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528 CHAPTER 23 1950 Guide stat so-as to turn heat on or off, when the desired temperature of 80 F,or any other predetermined surface temperature of the cylinder, decreases or increases as a result of changes in the Operative Temperature. For. testing work, the globe thermometer is a useful instrument.' It con sists of an ordinary mercury thermometer,'with its bulb placed in the center of a sphere from 6 to 9 in..in diameter, usually made of thin copper, and painted black, and sometimes covered with cloth. The' temperature recorded by a thermometer with its bulb in the center of the sphere is termed the radiation-convection temperature. See Chapter 49. CONTROL OF PANEL AND RADIANT HEATING The effectiveness of any type of control, will depend largely on the time lag of the system. With warm air passing through floor ducts, the time lag :> Fig. 11. Typical Panel and Radiant Heat Control System is usually too long for any kind of room thermostat; in fact:a thermostat will mot'prove suitable with any system if the building is constructed with massive brickwork and masonry, unless it operates in. conjunction-with a time control responsive to changes in outside conditions. The heat emitted by hot water pipes imbedded in the plaster of the ceiling mid walls or in the concrete base of a floor, can be effectively controlled by an instrument designed to modulate the'temperature of the water circulat: ing in the system according to the outside conditions. .Metal panels which cad be installed in the ceiling or side walls* may be either-controlled by an instrument responsive to outside weather conditions, or by a specially de signed instrument responsive to both air temperature and radiation. Any purely on or off control system is ndt recommended for'panel heating. ; A typical control system operated from an outside thermostat and sup plemented with a room heat' control instrument, is illustrated in Fig. 11. The outside thermostat modulates the temperature of the circulating water in the coils by mixing some of'the hot water leaving the*boiler with a pro portionate amount of return water which is diverted to the three-way valve. . - One type.;of room instrument conasts, of a blackened copper sphere of 6 Panel Heating and Radiant Heating 529. or 8 in. in diameter, in which a cylindrical sump contains a volatile liquid. A small electric heating coil creates in the sphere a vapor pressure which remains constant as long as'the total heat loss from the sphere is at the desired rate. If the Operative Temperature becomes too high for comfort, a greater vapor pressure results from the smaller heat joss from the sphere. This acts on a diaphragm and reduces the supply of heat to the room.. With' too low an Operative Temperature, the reverse action occurs. A similar instrument, which has an electric heating element for warming the air inside the sphere and the thermostat-operated switch, is.also used for controlling room conditions. In addition to a thermostatically controlled device for modulating the: temperature of the circulating water, it is advantageous to insert :in each coil a locked flow control or adjustable resistance to give uniform condi tions throughout all rooms. Owing to unforeseen difficulties with varying frictional losses in pipes, emission factor, and exposures, it is an advantage to be able to regulate permanently the flow through each circuit by means of a key operated valve as indicated'in Fig74. " :'-------- REFERENCES 1 Standard Operative Temperature, A Generalized Temperature Scale, Applicable to Direct and Partitional Calorimetry, by A.' P. Gagge (American Journal Physiology. 1940, Vol. CXJCXI, p. 93). ' * The Influence of Air Movement Upon Heat Losses from the Clothed Human Body, by C.-E. A. Winslow, A. P. Gagge and L. P. Herrington (American Journal Phytidlm, 1939, Vol; CXXVH, p. 805).- : * Surface Heat Transmission, by R. H. Heilman (AJS.MJS. Transactions, Fuels and Steam Power Section, Vol. 51, No. 22, September-Decernher, 1929). BIBLIOGRAPHY Trend Curves for Estimating Performance of Panel Heating Systems, by B. F. Raber and F. W. Hutchinson (A.S.H.V.E. Transactions, Vol. 48, 1942, p. 425). A.S.H.V.E. Research Report No. 1192--Panel Heating and Cooling Performance Studies, by B. F. Raber and F. W. Hutchinson (A.S.H.V.E. Transactions, Vol. 48, 1942,p. 35). A.S.H.V.E. Research Report No. 1193--Radiation as a Factor in the Feeling of Warmth in Convection, Radiator and Panel Heated Rooms, by F. C. Houghten, Carl Gutberlet and E, C'. Hach (A.S.H.V.E. Transactions, Vol. 48, 1942, p. 55). Panel Heating and Cooling Analysis, by B. F. Raber and F. W. Hutchinson (A.S.H.V.E. Transactions, Vol. 47,1941, p. 285). Operating Results of a Residence Radiant Wall Heating System, by E. J. Rodee (A.S.H.V.E. Transactions, Vol. 47,1941, p. 123). Performance of a Residential Panel Heating System, by H. F. Randolph and J. B. Wallace (A.S.H.V.E. Transactions, Vol. 49, 1943, p. 235). Radiant Heating (Heating and Ventilating, March, 1941, p, 35). Radiant Heating and Cooling, by F. E. Giesecke (Heating, Piping and Air Condi tioning, June, July, August, September and October, 1940). Calculations for Radiant Heating, by T. Napier Adlam (Heating and Ventilating. . October, 1931). Radiant Heating and Cooling, Part I, by C. O. Mackey, L. T. Wright, Jr., R..E. Clark, and N. R. Gay (Cornell University Engineering Experiment Station, Bulletin ' No. 32,1943). Design Method for Panel Heating Systems Using Copper Tubing, by R. G. Vanderweil (A.S.H.V.E. Journal Section, Heating, Piping and Air Conditioning, Nov. 1947, p. 123), Air Temperature Gradients in a Panel Heated Room, by J. M. Ayres and B. W. Levy (A.S.H.V.E. Journal Section, Heating, Piping and Air Conditioning, Oct. 1947, p. 113).