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Heating Ventilating Air Conditioning Guide 1939 30 ft to as much as 200 ft from the unit. This makes possible the locatio of these units at considerable distances from each other, thus reducing greatly the piping and loss of floor space due to the heating equipment Propeller-type units, illustrated in Fig. 1, with outlet velocities of fro ' 300 to 1000 fpm are usually placed from 30 up to 100 ft apart. Two methods of application of unit heaters are commonly used. Floe,, mounted units, shown in Fig. 2 are available either with or without the air by-pass, and withdraw the cold air from the floor and discharge the heated air above the working zone. Suspended type units are located in an elevated position withdrawing air from this higher level and dis charging the heated air down into the working zone. In closely occupied spaces where direct air drafts into the working zone are not permitted 7~~T 22 Unit Heaters. Ventilators. Air Conditioning. Cooling Units AIR TEMPERATURES5 F r recirculating heaters with intakes at the floor level, the temperature v fflaintained in the room should be considered as the temperature of -air entering the heater. Where outside air is introduced, the temt"e ture of the mixture must be calculated and used as the entering air ' P^nerature to the heater. Where suspended heaters are used without ^v intake boxes extending down to the floor level, a higher entering air temperature should be used than that at which the room is to be main- ' ^Wfth suspended unit heaters taking air at some distance above the ` t[je temperature variation from floor to ceiling may reach as much - j jgg {or each foot of elevation during the periods when the maximum eabacity of the heaters is required. Thus this allowance should be made ' in calculating the capacity of suspended heaters. High velocity discharge Fig. 1. Suspended Unit Heater, Propeller Type Fan Fig. 2. Floor Mounted Unit Heater, Housed Type Fan the floor mounted unit will give more uniform temperature distribution. On the other hand, if opportunity is provided to deliver the heated air from suspended units down into the working zone, excellent temperature distribution is possible. A suspended high-velocity type unit heater con nected to an outside air intake with damper to control the volume of ventilation is shown in Fig. 3. A code4 governing the number of sizes of propeller type unit heaters offered for sale by a manufacturer, as well as a standard method of specifying outlet air velocities has been adopted. A wide variety of structural designs is available. All employ some form of convector, supplied with either steam or hot water, although occasionally equipped for gas or electric heat. Air is always forced over these con vectors by a fan of either the propeller or centrifugal type. Heating sur faces may be in the form of steel pipe coils, non-ferrous tubes or pipes with extended surfaces, cast-iron, and pressed or built-up sections .of the cart ridge or automotive type. `Standards for Propeller Type Unit Heaters prepared and adopted by the Industrial Unit Beater Asstr ciation, June. 1938. 434 units (blower type) will maintain slightly lower temperature differences than will low velocity units (propeller type). Unit heaters taking in recirculated 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 radiation. OUTPUT OF HEATERS It is standard practice to rate unit heaters in Btu per hour at a given temperature of air entering the heater and at a given steam pressure main tained in the coil. Steam at 2 lb pressure and air entering at 60 F are used as the standard basis of rating6. The capacity of a heater increases as the steam pressure increases, and decreases as the entering air temperature *A.S.H.V.E. Research 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. 1011--Tests of Three Heating Systems in an Industrial Type of Build` . h*?. by G. L. Larson, D. W. Nelson, and John James (A.S.H.V.E. Transactions, Vol. 41, 1935, p. 185). ,, /A.S.H.V.E. Standard Code for Testing and Rating Steam Unit Heaters (A.S.H.V.E. Transactions, --. -Vol. 38.1930,.p. 165). 435