IITK Compressor Analysis
Transcript of IITK Compressor Analysis
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
ME340A: Introduction to R&AC 25-10-2019
Instructor: Prof. Sameer Khandekar, SL-109, ME/IITK. 2
COMPRESSORS IN HVAC
The compressor is the heart in a refrigerationsystem» Only major component that has moving parts
Compressors have a cooling capacity and aCOP/EER» Compressors are often rated by cooling capacity
and COP/EER based on assumed system statepoints (e.g. 1 ton compressor with EER of 12)
The task of the compressor in a VC system is to provide continuous mass flow rate of refrigerant from a low pressure level to a high pressure level
Miniature compressor by Aspen (200 W)
REVIEW OF COMPRESSOR TYPES
Positive displacement compressors – increase pressure by displacing fluid in a shrinking volume
Reciprocating compressors
Rotary compressorso Rolling pistono Rotary sliding vaneo Screw
Scroll compressors
Low fluid volumes, high pressure ratios
ME340A: Introduction to R&AC 25-10-2019
Instructor: Prof. Sameer Khandekar, SL-109, ME/IITK. 3
REVIEW OF COMPRESSOR TYPES
Centrifugal compressors• Belongs to turbomachinery class• Increases pressure by adding kinetic energy to the fluid• High velocity impeller
High fluid volumes, low pressure ratiosStaging is required to reach higher pressure ratios
REVIEW OF COMPRESSOR TYPES
ME340A: Introduction to R&AC 25-10-2019
Instructor: Prof. Sameer Khandekar, SL-109, ME/IITK. 4
REVIEW OF COMPRESSOR TYPES:Differences with respect to Motor Location
Hermetic:» Welded steel shell houses both compressor and motor
» Usually smaller sizes; high manufacturing cost of larger sizes
Semi-hermetic:» Common, but bolted, housing for compressor & motor
» Intermediate capacities
» Can be opened and serviced, unlike fully hermetic
Open Drive:» Motor external to shell
» Largest capacities
» Shaft seal necessary
WHAT ARE COP AND EER?
• Efficiency of a refrigeration system is measured in COP or EER (SEER in United States for air conditioners)
• COP is Coefficient of Performance
𝐶𝑂𝑃 = 𝑃𝑜𝑤𝑒𝑟 𝑜𝑢𝑡𝑝𝑢𝑡
𝑃𝑜𝑤𝑒𝑟 𝑖𝑛𝑝𝑢𝑡
𝐶𝑂𝑃 = 𝐶𝑜𝑜𝑙𝑖𝑛𝑔 𝑝𝑟𝑜𝑣𝑖𝑑𝑒𝑑 (𝑘𝑊)
𝐶𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑜𝑟 𝑝𝑜𝑤𝑒𝑟 (𝑘𝑊)
• COP is an instantaneous measure of performance
ME340A: Introduction to R&AC 25-10-2019
Instructor: Prof. Sameer Khandekar, SL-109, ME/IITK. 5
WHAT ARE COP AND EER?
