Document 9Y09J5oqOMM24wv1o4Vazeqp

HEATING VENTILATING AIR CONDITIONING GUIDE 1940 standardization of fan motor types and the method of specifying outlet. velocities has been adopted. Ratings Standard practice is to rate unit heaters in Btu per hour at a given temperature of air entering the heater and at a given steam pressure maintained in the coil. Steam at 2 lb pressure and air entering at 60 F are used as the standard basis of rating3.. The capacity of a heater increases as the steam pressure increases, and decreases as the entering air temperature increases. The heating capacity for any condition of steam pressure and entering air temperature may be calculated approxi mately from any given rating by the use of factors in Tables 1 and 2. Table 1 is used for the blow-through type and Table 2 for the drawthrough type of unit. The formulae given'under unit ventilators for calculating capacities also apply to unit heaters. Fig. 3. Suspended Type Unit Heater, Housed Type Fan The temperature to be maintained4 in the room, lor recirculating heaters with intakes at the floor level, should be considered as the tem perature of the air entering the heater. Where outside air is introduced, the temperature of the mixture must be calculated and used as the entering air temperature to the heater. Unit heaters taking in recircu lated air at the floor level should maintain temperature differentials of less than 0.5 deg per foot of elevation when the maximum capacity of the heaters is required. This temperature difference per foot of elevation is less than the corresponding variations for spaces heated by direct radi ation. The temperature variation from floor to ceiling with suspended unit heaters taking air at some distance above the floor, may reach as much as 1 deg per foot of elevation during the periods when the maximum capacity of the heaters is required. Thus this allowance should be made in calcu- A.S.H.V.E. Standard Code for Testing and Rating Steam Unit Heaters (A.S.H.V.E. Transactions Vol4.A3.6S..H19.V30.E, p. .R1e6s5e)a. rch Report No. 958---Temperature Gradient Observations in a Large Heated Space, by G. L. Larson. D. W. Nelson, and O. C- Cromer (A.S.H.V.E. Transactions. Vol. 39. 1933. p. 243). A.S.H.V.E. Research Report No. 1012--Testsof Three HeatiagSystems ioaa Industrial Type of Build ing, by G. L. Larson, D. W. Nelson, and John James (A.S.H.V.E. Transactions, Vol. 41, 1935. p. 185). 396 CHAPTER 22. UNIT HEATERS, UNIT VENTILATORS, UNIT HUMIDIFIERS lating the capacity of suspended heaters. High velocity discharge units (blower type illustrated in Fig. 3) will maintain slightly lower temperature differences than will low velocity units (propeller fan type illustrated in Fig. 2). Unit heaters are customarily rated as free delivery type units. If outside air intakes, filters, or ducts on the discharge side are used with the heater, proper consideration should be given to the reduction in air and heating capacity that will result because of this added resistance. The percentage of this reduction in capacity will depend upon the characteristics of the heater and on the type, design and speed of the fans so that no specific percentage reduction can be assigned for all heaters at a given added resistance. In general, however, disc or propeller Ian type units will experience a larger reduction in capacity than housed centrifugal fan units for a given added resistance and a given heater will have a larger reduction in capacity as the fan speed is lowered. When confronted with this problem the ratings under the conditions expected should be secured from the manufacturer. Boiler Capacity The capacity of the boiler should be based on the rated capacity of the heaters at the lowest entering air temperature that will occur, plus an allowance for line losses. Ordinarily for recirculating heaters the lowest entering temperature will occur at the beginning of the heating period and is usually taken as 40 F, while for ventilators taking air from outdoors the lowest entering temperature will be the extreme outdoor temperature expected in the district. No greater allowance in boiler capacity beyond the calculated heat demand need be added in order to supply unit heaters than for any other type of system. It is unwise to install a single unit heater as the sole load on any boiler, particularly if the unit heater motor is started and stopped by thermo static control. The wide and sudden fluctuations of load that occur under such conditions would require closer attendance to the boiler than is usually possible in a small installation. Where oil or gas is used to fire the boiler, it is possible by means of a pressurestat to control the boiler, in response to this rapid fluctuation. In most cases, however, and particularly where the boiler is coal-fired, it is advisable to use two or more smaller heating units instead of one large unit. Steam pressures below 5 lb can be used with safety for recirculating unit heaters when their coils are designed for the purpose and when proper provision is made for returning the condensate. If ventilators are to take in air that may be at a temperature below freezing, however, a steam pressure of not less than 5 lb should be maintained on the convector or a corresponding differential in pressure between the supply and returns be maintained by means of a vacuum. Piping Connections Piping connections for unit heaters are similar to those for other types of fan blast heaters. The piping around the unit heaters must strictly conform to the system requirements while at the same time permitting the heaters themselves to function as intended. The basic piping princi ples for steam systems are discussed in Chapter 16. 397