Book - School of Science and Technology

Indirect systems ± liquid media 113SuspensionFanElementFigure 4.17 Electrical forced convectors (GEC-Xpelair). Simple multi-switch control of any appliance which is fitted with a fan, i.e. a forcedconvector, including speed control if available.. Automation of any of the foregoing via a thermostat which may be either integral tothe unit or wall mounted as a remote room sensor.Groups of units in larger systems. Any of the above, applied to whole or part of the group, i.e. by zoning to suit aspect orsub-division.. Further automation by sensing outside weather and use of time switches to suitcircumstances.The above methods may be applied to liquid, gaseous and electrical equipment using selfactingcontrollers in the less complexapplications and electric or electronic devicesotherwise. Most equipment will be fitted with override or other safety controls as maybe appropriate to the particular fuel or energy source.Indirect systems ± liquid mediaDistribution of heatAlthough non-toxic thermal fluids such as particular types of petroleum oil and a varietyof synthetic chemical mixtures are available, the use of these is confined to high temperatureprocess systems. Hot water, therefore, is the only liquid medium which needs to beconsidered under this heading.Water has a high specific heat capacity per unit volume; it is non-toxic unless adulterated;it offers great flexibility in that the temperature may be adjusted easily to match thechanges in demand imposed upon a heating system as a result of variations in the weatherand it is extremely cheap. It may have the disadvantages of being corrosive to metals, ofcarrying mineral salts which produce scale deposits and of freezing at a relatively hightemperature but each of these may be overcome without very much difficulty given anintelligent maintenance effort.For the common run of building size in the domestic and commercial sectors,systems are usually designed to operate at atmospheric pressure. That is to say that