Heating of Buildings – Factors and Types:
The purpose of most forms of space-heating installation is to provide comfortable conditions in which people can work or relax. Space heating of buildings is far from being an exact science because opinions vary considerably as to what constitutes a comfortable environment. It is, however, reasonable to say that in order to be comfortable the feet and head should be at about the same temperature. The head should not be subjected to intense radiation and there should be as little draught as possible. Under these circumstances an ambient temperature between 20°C and 22°C will be acceptable to most people.
Factors Affecting The Choice of Heating Systems
The type of heating of buildings equipment used is influenced by several considerations. Probably the most significant of these is whether heat is to be required for long or short periods. Short-term heating generally relies on radiation which can be very effective in no time. Convected heat is best suited to long-term heating in which the time taken to ‘warm up’ is not so important.
The construction of building and the conditions existing in it are important. Where ceilings are low and changes of air are limited it may be practical to heat all the air in the room. If on the other hand the air is changing continually or there is a very
high ceiling some sort of directional heating may be required.
Some forms of heating can only be installed effectively and economically during the construction of a new building whilst other may be used to improve an existing building.
Complete or Partial Heating Systems: In the past when coal or wood fires were the normal heat source it was customary to treat each room as a separate room. This persisted for many years after electricity was first used for space heating and in many cases a portable electric heater simply took the place of an open fire. This approach to heating produced result that tended to be both uncomfortable and uneconomical. Heat being continually lost from warm parts of the building to the cold and the wide differences of temperature experienced preventing all parts being used to equal advantage. Better results are obtained by designing a heating installation to suit an entire building. With this is done, a comfortable temperature can be achieved throughout the building and the points at which heat losses occur can be readily identified and the necessary steps taken. These steps would normally include double glazing of windows, draught-sealing doors and insulating the roof space with glass wool or some other thermal insulant.
One- and Two-Level Heating: Having decided to design a comprehensive heating system for a building another choice must be made. This is whether to settle upon a single value of temperature and heat the entire building to it or not. In office blocks and factories where every effort is made to get the maximum use out of the available floor space there is little waste involved in heating to the same temperature throughout.
In domestic installations the amount of use given to different parts varies largely and also there are considerable differences in levels of activity from one time of the day to another. To allow for such variations a two-level heating system is often employed. In such a case general heating is provided to a fairly low comfort level in the region of 15°C. This is called the background heating and keeps all the house or flat comfortable for housework or similar activities. Additional appliances are available, controlled by the consumer, to raise the temperature of specific areas a further 5° or 6° when he or she is sitting still for any length of time. This is called topping up. Background heating is usually provided by convection heaters and radiant heaters are used for topping up.
Types of Heating Equipments Used For Space Heating:
The different types of equipments used for heating of buildings are given below :
1. High Temperature Radiators: The high temperature radiator essentially consists of a high resistance element wound on porcelain or refractory former and mounted into decorative frames. Resistance elements get heated to high temperature (1,400°C to 1,600°C) due to flow of electric current through them and 50% to 70% of heat so produced is dissipated by radiation and remaining by convection. Advantages of this type of heater are that it is portable, cheap in initial cost, simple in construction and attains its highest temperature quickly. The main drawback of this type of heater is that heat dissipated is localised. These are used for domestic purposes or where intermittent heating is required. The usual sizes are from 0.5 kW to 3 kW.
2. Tubular Heaters or Low Temperature Convectors: The most common type of low temperature heater is 5 cm diameter steel tubes of length ranging from 0.6 to 5 metres and containing resistance elements arranged on mica, fire clay, or porcelain former. Such tubes are mounted around the skirting board of room to be heated. The normal loading is 200 watts per metre length of the convector. The temperature of the external surface is maintained at about 93° C. About 90 per cent of the heat is transferred by convection. These are spaced to produce convection currents in the room so as to heat whole of it. These are specially suitable for thermostatic control, by which means they are switched on and off automatically as the room temperature falls below or rises above the desired value. When fixed at the base of a cold wall or underneath a window they prevent cold down draughts, which cause cold feet to the occupants of the room.
