THE IMPORTANCE OF GOOD GLASS HOUSE HEATING SYSTEMS

A range of fuels have been used as sources of energy to power boilers. The most common type of fuel used today is OIL. Heavy oils are the cheapest, these are called heavy because they are THICK and only flow slowly. They are also called crude oils. Because they are thick and only flow slowly, they have to be heated by a special apparatus before they can be fed to the burner unit of the boiler.

Lighter oils are available, which are more refined, and therefore thinner. An example of this type of oil would be PARAFFIN. These sorts of oil are thinner, and easier to burn, although they are more expensive.

Solid fuel – COAL – has often been used in the past, in the days of the hand-stoked boiler. However, AUTOMATIC STOKING equipment is now available. The increasing price of oil, especially as the North Sea supply declines over the next 10 to 20 years, has resulted in many growers finding it economic to install solid fuel firing equipment, and reverting to this older fuel.

GAS is a common fuel especially where the gas main pipes are close to the nursery. If there is no local supply, it is very expensive to have it installed, so generally, although gas is a clean and efficient fuel, where it is not directly available, then other sources of fuel are used.

All possible sources of wastage should be examined and if found corrected. Most of the checks are easily carried out on a regular basis but will save the grower wasted energy and therefore money.

Tests, servicing, maintenance

a) Weekly combustion efficiency tests to the boiler.

b) Regular checks on water pumps and oil pressure.

c) Flue temperatures should be taken daily with a thermometer permanently installed.

d) Check tank height and insulation to see if this layout can be improved for the flow of oil.

e) Boiler cleanliness.

f) Check glasshouse temperature gradients by using temperature integrating jars.

g) Airtightness of the glasshouse. Check ventilator searings and broken glass.

The thermo-syphon heating system was expensive to use because there was a great volume of hot water in the 100mm cast iron pipes which was no longer required when the level oi warmth needed had been achieved. The water was used as the means of moving the heat from the boiler to all the circulation system – the heating pipes – laid out in the glasshouse. Smaller bore pipes with hot water circulated rapidly will do a better job than the 100mm pipes. Small pipes don’t work well with thermo-syphon systems hence the circulation pump.

Heating Systems

Hot water gravity circulation systems are not satisfactory where precise control of temperatures are critical. These systems are neither flexible or responsive to required changes, and large cast iron pipes take too much space. High speed hot water systems have flexibility and use small diameter steel pipes. Pipe sizes range from 2.5 – 5cm in diameter. This system can be used with a steam boiler. Two other systems are commonly used, the medium pressure hot water system and the low pressure steam system.

Warm air heating is an alternative sometimes used instead of pipe systems. The warm air is distributed by means of perforated plastic ducting and main advantages of this system are the low initial cost of the heaters and ducting, the positive air movement and flexibility to locate the ducts either at ground level or above the crop. With a good layout the air temperature can be almost as evenly spread as that from the best pipe systems.

An aspirated screened thermostat is frequently used to switch on and off the valves releasing hot water into the heating system from a constantly heated hot water tank system thoroughly lagged against heat loss. In this way a very fast response of hot water in the pipes can be achieved.

Where STEAM is used to heat the pipes from a steam boiler, the temperature will clearly be much higher, the temperature of STEAM being above 100°C. The higher temperature of steam pipes means that FEWER pipes are needed compared with hot water pipes to produce ar equivalent amount of heat; this therefore reduces the cost of piping systems. When piping is installed for steam heating a slight slope has to be incorporated so that water produced wher the steam condenses can flow back to the boiler; this condensed steam is called Condensate’,

On small nurseries, the use of a HOT WATER BOILER is satisfactory although on large] holdings, STEAM BOILERS are often used. This is because steam boilers are more efficient and produce more heat per unit of fuel than hot water boilers. Also the steam is available foi sterilization of soil on the nursery. Another important point is that steam boilers last longe: than hot water boilers because there is less metal corrosion at the higher temperatures founc in steam boilers. Usually steam boilers have a boiler man on duty all the time – hence then are cost implications.

Most of these can be overcome by even distribution of heat which is possible at a very low initial cost by installing soil warming cables.

Electricity is also used as a heat source for small installations for soil warming, and for warm air space heating, but for larger areas the costs mount up quite dramatically.

Soil warming cables provide very easy heating with thermostatic control for the humblest amateur willing to part with his cash – may be £60 or so for the most basic system.

Soil warming cable Soil Thermostat Metal or concrete

(Rod type) bench

Glasshouses with no ventilation tend to sweat inside and this builds up the opportunity for botrytis. Some ventilation is desirable to assist air changes in modern, very air tight glasshouse designs. There are two other important ways of reducing the relative humidity without using ventilation:-

(I) Raise the air temperature. (ii) Dehumidify the air.

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