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580 CHAPTER 38 1960 Guide where it drops to zero. As previously stated, steam-jet equipment requires more condensing water than other types of compression systems. Consequently, steam-jet systems are well suited to those applications where condensing water is cheap, or where condensing water is rather high in temperature. The selection of proper refrigeration equipment for any air-conditioning job is of utmost importance for satisfactory results. The most important factors in the selection of the equipment are: 1. Loads (as determined by the conditions of the space to be cooled). 2. Economics (both initial and operating costs). 3. Codes (local safety codes must be adhered to and in fluence the type of system to be used). A broad division of equipment to be used for a particular installation or application may be made on the basis of the magnitude of the load. Current general practice is outlined in Table 8. Unit or packaged systems, consisting of a reciprocating compressor, condenser, evaporator, and fans, are generally used in the smaller sized jobs where electric power is avail able, as they are manufactured complete, ready to install, and are the most economical (See Chapter 16) . The reciprocating compressor in the built-up central sys tem (see Chapter 19) covers the widest range of application since it is applicable to either the direct-expansion or in direct systems, and can be driven by steam or gas engines, or by electric motors. The quantity of condensing cooling medium required is also less than for any other system, with the exception of the centrifugal compresor, which uses the same amount. Centrifugal compressors are used for large installations, and usually where the indirect system is required. The driving mechanism can be a steam turbine or electric motor. The steam-jet system is used where steam is available and cooling water can be had in large quantities. It will be noted by referring to Fig. 16 that all systems using compressors have a common characteristic, namely, that the capacity varies with the evaporating temperature. Not only can the equipment be selected to produce a given result, but the performance can be predicted under varying load conditions by the simple expedient of using the vari able of evaporating temperature as the abscissa, and the load or capacity as the ordinate in a series of curves. Manufacturers of compressors and cooling coils furnish performance data for apparatus that can be plotted in the form of curves similar to those shown in Fig. 18. The per formance of a compressor is plotted as a series of curves, each curve being drawn for a given condensing pressure. The performance of a direct-expansion coil at two different air velocities is plotted on the same graph. The operating point will be, of course, where the two curves cross. Data given in Table 9 illustrate two types of conditioned enclosures having the same total load of 148,000 Btu per hour, but with two different ratios of sensible to total heat. In the case of the office with a ratio of 82 percent sensible to total heat, the operating point A in Fig. 18 is found to be 42.2 F evaporating temperature, with a face velocity, of 500 fpm. In the case of the restaurant, with a ratio of 69.5 percent sensible to total heat, the air velocity is lowered to 300 fpm, and the evaporating temperature is lowered to 34.4 F as shown in point B of Fig. 18. In order to obtain the same capacity, a larger condensing unit is used. This illustration assumes zero pressure drop through the suction line. The pressure drop can be taken intoaccount by shifting the compressor performance curves by the amount of pressure drop expressed in Fahrenheit degrees. REFERENCES 1 P. C. Scofield: Air cycle refrigeration (Refrigtraling En gineering, Vol. 57, June 1949, p. 558). * A. R. Mumford and A. A. Markson: Application and econ omy of steam jet refrigeration to air conditioning (ASHVE Transactions, Vol. 44, 1938, p. 33). * A. A. Berestoeff: A new development in absorption re frigeration (Refrigerating Engineering, Vol. 57, June 1949, p. BIBLIOGRAPHY Refrigerating Data Book, Vol. 1 (American Societt or Refrigerating Engineers). H. J. Macintire: Refrigeration Engineering (John Wiley A Sons, New York, 1937). N. R. Sparks: Theory of Mechanical Refrigeration (McGrawHill Book Co., New York). B. F. Raber and F. W. Hutchinson: Refrigeration and Air Conditioning Engineering (John Wiley A Sons, New York. 1945). J. A. Moyer and R. U. Fitts: Refrigeration (McGraw-Hill Book Co., New York). W. R. Hainsworth: Refrigerants and absorbents (Refrig erating Engineering, August, September 1944). B. H. Jennings and S. R. Lewis: Air Conditioning and Refrig eration (International Textbook Company, Scranton, 1944). Jordan and Priester: Refrigeration and Air Conditioning (Prentice-Hall, Inc., New York, 1948). CHAPTER 39 THE HEAT PUMP History and Development Basic Circuits; Fundamentals; Types; Heat Sources and Sinker Performance Characteristics; Tem perature Level*; System Balance; Seasonal Performance; Annual Operating Cost; Components; Applica tion; Criteria for feasibility; Design and Selection; Systems for Larger Buildings; Sensible, Latent, and Low-Side Heat Storage HE term heat pump as applied to a year-rcund air- History in the