3. Convector Heaters: In these heaters, heating elements are mounted within sheet-metal cases which admits cool air from bottom. Air after getting heated up, comes out from grills at the top. The front and top may be of moulded plastic material for the sake of pleasing appearance. These heaters may be either portable or tied, and may be handled without fear of burns. In these type of heaters, heating clement attains low temperature of 66° to 93°C and proportion of heat transmitted by convection is large as compared to heat transmitted by radiation.
A very satisfactory method of heating a room is to use convector heaters to warm the room to about 13°C, together with a small radiant heater for local heating.
4. Panel Heaters: In this system of heating large panels of fire clay or some other suitable heat resisting materials are used along the walls or ceiling of a room. The resistance elements are embedded in these panels. The heat produced is utilised in the form of convection or radiations. This system of heating of buildings is advantageous from the decoration point of view and gives a uniform distribution of heat. Loadings are of the order of 400 watts/m2 for the low temperature panels and 6,000 watts/m2 for the high temperature panels. Low temperature panels operating at temperature from 27°C to 65°C may be fixed in walls or ceiling of a room and plastered over. High temperature panels operating at temperature up to 288°C are used on walls and ceiling but are not fixed in the wall surface. They are sometimes suspended from the ceiling.
5. Underfloor Heating: This is one of the systems that may be installed in a building whilst it is being built.
In this system a number of heating cables are embedded in the floor of a room or otherwise contained in the floor space. Thus heat is given to the concrete or other floor material slowly and heat from the floor is used in warming the air in the room, and in warming the walls and the furniture. The electrical power intake is controlled by thermostat fixed on the room wall in a suitable position. The cables used for heating are made from alloys of nickel, chromium, iron or other resistance materials and are insulated with asbestos mineral insulation, butyl rubber, or other heat-resisting insulating materials. For mechanical protection lead, copper, aluminium, or PVC sheathing can be used. Loading of from 100 to 150 watts per metre2 of floor area are generally satisfactory.
In this system also the advantage of cheaper power available during night hours can be taken but in such a case it will be desirable that one or more convectors or radiant heaters for use towards the end of the floor-warming output cycle are installed.
6. Thermal Storage System: Roughly this system is similar to well-known central-heating system employing a coal- or coke-fired boiler except that the heat is supplied to the storage tank electrically. Hot water is circulated through the heating pipes and radiators situated throughout the building. It must be noted that in electrical heating system the tank and piping must be lagged to prevent loss of heat, a feature which is usually omitted with fuel-fired systems, since with such systems the efficiency is so low that the additional loss due to lack of lagging is relatively insignificant. There are two types of heaters used in this system : the immersion heater for a load up to 100 to 200 kW and the electrode boiler type for loads exceeding 200 kW.
The immersion heater used for heating the water in a storage tank consists of a spiral of resistance wire embedded in refractory insulating medium like magnesium oxide inside a copper or steel tube, which is bent to the desired shape. The tube is provided with a flange at one end, so that it can be bolted to the vertical side of the tank with the heating element projecting horizontally into the water. The loading of such a tube is usually between 16 and 24 kW/m2 of face, so that a tube of length 305 mm and diameter 51.0 mm will be rated at about 1 kW. Single tubes in ratings of 4-5 kW are available. For higher ratings group of tubes are used.
The electrode boiler is only suitable for use on ac, usually three phase, since dc would electrolyse the water. In electrode boilers water is heated by placing cast iron electrodes directly in the water. For 3-Φ supply three electrodes are placed in the water and the tank is earthed so as to form a star-connected system. The resistance of average cold tap water varies between about 20 and 50 Ω-m. Sodium carbonate (soda) is, therefore, added for increasing the conductivity of water. Another noteworthy point is that resistance of water decreases with the increase of its temperature, the value at 100°C being one-third of that at 20°C. Automatic control of the power supply is, therefore, required if a constant input is required. Control of power input may be carried out by varying the height of the electrodes in the water, by means of a movable shield surrounding the electrodes, by moving one electrode relative to the other, or in large units by varying the water level by means of a pump. A low-voltage boiler operating at 400 V is usually employed for a requirement up to 500-700 kW while for larger requirement a high-voltage boiler operating at voltages up to 11,000 V or even higher is used.
The advantage of this system is that the water can be heated during night (off-peak periods) and is supplied throughout the day, thus use of cheap power available during off-peak period can be made. This heating of buildings system also provides hot water service for the building.