co2 cascadecomp_studi galileo 1 semi-hermetic compressors for co 2 cascade systems hermann renz...

CENTRO STUDI GALILEO

INDUSTRIA & FORMAZIONE

CENTRO RICERCA AMBIENTE ED IMPRESAUNIVERSITA’ DEGLI STUDI DI MILANO

European seminar: carbon dioxide as a refrigerantMilan, 27th November 2004

CO2 CascadeComp_Studi Galileo 1

Semi-Hermetic Compressorsfor CO2 Cascade Systems

Hermann RenzBitzer Kuehlmaschinenbau GmbH Germany






Semi-Hermetic Compressorsfor CO2 Cascade Systems

Contents

Introduction

Example of CO2 cascade system

Properties of CO2 vs. R22

Requirements on compressor design

Development Stages / Design Features

Summary






Introduction

Due to its environmentally benign characteristics and specific properties CO2 has good potential for:

The extended use in subcritical cascade systems like for example: Supermarket applications Industrial refrigeration systems

Specific features Very high volumetric cooling capacity Favourable TEWI, energy demand & installed cost Reasonable safety requirements






Example of Supermarket Application – MT with HFC / LT with CO2 Cascade






Comparison of Properties –Low Temp CO2 Cascade vs. R22

0

100

200

300

400

500

600

700

800

900

1000

R22 Reference

CoolingCapacity

COP SuctionPressure

DischargePressure

VapourDensity

(LP)

Co

mp

aris

on

CO

2 vs

. R2

2 [%

]

R22 / to -35°C, tc -10°C

CO2 / to -35°C, tc -10°CCompressor displacement counter-proportional to relative cooling capacities






Pressure Ranges –CO2 Cascade vs. R22 and R410A

Comparison of pressure levels CO2 & R410A vs. R22

5

10

15

20

25

30

35

40

0 5 10 15 20

Suction pressure [bar]

Dis

ch

arg

e p

res

su

re [

ba

r]

'

R410ACO2

Cascade

R22






Requirements on Compressor Design

Consideration of CO2 properties High Pressure levels

Strength of housing and crank gear High volumetric cooling capacity

Adaptation of displacement relative to motor power High vapour density / absolute mass flow

Adaptation of working valves & cross sections High gas solubility in oil

General requirements High isentropic efficiency Compact & solid design, use of “standard” parts






Design Features Pressure Levels

High pressure levels of CO2

Compressor housing with high strength pressure

Safety factor “3“ acc. (pr)EN12693

Required pressures (approx.)

HP 40 bar

LP 25 bar (standstill pressure)

Realised compressor construction

Compact design

Closed housing w/o bottom plate






Adaptation of displacement Optimum match to

“standard” motor sizes Potential for the use of

“standard” parts cost issue

Smaller displacement allowing for generously sized crank gear

Design FeaturesDisplacement / Crank Gear (1)






Design FeaturesDisplacement / Crank Gear (2)

Adaptation of displacement by modified piston stroke Allowing for larger piston diameter

Potential for generously sized piston pin and for special piston pin bearing

Connecting rods with closed big end (large diameter) Potential for large cross sections of working valves

Adaptation to high vapour density Improvement of compression process

Reduced piston velocity Improved running smoothness






Lubricant Issues with CO2 Compressors

Relevant to compressor Heavy mechanical load on crank gear High gas solubility in oil Strong oil foaming tendency with pressure fluctuations

High danger of boundary friction

Relevant to (commercial) refrigerating systems Oil return from system / avoiding strong oil migration

and fouling effects in heat exchangers require: Good flow characteristics, low viscosity on suction side Sufficient miscibility with CO2

Suitable Solution especially formulated POE lubricant






Oil / Refrigerant (CO2) Properties

-40 -20 0 20 40 60 80 100Temperatur [ oC]

5

10

20

50

100

Dam

pfdr

uck

[ba

r]

9595

9090

8585

8080

5

10

20

50

100

5001000

10000

kin.

Vis

kosi

tät

[ m

m2 /s

]

100 M-% Öl100 M-% Öl

9595

9090

8585

BSE60K

0 20 40 60 80 100Konzentration [M-% Öl in CO2 ]

-50

-40

-30

-20

-10

0

10

20

30

Tem

pera

tur [

o C]

PhasentrennungPhasentrennung

NominalConditions

Miscibility Limits

Pull down

Pressure / Temperature / Viscosity

BSE60K

Vap

our

Pre

ssur

e [b

ar]

kin.

vis

cosi

ty [

mm

2 /s]

Temperature [°C] Concentration [M-% Oil in CO2]

Phase separation

Tem

pera

ture

[°C

]






Compressor Lubrication

Enhanced centrifugal lubrication Quick oil supply after start Enlarged oil pocket at

the shaft end Separation of oil foam

by gravity principle Reduced oil carry over

minimum oil side flow Potential for oil monitoring

system in oil pocket






Summary

Convincing arguments for the application of CO2 in low temperature cascade systems

Very high volumetric cooling capacity – allowing for: Small compressor displacement Small pipe sizes, favourable installed cost

High specific load on compressor housing and crank gear to be considered High pressure levels & vapour density

Need for Adapting compressor design and Lubrication system

co2 cascadecomp_studi galileo 1 semi-hermetic compressors for co 2 cascade systems hermann renz...

Documents

co2 cascadecomp

studi galileo

cascade slide

plate slide

r410a slide

standard parts slide

crank gear high gas

compressor housing