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



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




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: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



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



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



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




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%




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



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



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



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



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



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



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



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


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.)




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

IITK Compressor Analysis

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