Getting Back To Basics With NGMA Part III

January 30, 2001 - 01:00

Heating Systems


How does heat loss occur?


Heat loss occurs from a greenhouse structure whenever the interior temperature exceeds the exterior temperature. The rate at which it occurs is affected primarily by the efficiency of the covering materials (glazing) installed on each surface (roof, side walls and end walls). The most commonly used covering materials all have published heat transfer factors called "U" factors that provide a means of calculating their impact on heat loss in different scenarios.


What are U factors?


U factors are the inverse of the commonly used "R" factors, where U equals 1/ R. The lower the U factor, the less the ability your glazing material has to transfer heat, and the lower the heat loss.


What is a British Thermal Unit?


In North America, heat loss is typically expressed in terms of total British Thermal Units per Hour (BTUH) loss. British Thermal Unit (BTU) equals the amount of energy it takes to warm one pound of water to 1° F.


Are there any special considerations for calculating heat loss?


There are unique crop considerations. Many growers do not use the total volume of their structures to grow crops. Consequently, they may not need to have the total conductive heat load available in their heating system. While the standard means of calculating heat loss are well accepted and documented, considerations should be made as to whether your crop requirements justify installing the total number of BTUH indicated by this calculation.


What is zone heating?


A zone heating system is one that places the BTUs in a specific portion of the structure.


How important is good ventilation?


Ventilation essentially provides the same benefit regardless of the season. Ventilation, in addition to removing excess heat in the summer, replenishes carbon dioxide and assists in the control of humidity levels.


How is air circulation affected by cold weather?


During cold weather, when greenhouses are closed in, there is often insufficient air circulation to maintain desired conditions. The appropriate type of air circulation equipment will help obtain a more uniform relative humidity and provide the proper air movement. Continuous circulation produces gentle air movement, which has been reported to maintain better leaf surface microclimates and prevent pockets of disease-producing high humidity. Gentle air circulation may result in slightly higher heating demand, yet many regard it as advantageous from a plant production and quality standpoint.


When do I need perimeter insulation?


A substantial amount of heat energy can be lost out of the perimeter of a greenhouse through the ground below the perimeter walls and ends. This is conductive heat loss that can be minimized by installation of perimeter insulation below the frost line.


Does a thermal blanket save on heating bills?


Installation of a thermal blanket system can affect the total heating requirement of your greenhouse by reducing the heat loss. These systems are typically designed to automatically retract in the daytime and close in the nighttime to trap energy.


What are the different heating types?


All forms of heating can be explained using three basic principles: convection, conduction or radiation.


Convection heat utilizes the forces of natural air circulation currents to transfer heat. Convection involves two basic principles: First, cold air displaces warm air, and second, warm air rises in the presence of cold air. With convection, heat is transferred by air currents, which transport energy throughout the structure. When these air currents pass by plant material, energy is transferred to the plant. Because of this, it is very important that some means of air circulation is used to assure the maximum amount of warm air is transferred to the plant environment.


Conduction heat utilizes direct application to transfer heat energy to the plant. Physically touching any warm object demonstrates the principle of conductive heating. In greenhouses, this type of heating is most commonly distributed with hot water tubes, and occasionally electric resistance strips, which are placed directly on the growing surface or in the growing media. The soil, containers and growing surface in direct contact with the warm tubes or strips are heated and subsequently transfer that heat energy to adjacent materials.


Radiant heat utilizes electro-magnetic infrared waves to transfer heat. Anyone who has warmed themselves by a hot wood stove or their hands at a campfire has experienced radiant heat. Although the air may not be warm, you are able to feel the heat energy radiating from these surfaces.


What are the main principles of heating a greenhouse?


The conversion of fuel to heat energy is typically accomplished through combustion with a burner installed in a boiler or heater combustion chamber. That heat energy is then distributed through the greenhouse via pipes, ducts, tubes or air. Once the energy is distributed through the house, it must then be transferred to the plants and soil by convection, conduction or radiation. Finally, once transferred to the plants and soil, they must in turn absorb its energy and convert it to usable heat. How each of these functions is accomplished has a significant effect on both the efficiency and effectiveness of the heating system.


What is a unit heater?


The definition of a unit heater is a fan-equipped device with a means to heat air being provided by the fan. Unit heaters are the most commonly used piece of greenhouse heating equipment. They are valuable in that they provide warm air temperatures that are imperative for leaf transpiration and for melting snow load. Unit heaters gently circulate warm air to prevent temperature stratification, reduce mold and limit fungal disease.


Unit heaters are available in oil fired, electric, hot water or steam, and gas fired. The most popular is the gas-fired unit. Unit heaters are typically suspended from greenhouse framing. Floor mounted units are also available.


What are the benefits of a hot water system?


Hot water or "hydronics" systems are available for providing heat to a greenhouse space. Because of their initial cost, many growers feel they cannot justify the investment required to install hot water. However, many advancements in technology over the past decades have made these systems more affordable.


Hot water systems do require specific engineering for each and every application, and installation is complex. However, the energy efficiency advantages they offer and the potential to enhance evenness and growth in a greenhouse facility make them attractive enough to warrant researching whether or not hot water is a logical alternative for you.


