer having a wet wick on its bulb and a stream of air blowing over it. At 100 percent RH the dry and wet bulb temperatures of air are the same, but at any other condition less
During the heat of the day when both the dry and wet bulb temperatures are at their peak, the difference between them is greatest. Consequently the greatest amount of potential for cooling is obtained during the heat of the day when it is needed most.
(ii) Movement of air – Aerodynamics
A brief study of some of the principles of aerodynamics will aid in the understanding of proper greenhouse climate control. There are several basic types of air flow that are important in a greenhouse climate control system and these are explained in the following paragraphs.
(a) Jet flow Jet flow is the rapid flow of air in a perpendicular direction from a hole into an open space due to the difference in air pressure. The jet, being a relative high velocity stream of air, is turbulent with the air particles swirling about as they travel in the jet stream. Due to its extremely turbulent flow, the jet immediately begins mixing with the surrounding air and gradually slows down. At a distance of 20 diameters from the opening, the jet stream has slowed down to a centerline velocity of about 20 percent of its original outlet velocity, but it is 90 percent mixed with the inside air. It expands at an included angle of about 22 ̊ due to its mixing action and at 20 diameters from the opening becomes about 8 diameters wide. For example: A 6 mm diameter hole would throw a jet distance of about 120 mm. A jet stream from a 400 mm diameter hole, such as a missing pane of glass, would travel over 8 m before being completely mixed with the surrounding air and would produce a very objectionable draft if cold. It is apparent from this that jets of proper size and velocity that are well distributed will produce good mixing, uniform distribution and thorough circulation within the greenhouse.
(b) Sink flow – air flow to an exhaust fan In contrast to the characteristics of the jet flow from an opening, the flow of air to an opening, such as to an exhaust fan, the sink flow, is of an entirely different nature, although still dependant on a pressure differential. This type
of flow is called a sink flow. The air having no momentum, flows uniformly to the fan from all directions just as water moves from all directions toward the drain hole in a wash basin. At a distance of only one fan diameter from the inlet of the fan, the velocity of the air moving toward it is less than 12 percent of its exit velocity.
(c) Potential flow A potential flow consists of a relatively large quantity of air flowing smoothly in one direction with each particle of air moving in a straight line. This type of flow is used in the greenhouse fan and pad system, when a large mass of cool air is flowing smoothly and at a low velocity through the growing region of the house with a minimum of mixing with other air in the house. To achieve this, the location and type of inlet and outlet openings are very important. In greenhouse cooling the air enters the house at a relatively low velocity through a large pad opening at crop level and leaves through exhaust fans in the opposite wall.
(d) Location of exhaust fans Some exhaust systems are based on the principle that all flow is due to pressure differences and that the pressure is constant within an enclosed space. Exhaust fans create a slight vacuum that is essential uniformly throughout the entire greenhouse, therefore air will enter at the same velocity through any and all openings, regardless of their location.
of high insulation, but also intensity may be reduced unnecessarily on cloudy days. Several researchers and greenhouse operators have tried blinds, movable shading cloth, coloured water and many other non-fix materials. The best system is still the unrolling of wood or plastic shade mats. They are fixed at the top of the greenhouse and can be unrolled downward along guides. They are used on the outside as well as inside. Nowadays they are completely automatically controlled by light and temperature sensors, which control the motor that starts the unrolling process.
Alunet screens in greenhouses
Thermal screen (Writtle College –UK)
9.1 E The greenhouse climate control (edited)
Course: Greenhouse Technology (GHT100S)
University: Cape Peninsula University of Technology
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