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SIMPLE APPARATUS FOR REFRIGERATION EXPERIMENTS M. A. GOVINDA RAU' and M. G. SUBBA RAU University of Madras, Madras, India
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usual equipment commercially available for refrigeration experiments of one-half ton or more capacity, and generally using ammonia as a refrigerant, are rather elaborate and costly. A simple piece of equipment can easily be set up in any engineering laboratory, using the ordinary, readily available household Freon refrigeration units of about one-fourth ton capacity. The detachable evaporator coil is mounted inside a coil of three-quarter-inch copper tube forming a double-tube heat exchanger system. The usual aircooled condenser in such a unit is similarly replaced by another double-coil spiral heat exchanger. With the unit used in our chemical engineering laboratory, each coil had 10 turns of 18-inch diameter, making the total leugth of the tube about 50 feet. The evaporator coil mas completely lagged and embedded inside a cylindrical wall (3 inches thick) of lagging of asbestos magnesia, and painted over with aluminum. The condenser coil was only painted with aluminum. Water from a constant-level overhead tank was passed through the two coils so that the discharge from the evaporator coil was cooled and that from the condenser coil heated up. I t is simple to determine the heat absorbed in one and rejected in the other. From the usual pressure gages supplied with such equipment, all necessary data for calculating the c.0.p. (coefficient of ~erformance)can be obtained. An energy meter in EDITOR'SNOTE: We regret to note that one of the authors of this paper, Dr. M. A. Govinda Rau, died suddenly in November of last year, after this manuscript was submitted.
the electrical mains gives a measure of the actual power consumed and thus the actual c.0.p. in the system can be calculated. It is obvious that the same experiment can also be used t o illustrate the working of a heat pump. I n this case, heat is pumped from one stream of water to another, making the former cold and the latter hot. Indeed, since both streams come from the same source, the experiment illustrates a method of pumping heat from one part of the liquid to another. The efficiency of this heat pump can be evaluated from the same data, as shown in the calculations. A specimen set of data obtained with this equipment is given below. Refrigerant used: Freon 12. (Thermodynamic data for Freon 12 taken from Perry, "Chemical Engineers Handbook," 3rd Edition, 1951, p. 261.) (1) Condenser pressure: 130 p.s.i.g. (2) Evaporator pressure: 26 p.8.i.g. (3) Heat absorbed in evaporator: (a) Inlet temperature of water: 8 9 T . (b) Outlet temperature of water: 71°F. (c) Rate of water flow: 202 lhs./hr. ( d ) Heatabsorbed: 202.5 X l X (89-71) 3645 B.t.u./hr. (4) Heat rejected in condenser: (a) Inlet temperature: 89-F. (b) Outlet temperature: 9 7 T . (el Rate of flow of water: 495 lhs./hr. id) Hest rejected: 495 X 1 X (9i'-89) 3960 B.t.u./hr:
A . Motor; B , eomnressor: C, condenser; D, liquid receiver; E, throttling valve; F, eaaporator; 0 , evaporator prcsmre gage: gape; I , prearure svitch (Presaostat).
H , condenser pressure
JULY, 1955 (5) Power consumed: 0.533 kw.-hr./hr. 1820 B.t.u./hr.
REFRIGERATION CALCULATIONS Heat ahsorbed/lh. of Freon = Hc - Ha = Hc - H.4 = 81.28 - 33.65 = 47.63 B.t.u. Mechanical work done/lb. of Freon (assuming adiabatic compression of dry saturated vapor from C ) = HD - H c = 90.25 - 81.28 = 8.97 B.t.u. Theoretical c.0.p. = Hc -- - H B H" - H c e 47.63 - - - 5.3 8.97 Actual c.0.p. = E heat s,bsbsorhed in evaporator work done
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Relative c.0.p. (actual/theoretical) =
2 - = 38% 5.3
HERT-PUMP CALCULATIONS Heat delivered/lb. of Freon = HD - H A = 90.25 - 3 3 . 6 5 = 56.60 B.t.u. Mechanical work done/lb. of Freon (assuming adiabatic compression of dry saturated vapor from C ) = HD - Hc = 90.25 - 81.28 = 8.97 B.t.u. HD - H A 56.60 - 6.32 Theoretical e.o.p. = -- HD - Hc 8.97 heat delivered in condenser Actual .le.o.p. = work done -= 2.175 - 3960 1820 Relative c a p . = actual/theoretical 2.175 - 34.4% 6.32
Besides providing a simple quantitative experiment, this apparatus alloms the students to get acquainted with the automatic pressure controls and throttle adjustment which are supplied with such units.
