HCB 3- Chap 17A: Compressors and Exp Devices 1 Chapter 17A: COMPRESSORS AND EXPANSION DEVICES Agami Reddy (July 2016) Compressor types Reciprocating compressors: - volumetric efficiency - compressor efficiency Rotary compressors Scroll Screw Centrifugal Expansion devices: capillary, TXV

Compressors 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 HCB 3- Chap 17A: Compressors and Exp Devices 2 W9v8 -Reciprocating -Rotary -Screw -Scroll FO8&playnext=1&list=PLoXgevn2SQ_Z4mCf 9C-Sw6LtH0CW0dozX&feature=results_main

HCB 3- Chap 17A: Compressors and Exp Devices 3 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 Intake or suction stroke 2-3 compression stroke 3-4 Exhaust or discharge stroke Expansion and compression are polytropic: pv n = cte Distinguish between theoretical and actual polytropic exponent

HCB 3- Chap 17A: Compressors and Exp Devices 5 What happens to m r as the suction pressure is decreased? As p suction decreases, also decreases Eventually we will reach a point when Also, temperature of compressed gas much higher than the incoming gas Capacity loss due to high temperature and re-expansion

HCB 3- Chap 17A: Compressors and Exp Devices 6 Efficiency Measures (a) Volumetric efficiency (b) Compressor efficiency (same as isentropic effy but with motor effy included) Measure of effectiveness of compressor as a refrigerant “mover”, > 90% Measure of thermodynamic efficacy of the compression process: 75-80%

HCB 3- Chap 17A: Compressors and Exp Devices 7 Rolling piston Rotating vane: 2 vane and 4 vane Rotary Compressors

HCB 3- Chap 17A: Compressors and Exp Devices 8 -Low noise -Low vibration 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

HCB 3- Chap 17A: Compressors and Exp Devices 9 Scroll compressors 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 10 -Needs precision machining -COP: 2.9 – 3.2 -First patented in First manufactured by Hitachi in 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 upto 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

HCB 3- Chap 17A: Compressors and Exp Devices 11 Typical Scroll Compressor Efficiency- Ammonia Effect of variable inlet port volume

HCB 3- Chap 17A: Compressors and Exp Devices 12 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)

HCB 3- Chap 17A: Compressors and Exp Devices 13 Centrifugal Compressors 200 kW to 10 MW

Centrifugal Compressor Slide 14 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 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 Chiller Plant Multistage compressor Impeller schematic HCB 3- Chap 17A: Compressors and Exp Devices 14

HCB 3- Chap 17A: Compressors and Exp Devices 15 2) Centrifugal compressor will maintain a near constant evaporator temp. when cooling load changes Centrifugal vs Reciprocating Performance 1) Effect of evaporator temperature for centrifugal chillers: Capacity reduces from 240 to 100 Tons when evaporator temp reduces by 10 0 F

HCB 3- Chap 17A: Compressors and Exp Devices 16 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) Centrifugal vs Reciprocating- Effect of Condensing Temperature Power consumed by centrifugal compressor decreases with condenser temperature, but that of reciprocating compressor actually increases

HCB 3- Chap 17A: Compressors and Exp Devices 17 Chiller Type Capacity Range (kW) Operating Cost Full-Load COP Part-Load Efficiency Part-Load Limit Compression Ratios Up to Recip2-600High Very good30%10:1 Rotary vane0.5-10Low3.0 – 4.0Very good20%7:1 Scroll5-200Medium Good50%10:1 Twin-screw High Highest5%15:1 Centrifugal400-10,000High Poor20%3:1 Table 17.1 Comparison of Different Chiller Compressor Types

HCB 3- Chap 17A: Compressors and Exp Devices 18 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 19 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 20 - 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 Capillary Tubes

HCB 3- Chap 17A: Compressors and Exp Devices 21 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

HCB 3- Chap 17A: Compressors and Exp Devices 22 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 23 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