ME HVAC Systems, Topic 14, Cooling Equipment

Transcription

1 ME HVAC Systems, Topic 4, Cooling Equipment. Vapor compression cycle (VCC) Ch Introduction to the absorption cycle Ch Evaporative cooling Ch. 20, Section Cooling towers Ch. 7, Section Research and development on alternative cooling approaches Vapor Compression Cycle (VCC) The ideal vapor compression refrigeration cycle is a Rankine cycle operated in reverse. Transition :2 isentropic compression Transition 2:3 isobaric condensation Transition 3:4 isenthalpic expansion Transition 4: isobaric evaporation Real refrigeration effects include non-constant T and P during condensation (subcooling occurs) and evaporation (superheating occurs), and compressor efficiency. Friction losses and heat loss/gain occur in all the components and plumbing. The power you have to put in to the compressor isw in. Capacity is the rate at which energy can be extracted, Q L. The performance measure of the cycle is the coefficient of performance. COP R energy removed energy in h h 2 h 4 h Q W L i Example 4. walks through the calculations for an ideal VCC.

2 In addition to cooling industrial space directly (walk-in freezers, refrigerator trucks), vapor compression cycles are used in HVAC to chill water that is then used to cool air in the air handlers. The coupling of a VCC with water/refrigerant heat exchangers is called a chiller. Many different working fluids refrigerants are available (R2, R34a, R22, R40A). We are fortunate to have access to actual refrigerant properties in EES. Several types of compressors are in common use reciprocating, centrifugal, screw, rotary, and scroll (the scroll compressor was patented by Neils Young, an inventor who retired to Boise in the 990s). The work input for an actual polytropic compression cycle (pv = constant) is n n np ivi p o W in n pi n is the polytropic exponent and in general it is not the ratio of specific heats. Example 7. compares work in using a polytropic compression analysis in comparison with isentropic compression. Specific characteristics of different compressors are related to their performance (reciprocating compressors: rpm, bore, stroke; centrifugal compressors: torque, rpm, rotor radius). Figure 4.2 gives actual performance curves for typical real VCC equipment. Chillers A chiller is equipment to produce chilled water for cooling within building zones. Typically, a VCC unit with the evaporator is water coupled. Because chillers have large thermal inertia they require part load modeling when cycled to meet loads less than their capacity. The part load ratio (PLR) of a chiller is defined as the actual cooling load at any particular time divided by the capacity at load as rated by the manufacturer. Q PLR Q L, act L, Q Q cool The power input to the compressor at part load is then W in Q COP A B PLR CPLR 2 where A, B, and C are curve fit constants from vendor data. Table 4.0 includes these for sample chillers. It is better to use PLR data from the manufacturer of the actual compressor of interest. Example 4.9 illustrates the use of actual PLR data. 2

3

4 4.4 A chiller uses refrigerant 22 and operates between a low-side pressure of 65 psia and a high-side pressure of 250 psia. The chiller capacity is 200 tons. Find the refrigerant flow rate, power input, and COP of this device as well as the evaporator and condenser temperatures? P (psia) T ( o F) h (BTU/LBm) P-h Diagram h (BTU/LBm) EES ASHRAE or HCB tables ṁr = 37,850 LBm/hr = 0.5 LBm/s Ẇi = 5.49 x 0 5 BTU/hr = 6 kw = 26 horsepower COP = 4.37

5 File:D:\HVAC temp\p4.4.ees 3/9/207 7:02:36 PM Page EES Ver : #29: For use only by students and faculty in Mechanical Engineering, Univ. of Idaho, Moscow, Idaho SOLUTION Unit Settings: Eng F psia mass deg COP = h = 73.6 [Btu/lb m ] h2 = 88. [Btu/lb m ] h3 = 0.2 [Btu/lb m ] h4 = 0.2 [Btu/lb m ] mr = [lb m /hr] P = 65 [psia] P2 = 250 [psia] P3 = 250 [psia] P4 = 65 [psia] Phigh = 250 [psia] Plow = 65 [psia] QL = 2.400E+06 [Btu/hr] s = [Btu/lb m -R] s2 = [Btu/lb m -R] s3 = [Btu/lb m -R] T = [F] T2 = 5.3 [F] T3 = 2.7 [F] Wi = [Btu/hr] No unit problems were detected. EES suggested units (shown in purple) for h[] h[2] h[3] h[4] h_ h_ R F Btu/lbm-R P [psia] F 25 F 75 F h [Btu/lbm]

6