How important is insulation?


Insulate! It cannot be stressed enough that a heated concrete slab floor should be insulated around the perimeter and as deep as the frost line in your area. Some reports indicate that up to 50 percent of your heat can be lost out of the perimeter of a heated slab if no below-grade insulation is installed. Some growers have even installed insulation below the slab. The best type of insulation to use is one of the extruded styrene boards, usually 1.5 to 2 inches thick.


How does bench-top heating work?


Bench-top heating systems use conductive heat transfer to deliver heat to plants placed directly on top of a multiplicity of parallel tubes. Small synthetic rubber tubes with high UV, heat and chemical resistance are placed on the growing surface of benches and contain warm water that is circulated to and from a hot water supply, typically a boiler.


How does infrared radiant heating work?


Any object that is warmer than absolute zero radiates energy. Approximately 50 percent of the sun’s energy is infrared radiation or energy in the "far-red" spectrum. This spectrum of energy is not visible so it does not interfere with photosynthesis or photo period-sensitive plant material. Energy, when absorbed by Earth and its objects, is converted to heat.


Should I install more than one heating system?


Many growers are installing multiple heating systems in an attempt to get the best of all worlds. For example, on temperate nights, they can rely on a bench-top system only, and on cold nights, unit heaters are utilized to carry the worst caseload and melt snow. Others are installing infrared systems in conjunction with bench-top or floor heating systems.


Environmental Control Computers


How are environmental control computers important


in raising a crop?


In addition to controlling humidity and temperature, they can manage shade systems and supplemental lighting, control CO2 concentration, schedule and sequence irrigation valves, and control the pH, EC and temperature of the irrigation water. Because they integrate these functions into a single controller, environmental computers can manage the interactions between these elements, which gives the grower a tool to optimize production.


Do they respond to both the greenhouse and outdoor


environments?


Yes, they monitor wind speed and direction to protect vents from damage. Many can use wind speed, outdoor temperature and light level to anticipate heating and cooling demand and to control the firing of boilers. By measuring outdoor humidity, they can compute whether dehumidification of the greenhouse is possible.


How can they minimize energy costs?


Computerized environmental controls help minimize energy costs by closely controlling the environment and by integrating and coordinating the use of all the climate control equipment in the greenhouse.


Can labor be reduced through an environmental control system?


Yes, environmental control computers can create a model of how the environment should be controlled to dramatically reduce the manual intervention needed from the grower to maintain the desired conditions.


Can an environmental control computer change crop quality?


Because they constantly monitor and control temperature, humidity and light intensity, environmental control computers create the most uniform conditions possible. This leads to increased uniformity in the crop.


Does the computer keep records and how can they be used?


Environmental control computers keep records of their sensor readings and equipment usage. This allows the grower to monitor past conditions and events on nights and weekends. This data can be carried over from crop to crop and used as a tool in crop timing and quality control. Equipment usage records can also help in planning maintenance schedules.


Does the computer generate alarms?


An environmental control computer will generate alarms when it detects conditions that threaten the crop. Examples of alarm conditions include high and low temperatures, power failure and any failure of the computer to operate properly. If you have equipment that operates on three-phase power, consider a computer that can detect phase failures and generate an alarm to protect your equipment.


How will the computer respond during a power failure?


Environmental control computers retain all your settings during power failures that last several hours. It is not essential for the computer itself to continue to operate through an outage since the equipment it controls cannot respond. If your standby generator doesn’t have enough capacity to operate all the equipment, consider a computer that can selectively shed load while on standby. When power returns, be sure your computer will gradually turn on the equipment that’s needed —- turning everything on at once can stall a generator or trip the overload protection on your electrical service.


What are the advantages of a centralized single system?


A single central computer consolidates all control and supervisory functions at one location. Information and control functions that are common to all zones in a greenhouse range, like weather data or boiler control, need not be communicated between multiple computers. Because only one computer is needed, a single central unit can be the most economical choice.


However, in a single computer system, if there is a problem with the computer, it affects the whole greenhouse range. Also, wiring of sensors can become complex. Either all the wiring must be brought to the central computer or to concentrators that send and receive signals from the central control. A single central location can be distant from some zones and therefore less convenient. Furthermore, for a large range, a powerful enough computer may actually be more expensive than multiple smaller computers.


What are the advantages and disadvantages of a


multiple computer system?


In case of a problem with one computer, other computers in the greenhouse range are unaffected. Multiple computers can be located near the areas that they control. This can reduce cost and complexity of wiring and improve convenience of use.


However, multiple computers require a communication link in order to share information about weather or equipment common to zones controlled by more than one computer. Because you are buying more than one computer, it may be more expensive than a single, centralized unit. In practice, many greenhouses need only one unit of a multiple computer system, and they do not gain the benefit of the multiple system.


More Information


For more information about greenhouse-related topics, look to any NGMA structural manufacturer member or log onto the NGMA Web site at www.ngma.com.


Look for upcoming installments of this series in future issues of GPN. Articles will address issues such as insect screening and internal and external curtain systems.

About The Author

All information provided by the NGMA, Littleton, CO; Phone(800)792-NGMA; Fax(303)798-1315

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