ME340A: Introduction to R&AC 25-10-2019 Instructor: Prof. Sameer Khandekar, SL-109, ME/IITK. 1 ME 340A - Introduction to Refrigeration and Air Conditioning Lecture 22: Compressor Analysis Dr. Abhijit A Sathe IIT Kanpur E-mail: [email protected]ABOUT ME M.Tech from IIT Madras at Refrigeration and Air Conditioning Laboratory PhD in Mechanical Engineering from Purdue University Thesis: Compressor analysis for a miniature refrigeration system for electronics cooling • Parker Hannifin Corporation – Liquid cooling of Power Electronics (7 years) • Goodman Manufacturing – Air Conditioning Engineer (9 months) • Currently at SIDBI Innovation and Incubation Centre, IIT Kanpur working on entrepreneurship development
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ME340A: Introduction to R&AC 25-10-2019
Instructor: Prof. Sameer Khandekar, SL-109, ME/IITK. 1
ME 340A - Introduction to Refrigeration and Air Conditioning
For scroll and screw compressors, the volumetric efficiencies
are generally higher due to absence of valves.
For centrifugal compressors, the volumetric efficiency
is not defined.
ISENTROPIC EFFICIENCY (S)
Ratio of actual compressor power consumption to power
consumption needed for an isentropic and reversible
compression process
s = f ( heat loss, pressure drop, friction, suction superheat, motor speed)
p2T
cs
h
W
˙ r 2sm h1 2
Tcond
Tevap 1
2s
s𝜂 =
ℎ − ℎ
ℎ − ℎ
ME340A: Introduction to R&AC 25-10-2019
Instructor: Prof. Sameer Khandekar, SL-109, ME/IITK. 10
ISENTROPIC EFFICIENCY (S)
For hermetic and semi-hermetic compressors the motor
efficiency is included in an “overall isentropic efficiency”.
For reciprocating compressors, the isentropic
efficiency ranges are
» Small size hermetic compressors (domestic appl.) [0.4 - 0.6]
» Medium size (semi-) hermetic compressors [0.5 - 0.7]» Large size open compressors (at full load) [0.6 - 0.8]
• Isentropic efficiencies of rotary compressors are
generally higher than for reciprocating compressors
ACTUAL CYCLE IS FAR MORE COMPLEX
Several irreversible processes affect system performance
ME340A: Introduction to R&AC 25-10-2019
Instructor: Prof. Sameer Khandekar, SL-109, ME/IITK. 11
COMPRESSOR OIL
Compressor oil is a necessary component in all vapourcompression systems
• Oil reduces friction on metal parts, reducing wear on the compressor and prolongs the life of the system.• Piston rings of reciprocating compressor
• Sliding vanes of rotary compressor
• Oil also maintains a seal between the high and low side of the compressor.
• Oil also acts as a noise dampener within the compressor and transfers heat away from moving and rotating parts within the compressor.
COMPRESSOR OIL
Important oil properties• Miscibility: Ability of oil to mix and move with refrigerant
• Viscosity: Measure of oil’s flow resistance. Bigger compressors with larger gaps need thicker oil
• Dielectric strength: Oil must sustain very high electrical voltages
• Chemical stability: Oil’s ability to prevent chemical reactions with the refrigerant at high temperatures
• Hygroscopic: A tendency for refrigeration oils to absorb moisture from the atmosphere
ME340A: Introduction to R&AC 25-10-2019
Instructor: Prof. Sameer Khandekar, SL-109, ME/IITK. 12
TYPES OF COMPRESSOR OIL
• Mineral oils: CFC refrigerants such as R12, R13, R113, R114 and R115 are using mineral oil or alkylbenze as lubricant
• Polyol esters (POE): Synthetic oil used with new HFC refrigerants such as R23, R32, R134a, R407A, R407C and R410A
• Polyalkylene Glycol (PAG): A synthetic oil primarily used in R-134a automotive air conditioning systems
• Polyvinyl Ether (PVE) – a synthetic oil that is being used as an alternative to POE oil.
WHY STUDY OF OIL IS IMPORTANT?
• In automotive engines motor oil stays in the crankcase
• However, in refrigeration systems oil travels with refrigerant through the entire system
This may lead to shortage of oil in compressor causing lack of proper lubrication
Oil tends to deposit inside evaporator tubes causing loss of cooling capacity
Oil in the refrigerant may cause restrictions in expansion valves or capillaries
Oil may break down at hot spots in the system and react with refrigerant to form acids
ME340A: Introduction to R&AC 25-10-2019
Instructor: Prof. Sameer Khandekar, SL-109, ME/IITK. 13
OIL SEPARATORS
• Traps oil from compressor discharge line and returns it to compressor suction
OIL SEPARATORS
• Insulated and sealed shell consisting of baffle plates to separate liquid oil from vapour refrigerant
ME340A: Introduction to R&AC 25-10-2019
Instructor: Prof. Sameer Khandekar, SL-109, ME/IITK. 14
COMPRESSOR OPERATING CONSIDERTIONS
Risk of compressor flooding• Flooding is liquid refrigerant entering the compressor’s
crankcase while the compressor is running.• A compressor is designed to move vapour refrigerant
NOT liquid
• Causes of flooding Wrong TXV settings Overcharging of refrigerant Very low load on evaporator
SUCTION LINE ACCUMULATOR
• A suction line accumulator prevents compressor damage from a sudden surge of liquid refrigerant and oil that could enter the compressor from the suction line
• The accumulator is installed in the suction line close to the compressor.
• It is nothing but a temporary storage tank for storing liquid
ME340A: Introduction to R&AC 25-10-2019
Instructor: Prof. Sameer Khandekar, SL-109, ME/IITK. 15
CAPACITY CONTROL OF COMPRESSOR
Most compressors in regular refrigerators are single speed refrigerators.
They are either “on” or “off” based on the temperature in the refrigerator and the setting in the thermocouple.
Most compressors are designed to handle peak load conditions
Additionally, split motors require high starting current every time the compressor turns ON
ADVANCES IN COMPRESSOR TECHNOLOGY
Inverter Compressor A refrigerant compressor operated with an inverter
Typically a Variable Frequency Drive (VFD) is used with a 3-phase induction motor
Motor speed can be varied to control refrigerant flow rate resulting in more efficient capacity control
Inverter compressor is almost always ON and hence eliminates startup losses
Significantly higher efficiency and reduced electrical consumption
ME340A: Introduction to R&AC 25-10-2019
Instructor: Prof. Sameer Khandekar, SL-109, ME/IITK. 16
INTRODUCTION TO ENGINEERING EQUATION SOLVER (EES)
Software specifically developed for thermal engineering
Developed by Prof. Sandy Klein of University of Wisconsin Madison
Refrigerant property tables in-built, properties available with a simple command
Both Imperial and SI units available
h = enthalpy(R134a, T=25, x=1)
will return enthalpy of saturated vapor of refrigerant R134a at saturation temperature of 25 C
INTRODUCTION TO ENGINEERING EQUATION SOLVER (EES)
Equations need not be written in the form of
Unknown variable = f (known variables)
EES will solve equations in any form,
e.g. Q = m Cp (T2 – T1) will solve for T2 if other variables are known
Formatted equations
Property plots (T-s diagram, P-h diagram, etc.)
ME340A: Introduction to R&AC 25-10-2019
Instructor: Prof. Sameer Khandekar, SL-109, ME/IITK. 17