United States Tconditioning system, commonly denotes a system in Pioneering work on heat-pump development in the United which refrigeration equipment is used in such manner thatStates was conducted through the early 1930's mainly under heat is taken from a heat source and given up to the con ditioned space when heating service is wanted, and is re sponsorship of a few investor-owned electric utility com panies, with the objective of promoting wider use for their moved from the space and discharged to a heat sink when service. The character of electric load from sum cooling and dehumidification are desired. The thermal cycle is identical with that of ordinary refrigeration, but the ap mer air conditioning, after a record of steady growth, was then already making evident its likelihood of creating new plication is concerned alike with the cooling effect produced P^ak HpmanHs on utility systems. This prompted the in at the evaporator and with the heating effect produced at vestigation of methods whereby such load could be made the condenser. In some applications, both the heating and more favorable both to the customer and to the electric the cooling effects obtained in the cycle are employed simul company. taneously. The first major installation in the United States was made The heat-pump principle is also applied for purposes other about 1931, for a new 3,800,000 cu ft office building of than air conditioning, such as: supply of domestic hot water Southern California Edison Company in Los Angeles. Re in residences and commercial buildings, recovery of low- frigeration equipment of the already existing air-condition temperature heat as a useful byproduct in industrial opera ing system was modified to permit heat-pump operation tions, and disposal of heat from processes involving evapo with outdoor air in a conventional cooling tower serving as ration of water or other fluids at depressed temperature heat source. An installation made in Salem, New Jersey* in and pressure. Defrosting of evaporator coils in refrigerating 1934 was the first commercial heat pump in this country systems by intermittently admitting hot gas directly from designed for complete heating and cooling. A number of the compressor is another application of the heat-pump buildings occupied by subsidiaries of American Gas and method. Descriptions and diagrams of various heat-pump Electric Company in Ohio, West Virginia, Virginia, and systems and criteria for feasibility are given later in this Kentucky were equipped during the middle and late 1930's. chapter. The home office of Louisiana Power and Light Company at HISTORY AND DEVELOPMENT Over a century ago, William Thomson (Lord Kelvin)* sug gested the use of a compressor as a warming engine to heat buildings, as an alternate for combustion of fuels. Somewhat prior to 1910 Dr. S. Z. Ferranti, Prof. J. Perry and Mr. T. V. Morley, in England, strongly advocated the undertaking of development in the design and.application of heat pumps, but the economic outlook, then unfavorable, deferred fur ther investigation. It was not until a paper on the principles and economics of heat pumps, prepared in October 1928 by T. G. N. Haldane, was presented before the British Institute of Electrical Engineers in December 1929,* that develop ment was started in the United States. Haldane reported tile trial results .from early installations made in Scotland. Ap plication for water heating was then as prominent abroad as use for space heating. Dr. A. R. Stevenson, Jr. was an early advocate in. this country, having first presented the possibility to the Re frigeration and Gas Committee of the American Gas Asso ciation in April 1926, and later to the Franklin Institute* The concept held in this country today, that heat pumps for the large majority of space-heating applications should Algiers, Louisiana was equipped in 1936, using purchased city water as source of heat. An office building* of United Illuminating Company at New Haven, Connecticut, erected in 1939, was equipped with a water-source heat pump. It utilized as a standby heating system a large water storage t^nk equipped with immersion electric heating elements. Throughout this early period all installations were custom built, employing standard equipment components modified where necesary to suit the dual-purpose functions. After World War II the use of heat pumps increased, particularly with the introduction of unitary equipment, in which the re frigeration system, fans, pumps, and the necessary automatic controls were assembled within a angle housing. In recognition of the status attained by beat pumps and their growing engineering importance, the American So ciety or Heating and Ventilating Engineers appointed a Technical Advisory Committee on the Heat Pump in 1947. An independent group from the electric utility industry, known as Joint AEIC-EEI Heat Pump Committee, was founded in February 1947. This body has worked closely with equipment manufacturers, giving advice on matters re lated to suitability of equipment types and control methods, be regarded essentially as year-round air-conditioning de to experience resulting from trial installations, and espe vices, did not become generally accepted until about 1950. cially to the characteristics of electric loads. It has also under- 581