Document wKVzMQJXJB1xrxZJ6OY1vpYjd

American Society of Heating and Ventilating Engineers Guide, 1934 In selecting a heating unit for any particular service, the choice should be based on the desired requirements as follows: 1. Final temperature desired. 2. Loss in pressure (or friction) of air passing over the heating unit. 3. Air velocity over the heating unit. 4. Free area or face area of heating unit. 5. Ratio of heating surface to net free (or face) area. 6. Air volume required. 7. Rows deep of pipe, tubes or sections. 8. Amount of heating surface. 9. Steam pressure drop through the heating unit. 10. Weight of heating unit. Final Temperature Desired. The choice of a heating unit is largely influenced by the final temperature desired, when the entering air tem perature and steam pressure available at the heating unit are specified. These data are obtainable from manufacturers' catalogs. Loss in Air Pressure {or Friction). The allowable friction through the heating unit is one of the first factors to be determined in the selection of the apparatus. The velocities of air through various types of heating units will not necessarily be the same, but for any particular job the velocity through the heating unit should be a secondary consideration and the allowable friction or air pressure loss should be fixed approximately before proceeding with the selection of the heating unit. The loss in air pressure (or friction) through the heating unit should not exceed a pre determined maximum allowable amount for economical operation and for moderate size and first cost of installation. In public building work, the maximum allowable friction through both tempering coil and reheater coils should never exceed ^ in. of water and it is advisable that the friction be kept considerably lower than this figure if possible. A tempering coil friction ranging from 0.10 to 0.20 in. of water is considered satisfactory. The air pressure loss for reheaters ordinarily ranges from 0.20 to 0.40 in. of water. In factory work, the maximum friction through the heater should never exceed 0.8 in. or 1 in. of water and it is advisable to figure the heaters at lower frictions if possible. Velocity through Heating Unit. This velocity has generally been given in manufacturers' tables as being measured at 70 F and in most rasw refers to the velocity through the net free area of the heating unit, or through the net space between the pipes, tubes or sections. Although most manufacturers give suitable velocities measured at 70 F, certain manufacturers show velocities measured at 65 F and others indicate velocities measured at the average air temperature through the heating unit. Many new heating units, however, specify net face areas with cor responding velocities instead of velocities through net free areas. In either case, manufacturers publish the corresponding friction or air- pressure loss in tables. The velocity through the net free area of the heating unit averages about 1000 fpm and that through the net face area about 500 fpm. ., . The volume of air to be heated in any particular case is determined after . consideration of the ventilation requirements, heat losses, and quantity of ' air required for proper circulation, as explained in Chapters 2 and 7. 294 Chapter 22--Central Fan Heating Systems The number of rows of pipe tubes or sections or the amount of heating surface to be used may be selected from manufacturers' catalogs after the quantity of air handled and heat load are known. Savings in operating expense or cost of installation should result from a proper selection of heater and by-pass areas. For example, instead of having the entire air quantity go through a one-row heating unit, it may be advantageous to use a two-row heating unit and a properly sized by-pass. Thus, when no heating is being done, a suitable by-pass damper may be opened to place a lighter load on the fan. The steam pressure drop through the heating unit is also tabulated in manufacturers' data tables. The sizing of steam supply and return piping, allowing for drops through heating units, is explained in Chapter 32. Weight of Heating Unit. In the design of a heating system, the weight limitations of heating units are determined by the location of the units. Obviously, if there is no loading limitation imposed, any type of heating unit may be selected. On the other hand if the heating unit is to be hung from the ceiling, it may be desirable to use the lightest unit which will accomplish the work required. DESIGNING THE SYSTEM The general procedure for the design of central fan systems is as follows: 1. Calculate the heat loss for each room or space to be heated. 2. Determine volume of outside air to be introduced. 3. Assume or calculate temperature of air leaving registers or supply outlets. 4. Calculate weight of air to be circulated. 5. Estimate temperature loss in duct system. 6: Calculate heat to be supplied the heating, units and washer. 7. Select heating units and washer from manufacturers' data and performance curves. 8. Calculate total heat to be supplied. 9. Calculate grate area and select boiler. 10. Design duct system. 11. Calculate total static pressure of system. 12. Select fan, motor,'and drive. The heat losses (H) should, be calculated in accordance with the pro cedure outlined in Chapter 7. If a positive pressure is maintained by the central fan system in the room or space to be ventilated or conditioned, there will ordinarily be very little infiltration of cold outside air through the cracks and crevices of the space. Consequently, the volume of air introduced into the space at the assumed or calculated outlet temperature need only be sufficient to provide for the transmission losses, plus a part of the infiltration losses, about one-third. The exfiltration of heated or con ditioned air through the cracks and crevices of the space should be pro vided for by making the usual allowance for the infiltration losses in arriving at the total heat loss of the space. The air required to make up for this exfiltration of heated or conditioned air will be brought in at the outside air intake and may be included as a part of the outside air hecessary for the ventilating requirements. The heat required to raise this air to the 295