Chapter 17A: COMPRESSORS AND EXPANSION DEVICES Agami Reddy (rev- May 2017) Compressor types Reciprocating compressors: - volumetric efficiency - compressor efficiency Rotary compressors Scroll Screw Centrifugal Expansion devices: capillary, TXV HCB 3- Chap 17A: Compressors and Exp Devices

HCB 3- Chap 17A: Compressors and Exp Devices At the heart of the vapor compression air conditioner is the COMPRESSOR Two general categories: Positive displacement- increases the pressure of the vapor by reducing the volume of the compression chamber Centrifugal- increases the pressure of the vapor via exchange of angular momentum between the rotating element and the refrigerant vapor https://www.youtube.com/watch?v=pT2Y367W9v8 Reciprocating Rotary Screw Scroll https://www.youtube.com/watch?v=rB6mv9_3FO8&playnext=1&list=PLoXgevn2SQ_Z4mCf9C-Sw6LtH0CW0dozX&feature=results_main HCB 3- Chap 17A: Compressors and Exp Devices

HCB 3- Chap 17A: Compressors and Exp Devices Fig. 17.1 Polytropic compression process p–v diagram used for chiller compressor work calculation. HCB 3- Chap 17A: Compressors and Exp Devices

Reciprocating Compressors Most popular Sizes: ½ to 200 T Familiar piston/ compressor Moderate compression ratios 7:1 Speeds- up to 3600 rpm Different types - single acting - single/multiple cylinder Hermetic Open HCB 3- Chap 17A: Compressors and Exp Devices

HCB 3- Chap 17A: Compressors and Exp Devices Expansion and compression are polytropic: pvn = cte Distinguish between theoretical and actual polytropic exponent 1-2 Intake or suction stroke 2-3 compression stroke 3-4 Exhaust or discharge stroke Fig. 17.2 Reciprocating chiller: piston-and-valve arrangement and corresponding p–V diagram. HCB 3- Chap 17A: Compressors and Exp Devices

Capacity loss due to high temperature and re-expansion What happens to mr as the suction pressure is decreased? As psuction decreases, also decreases Eventually we will reach a point when Also, temperature of compressed gas much higher than the incoming gas HCB 3- Chap 17A: Compressors and Exp Devices

Efficiency Measures (a) Volumetric efficiency Measure of effectiveness of compressor as a refrigerant “mover”, > 90% (a) Volumetric efficiency (17.2) (b) Compressor efficiency (same as isentropic effy but with motor effy included) Measure of thermodynamic efficacy of the compression process: 75-80% HCB 3- Chap 17A: Compressors and Exp Devices

HCB 3- Chap 17A: Compressors and Exp Devices Rotary Compressors Rolling piston Fig. 17.3 Rotating vane: 2 vane and 4 vane HCB 3- Chap 17A: Compressors and Exp Devices

HCB 3- Chap 17A: Compressors and Exp Devices Primary applications: Refrigerators Room air conditioners Automotive air conditioners Sizes: 1/6 – 3 Tons Characterized by CIRCULAR or ROTARY motion to compress refrigerant Compression ratios limited to 7:1 (otherwise too much leakage) Good part load performance (down to 20%) Inherently balanced Relatively light and have long life Relatively low starting torques and need inexpensive motors Have high volumetric efficiencies Low noise Low vibration HCB 3- Chap 17A: Compressors and Exp Devices

HCB 3- Chap 17A: Compressors and Exp Devices Scroll compressors https://www.youtube.com/watch?v=iU5PVinDyNQ Fig. 17.4 Operation of a scroll compressor at different angles of rotation Scroll is the curve formed by end of a string as it is unwrapped from a cylinder HCB 3- Chap 17A: Compressors and Exp Devices

First manufactured by Hitachi in 1983 First patented in 1905 First manufactured by Hitachi in 1983 US manufacturers: Copeland and Trane Has potential to increase efficiencies by 10-15% Large compression ratios: 20:1 Principal applications 1.5 – 10 Tons (can go up to 100 T) Capacity control: variable discharge ports or speed control Quiet operation, low vibration and wear Main Features: Fixed scroll Orbiting scroll No suction or discharge valves Requires two revolutions to compress refrigerant Needs precision machining COP: 2.9 – 3.2 HCB 3- Chap 17A: Compressors and Exp Devices

HCB 3- Chap 17A: Compressors and Exp Devices Screw Compressors Developed in 1930s, appeared in 1960s Capacities: 2- to 1300 Tons Rotational speeds of male rotor ~ 3600 rpm Intermediate between reciprocating and centrifugal Good part load operation (down to 10% full load) https://www.youtube.com/watch?v=stjvbAO_6JQ Fig. 17.5 Dual-screw compressor HCB 3- Chap 17A: Compressors and Exp Devices

HCB 3- Chap 17A: Compressors and Exp Devices Fig. 17.6 Typical screw compressor efficiency curves for R-717. Vi is the compressor volume ratio which is varied by a slide valve that changes the size and shape of the discharge port. HCB 3- Chap 17A: Compressors and Exp Devices

Centrifugal Compressors 200 kW to 10 MW https://www.youtube.com/watch?v=s-bbAoxZmBg HCB 3- Chap 17A: Compressors and Exp Devices