• EER is Energy Efficiency Ratio
𝐸𝐸𝑅 = 𝐶𝑜𝑜𝑙𝑖𝑛𝑔 𝑝𝑟𝑜𝑣𝑖𝑑𝑒𝑑 (𝐵𝑇𝑈/ℎ𝑟)
𝐶𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑜𝑟 𝑝𝑜𝑤𝑒𝑟 (𝑊)
• Compressor power is often expressed as kW/ton of cooling (or heating)
𝑘𝑊
𝑡𝑜𝑛 𝑜𝑓 𝑐𝑜𝑜𝑙𝑖𝑛𝑔=
12
𝐸𝐸𝑅=
12
𝐶𝑂𝑃 × 3.412
• e.g. Compressor power consumption for a 1 ton A/C with EER of 12 would be 1 kW
SEER RATING OF AIR CONDITIONERS
• SEER is Seasonal Energy Efficiency Ratio
• Primarily used to describe performance of home air conditioners in the United States
𝑆𝐸𝐸𝑅 = 𝐶𝑜𝑜𝑙𝑖𝑛𝑔 𝑜𝑢𝑡𝑝𝑢𝑡 𝑓𝑜𝑟 𝑎 𝑡𝑦𝑝𝑖𝑐𝑎𝑙 𝑐𝑜𝑜𝑙𝑖𝑛𝑔 𝑠𝑒𝑎𝑠𝑜𝑛
𝑃𝑜𝑤𝑒𝑟 𝑖𝑛𝑝𝑢𝑡 𝑓𝑜𝑟 𝑡ℎ𝑒 𝑠𝑎𝑚𝑒 𝑝𝑒𝑟𝑖𝑜𝑑
• Indicates the maximum possible performance efficiency for the air conditioning system
• In India, we used “Star Ratings” or ISEER
• SEER depends on compressor type, condenser size and TXV
ME340A: Introduction to R&AC 25-10-2019
Instructor: Prof. Sameer Khandekar, SL-109, ME/IITK. 6
SEER RATING OF AIR CONDITIONERS
• A 20 SEER 5-ton home A/C operates 8 hours per day for 120 days in a year
5 ton x 12,000 BTU/hr x 960 hours/year = 5,76,00,000 BTU/year
5,76,00,000 /20 = 28,80,000 W-hr/year = 2,880 kWh/year
2,880 * 0.2 ($/kWh) = $576 / year of electricity cost
• For a 15 SEER A/C unit, electricity cost becomes $768
• Higher SEER units are more energy efficient!
IDEAL THERMAL CYCLE
𝑄 = ℎ − ℎ
𝑊 = ℎ − ℎ
Isentropic compression
𝐶𝑂𝑃 =𝑄
𝑊
ME340A: Introduction to R&AC 25-10-2019
Instructor: Prof. Sameer Khandekar, SL-109, ME/IITK. 7
REAL WORLD PERFORMANCE
Several irreversible processes impact actual
performance
Efficiencies are used for a more general description of
the processes in a compressor.
» V : Volumetric efficiency [-] (Capacity)
» S : Isentropic efficiency [-] (Energy)
These efficiencies depend ONLY on the compressor
inlet and outlet conditions.
Therefore, efficiencies are more suitable for comparing
compressors than COP/EER.
VOLUMETRIC EFFICIENCY (V)
maximum volume of suction processnumber intake strokes per unit time (e.g., compressor RPM)
Ratio between the actual volume flow rate in the
compressor inlet and to theoretical flow rate:
density of inlet gas𝜂 =
��
��=
��
�� 𝑉 𝜌
refrigerant mass flow rate
ME340A: Introduction to R&AC 25-10-2019
Instructor: Prof. Sameer Khandekar, SL-109, ME/IITK. 8
VOLUMETRIC EFFICIENCY (V)
Theoretically, the volumetric efficiency is equal to one
However, in general practice several aspects affect
the volumetric efficiency
» re-expansion of gas in clearance volume
» pressure drops in valves and flow passages
» leakage from compression chamber
» internal superheat
» back-flow through valves
VOLUMETRIC EFFICIENCY (V)A reciprocating compressor has a bore size of 8 cm, stroke of 12
cm and operates at 3000 RPM. However, a clearance of 0.5 cm
has to be given for operation of valves and 8% gas escapes
because of dimensional tolerance between piston and bore.
• Displacement volume = π/4 D2 L = 6 x 10-3 m3
• Theoretical volume flow rate = 0.03 m3/s
• Actual displacement volume = (π/4* 0.082 *0.115)* 0.92 = 5.3
x 10-3 m3
• Actual volume flow rate = 0.0266 m3/s
• Volumetric efficiency = 0.0274/0.03 = 88.6%
ME340A: Introduction to R&AC 25-10-2019
Instructor: Prof. Sameer Khandekar, SL-109, ME/IITK. 9
VOLUMETRIC EFFICIENCY (V)
For reciprocating compressors, the volumetric efficiency
is in the following range:
» AC-conditioning (low pressure ratios) [0.7 - 0.9]
» Cooling (medium pressure ratios) [0.6 - 0.8]
» Freezing (high pressure ratios) [0.4 - 0.7]
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
INTRODUCTION TO ENGINEERING EQUATION SOLVER (EES)
Parametric tables
Arrays and functions
Polynomial curve fit
Linear regression (to be studied in next class)
NEXT CLASS
Example of compressor design
Compressor performance simulation
Centrifugal compressors
Novel compressor concepts