Document 70R6qOGZ1gjBORBDDdqx4jdNE

HEATING VENTILATING AIR CONDITIONING GUIDE 1943 is required, this must be considered similarly to a conventional air con ditioning system, except that the air temperature of the room will be much lower and will therefore require less moisture. Mean Radiant Temperature If the entire interior surface of a room were at the same temperature, this would be the MRT. Such a condition seldom exists, because in different parts of a room, some surfaces are exposed to the outer air while others are adjacent to heated rooms. The actual surface tem perature varies with the construction and exposure of different sides of the enclosures. It is therefore necessary to calculate the thermal mqan of these interior surface temperatures. This is not the same as the arithmetic average of the various actual OVER-ALL HEAT TRANSMISSION COEFFICIENT, U Fig. 7. Chart for Estimating Inner Surface Temperatures - of Outside Vertical Walls surface temperatures, but the radiant temperature which corresponds to the average of the several rates of heat emission (Btu per square foot) from the several surfaces. The emission at any given surface tempera ture, for any stated emissivity factor can be obtained directly from Table 1, while the emissivity factors for many materials may be found in Table 6 of Chapter 3. For example, from Table 1 it can be determined that if the emissivity of the surface is 0.90 then 1 sq ft of surface at 50 F will emit 104.9 Btu per square foot per hour to surroundings at absolute zero. Such a determination of the amount of radiant heating surface needed in a room, to maintain a desired MRT, requires knowledge of the type of heating, and the temperatures of the unheated surfaces. The latter can be estimated from Fig. 7 which is based on an inside air temperature of 65 F and an MRT of 71 F. There will be some variation in.surface 810 CHAPTER 45. RADIANT HEATING temperature with emissivity, but except in the case of reflective materials this may be neglected, as the variation due to ordinary building surfaces . will be small. Detailed. Computation Method Assuming the mean surface temperature of the exposed part of the human body and clothing to be 80 E and the emissivity factor to be 0.95, from Table 1 it can be determined that the body surface will give off 139.4 Btu: per square foot per hour to absolute zero surroundings. Since the average human body releases approximately 12.25 Btu per square foot per hour by radiation, the mean radiant emission from the surroundings must be 127.15 Btu per hour with an average emissivity factor of 0.93 which requires an MRT of approximately 71- F. If the body is covered less so that the mean surface temperature of the body, is 85 F with an emissivity factor of .0.95, the correct MRT for the room should be 74 F. Consequently, for baths and similar rooms the MRT should be slightly higher than for offices, etc. The mean radiant emission from walls, etc., Table 2. Highest Safe Surface Temperatures for Heating Panel Type of Panel Surface Temperature Deg F 115 120 85 120 160 180 200 aLow surface temperature radiation is recommended regardless of the heating medium employed. to give this desired rate can be determined from Table 1. Multiplying by the total surrounding area will give the desired total radiant heat effect. Therefore the MRT for. an ordinary living room, office, or similar room to give comfort conditions is 71 F. The location of the surfaces is generally decided according to the type of building and its use. For high ceilings it is advisable to select floor heating or install heated panels in the walls at low level. For exposed rooms it may be necessary to have some wall or ceiling panels in addition t;o floor heating. The temperature of the surface is controlled somewhat by the location. If the floor is chosen, then hot water pipes should be used as the medium; and the surface temperature should never be more than 85- F unless border heating is used. 'The latter comprises strips of heated surfaces where occupants will not usually rest their feet, such as portions of the floor adjacent to walls or windows, aisles of churches, halls, etc.: If iron panels are used on side walls, etc., a surface temperature up to 160. F may be used with hot water as the heating medium. Vapor or low pressure steam may also be used with a maximum surface temperature of 180 F. For ceiling or other plaster heating, hot water pipes should be used with a 811 X