Document 853EGeyDkvKb1mKxayjEyn2ZZ

American Society of Heating and Ventilating Engineers Guide, g__ (jNIT Heating and Ventilating Machines Chapter for temperature regulation. Another method of protection is to inunej in the return condensate a thermostat which controls a positive-acti^ by-pass valve which will admit additional steam if the temperature Jsi the condensate falls to the freezing point. This is purely a safety devi^f and assumes that no condition of normal regulation will require enough throttling to make freezing possible. If this were not the case this method5' would not be suitable, as it prevents regulation beyond the point when, the by-pass opens. As a precaution against allowing the unit heater motors to continue! to run if the steam supply fails or is for some reason shut off, either pressurestat or thermostat in the steam line may be installed to stop the! motor when the pressure or temperature in the supply line drops belo\1 a selected point. For a further discussion of this subject, see Chapter 20,j| Direct Fired Vnits The foregoing discussion relates generally to units in which steam vapor, or hot water are used as the heating medium. On rare occasions electrical resistances are used as the heating element. These are applied only where electric power is abundant and cheap and where other forms < r..i are scarce and expensive. Quietness of Operation \ In selecting unit heaters,, attention should be given to the degree of*' quietness required for the installation. i No given fan speed may be applied as a measure of relative quietness! to fans of different design and proportions. Quietness is a function of'- type, diameter, blade form and other variables besides speed, and all! these must be taken into account. In general small fans can be run at! higher motor speeds than can large fans with equal quietness. 5 Unit Heater Connections Piping connections for unit heaters are similar to those for other types! t of fan-blast heaters. Typical connections are shown in Figs. 1, 2 and 3.j i. One-pipe gravity and vapor systems are not recommended for unit! i heater work. fj On two-pipe gravity, or on pump and receiver systems the return from; ; each unit should be fitted with a heavy duty or blast trap and an air! 'f valve should be connected into the return header of each unit. Pressure-1 f drop must be compensated for by elevation of the heater above the water ] ! line of the boiler or of the receiver. J On vacuum systems the return from each unit should be fitted withal j large capacity trap to discharge the water of condensation and with aj j thermostatic air valve for eliminating the air, or with a heavy duty trap I s for handling both the condensation and the air, provided the air finally! f can be eliminated at some other point in the return system. ; I On high pressure systems the same kind of traps may be used as with | vacuum systems, except that they must be constructed for the pressure { j used. If the air is to be eliminated at the return header of the unit, a| ; high pressure air valve can be used; otherwise the air may be passed with j J the condensate through the high-pressure return trap. `j The connections for steam and return piping to unit heaters must: j always be calculated on the basis of the high heat emission or condensation | f rate of such devices, usually by reducing the heat to be supplied to square * j feet of equivalent radiation by dividing by 240, and then using the pipe-| f size tables given in Chapter 10. The supply valve must be at the high! f point, or provision must be made for draining the pocket above this | valve when the valve is closed. 140 v.r. l Unit Heater Connections Fig. 2. Unit Heater Connections Where Condensation Is Returned to Boiler Through Wet Return Fig 3 Unit Heater Connections Where Condensation Is Returned to Condensation Pump or Hot Well Where steam or hot water are not available the direct-fired type of heater is used, The heating element is a warm-air furnace in which coal, coke, oil or gas is burned directly. This heating element is enclosed in a casing through which a fan maintains a rapid air current by pressure. This type of heater is often used for temporary heat during building construction or where the installation of a steam or hot water plant is for some reason not justified. 141 Ill