Chapter 2

Greenhouse Heating

Greenhouse Hot Water Heating Systems

Today, hot water rather than steam is the most popular medium for carrying heat from the boiler (See Figure 2.4) to the greenhouse for several reasons. The uniformity of temperature across the greenhouse and over time is greater with hot water. There is a larger reserve of heat in a hot-water system in the event of boiler failure. The temperature of hot water can be sufficiently low to use it to heat pipes located in the greenhouse floor or in pipes suspended within the foliage canopy of cut flowers or vegetable plants, whereas steam would be too hot.

Advantages/Disadvantages of Heating Plant Boilers

There are a number of advantages to the central heating system: 1) a central plant offers greater flexibility to use alternative energy sources; 2) it uses less greenhouse space since the central plant is located in a separate building; 3) partial load performance might be much more efficient; 4) maintenance and control is easier and cheaper; and 5) since combustion is done outside the greenhouse, improper combustion does not increase the probability of damaging the crop due to toxic flue gases (e.g. ethylene).

Types of Heating Plant Boilers

Four general types of hot-water boilers are used in greenhouses today—cast iron, steel, copper fin tube, and condensing boilers.

Cast Iron Boilers

The first is the traditional cast iron sectional boiler commonly used in commercial space heating applications. These types of boilers don’t use tubes. Instead, they’re built up from cast iron sections that have water and combustion gas passages. The iron castings are bolted together, similar to an old steam radiator. The sections are sealed together by gaskets.

Steel Boilers

The second type is the steel boiler, which is generally divided into two types: fire tube and water tube. Fire tube and water tube boilers are essentially the opposite in design. The fire tube boiler has a series of tubes through which heated air is forced. These tubes are surrounded by water. As the heated air moves through the cold water by way of the tubes, the water is heated. Fire tube boilers are often characterized by their number of passes, referring to the number of times the combustion (or flue) gases flow the length of the pressure vessel as they transfer heat to the water. Each pass sends the flue gases through the tubes in the opposite direction.

Copper-fin Tube Boilers

The third is the newer low-mass boiler, which generally contains copper fin tubes. It is also known as a compact boiler (See Figure 2.5). The compact boiler burns only natural gas or manufactured gas, since soot from oil or coal could plug the narrow spaces between the fins of the heat-exchanging copper tubes. The compact boiler is typically cheaper to purchase and occupies considerably less space. A 3-million-Btu (877,193-W) compact boiler would contain only about 10 gallons of water, compared to several hundred gallons in a high-mass ferrous metal boiler of equal heat output.

Condensing Boilers

The fourth boiler is the condensing boiler. These boilers have an additional heat exchanger to extract more heat from the flue gas before it is released up the chimney. The flue gas temperature is lowered to the point where water condenses out of the gas. More expensive metal alloys are used in the additional heat exchanger. These boilers have thermal efficiencies up to 94 percent.

Location of Boiler

The boiler may be located in the greenhouse or in the service building. In either location, heat that escapes from the boiler jacket, the pipes carrying steam or hot water from the boiler, and the return lines carrying condensate or cool water back to the boiler serve a useful purpose at that location. If a boiler is located in the greenhouse, the high humidity can result in corrosion and premature breakdown of switches, pumps, and motors. Most growers today consider it more desirable to locate the boiler in the service building where the atmosphere is drier.

Distribution of Heat

Heat energy from the boiler can also be delivered to the greenhouse plant growing space by an array of water pipe which carry the warm water to each bay of the greenhouse, and distribute the heat through a series of pipe loops within the bay. The pipes are typically located overhead and above the crop, or alternatively may be located beneath the benches and on the perimeter walls, or in a combination of each. A set of unit heaters can be used in the place of the overhead pipe.

Types of Pipes

Two types of pipe are used, bare or finned. Perhaps the simplest of means of transferring heat from hot water is through bare pipes of steel, black iron, copper, or aluminum. Finned pipes have closely spaced plates or fins attached to the pipe (See Figure 2.6).

Perimeter Heating

Perimeter piping systems can be used in cold weather climates to protect plants growing near side and end walls to provide supplemental heat and contribute to a uniform thermal environment throughout the greenhouse (See Figure 2.7). Perimeter heating can be integrated into existing zone controls or can be controlled as a stand-alone system if extra perimeter heat is desired at night when a heat retention curtain is in use. The pipes should have a few inches of clearance on all sides to permit the establishment of air currents and should be located low enough to prevent blockage of light entering through the side walls.

Floor Heating

Many greenhouses today are constructed with a heated floor. As mentioned, different heating systems are available to heat greenhouses: from hot air, to hot-water, and radiant systems. The challenge of all these systems is to provide heat in the right amount at the right location, and as uniformly as possible at a reasonable cost. Generally, hot-water soil/floor heating systems are capable of providing the most uniform heat throughout the crop canopy, especially when floor or bench top heating is employed. Floor heating is ideal for crops grown directly on the floor such as bedding plants, containerized ornamentals, and bag-cultured vegetables, as well as greenhouse vegetables grown directly in the soil. With a cool-season crop (lettuce, spinach, Asian leaf vegetables), supplemental air heating may not even be required in a floor-heated greenhouse. Some of the benefits of soil/floor heating in protected cultivation environments are as follows:

Overhead Heating

Overhead heating is important in areas that get seasons with cold nighttime temperatures (See Figure 2.9). It provides the supplementary heating required to combat these cooler temperatures.

Under-bench Heating

Many greenhouse growers use bottom heat under their raised benches (See Figure 2.10). These systems offer growers the ability to control the soil temperature of their plants and they also contribute a portion of the heating energy required to meet the total conduction heat loss of a greenhouse structure. An under-bench hydroponic system uses a low-output finned tube (such as Delta FinTM TF or DuoFin) or a standard pipe to provide heat under the plants.

Bench-top Heating

Bench-top hydroponic heating systems distribute the heat via a mat or tubing placed on the bench (See Figure 2.11).

Hot Water (Hydroponic) Unit Heaters

This style of unit heater is used in conjunction with a boiler system or other hot water or steam sources. They extract heat from the water or steam and deliver the heated air to the greenhouse. Hot water unit heaters are very reliable and are inexpensive.

Steam Heating System

Although steam heating systems continue to be popular, hot water heating systems are becoming more common for large-scale greenhouse operations. Modern steam heating plants are closed circuit systems. Water is heated to 212 degrees F (100°C) or higher. The resulting steam is forced through the main pipes to the distribution pipes in the greenhouse. As the steam cools and gives up its heat, it condenses to water. The condensate (water) is returned, by pumps, to the boiler where it is reheated, and the process is repeated. The steam trap serves to prevent steam from returning to the boiler until it condenses to water. A steam system also allows the use of steam for soil pasteurization. A steam system requires a high initial investment; however, it has a long life expectancy.

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