Centrifugal Compressor Dynamic compression based on speed and type of impeller Can move large refrigerant volumes at moderate lift Compression ratio per stage up to ~4:1 Large capacities (up to ~20,000 tons) Multistage compression needed for ice-making and other higher lift applications Capacity control by inlet guide vanes or speed control Impeller schematic Chiller Plant Multistage compressor HCB 3- Chap 17A: Compressors and Exp Devices Slide 15

HCB 3- Chap 17A: Compressors and Exp Devices Centrifugal vs Reciprocating Performance 1) Effect of evaporator temperature for centrifugal chillers: Capacity reduces from 240 to 100 Tons when evaporator temp reduces by 100 F 2) Centrifugal compressor will maintain a near constant evaporator temp. when cooling load changes Fig. 17.8 Comparison of centrifugal and reciprocating compressors: effect of evaporator temperature on refrigeration capacity HCB 3- Chap 17A: Compressors and Exp Devices

HCB 3- Chap 17A: Compressors and Exp Devices Centrifugal vs Reciprocating- Effect of Condensing Temperature Fig. 17.9 (a and b) Comparison of centrifugal and reciprocating compressors: effect of condensing temperature on refrigeration capacity and brake horsepower The condenser temperature of centrifugal compressors has a much larger effect on cooling capacity. This underlines importance of having a cooling tower (air cooled condensers adequate for reciprocating compressors) Power consumed by centrifugal compressor decreases with condenser temperature, but that of reciprocating compressor actually increases HCB 3- Chap 17A: Compressors and Exp Devices

HCB 3- Chap 17A: Compressors and Exp Devices Fig. 17.10 Comparison of centrifugal and reciprocating compressors: effect of speed on (a) refrigeration capacity and (b) brake horsepower. The evaporator and condensing temperatures are held constant HCB 3- Chap 17A: Compressors and Exp Devices

HCB 3- Chap 17A: Compressors and Exp Devices Table 17.1 Comparison of Different Chiller Compressor Types Chiller Type Capacity Range (kW) Operating Cost Full-Load COP Part-Load Efficiency Part-Load Limit Compression Ratios Up to Recip 2-600 High 2.5-4.0 Very good 30% 10:1 Rotary vane 0.5-10 Low 3.0 – 4.0 20% 7:1 Scroll 5-200 Medium 2.9-3.2 Good 50% Twin-screw 150-4000 5.0-7.0 5% 15:1 Centrifugal 400-10,000 Poor 10% 3:1 Numbers shown are approximate HCB 3- Chap 17A: Compressors and Exp Devices

HCB 3- Chap 17A: Compressors and Exp Devices Ways to control capacity Cylinder loading Moveable slot at compressor entrance Inlet guide vanes to compressor Variable speed motors Hot gas bypasses HCB 3- Chap 17A: Compressors and Exp Devices

HCB 3- Chap 17A: Compressors and Exp Devices Expansion Devices Expansion devices serve two purposes in A/C: - Reduce pressure of liquid refrigerant - Regulate the flow of refrigerant to the evaporator   Expansion devices found in A/C systems: - Capillary tubes (small units) - Short-tube orifices (Large centrifugal chillers) - Valves (medium sized- widely used) (i) Thermostatic expansion valves (TXV) (ii) Electronic expansion valves (EEV) (iii) Float valves HCB 3- Chap 17A: Compressors and Exp Devices

HCB 3- Chap 17A: Compressors and Exp Devices Capillary Tubes - Used exclusively on small refrigeration systems - Used on older residential A/C - Valve replaced by long thin tube - Tube allows flow of liquid more readily than vapor - Operates best with one set of conditions - Simple, low cost, no moving parts - Consists of a small diameter (0.02 – 0.2 inch) line that connects outlet of the condenser to inlet of evaporator Length may vary from 3 to 5 feet Disadvantage; small tube subject to clogging HCB 3- Chap 17A: Compressors and Exp Devices

HCB 3- Chap 17A: Compressors and Exp Devices Thermostatic Expansion Valve Device to control the mass flow rate of refrigerant liquid entering the evaporator Its function is to assure that refrigerant is superheated vapor before entering compressor Misnomer: control not by refrigerant temperature but by magnitude of refrigerant vapor superheat leaving evaporator Fig. 17.13 Sketch slowing the location, operation, and control of a thermostatic expansion valve. HCB 3- Chap 17A: Compressors and Exp Devices

HCB 3- Chap 17A: Compressors and Exp Devices Major components Filler bulb: Filled with same or similar refrigerant to that used in system Capillary tube: used to transmit pressure (superheat) signal back to diaphragm Diaphragm: Focus point of force inside valve Spring: keeps valve closed until pressure above diaphragm overcomes the spring force plus evaporator pressure HCB 3- Chap 17A: Compressors and Exp Devices

HCB 3- Chap 17A: Compressors and Exp Devices Outcomes Understanding the importance of compressors in VC systems Knowledge of the different generic categories of compressor types and understanding differences between them in capacity, efficiency, part-load Understanding how a reciprocating chiller operates and knowledge of relevant performance measures Understanding how rotary, screw and scroll compressors operate Understanding of how centrifugal compressors operate Knowledge of different ways of capacity control Knowledge of interaction of cooling capacity and evaporator and condenser temperatures for reciprocating and centrifugal chillers Understanding of the purpose of expansion devices Understanding the operating principles of capillary and TXV HCB 3- Chap 17A: Compressors and Exp Devices