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Refrigeration:

Back to the BasicsSeminar 32Sponsor: TC10.1, and Refrig.

Committee


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Economizing forIndustrial Refrigeration

Screw Compressors

Joseph W. Pillis P.E.York Refrigeration, (Frick)

Waynesboro, PA

All Rights ReservedCopyright 2001 by YORK International Corporation


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DisclaimerTHIS INFORMATION WAS PRESENTED AT ASHRAE SEMINAR 32

HELD IN CINCINNATI, OH. THE SEMINAR FORMAT IS TO PRESENT

INFORMATION OF CURRENT INTEREST AND TO PROVIDE A VENUE

FOR INTERACTION BETWEEN ASHRAE MEMBERS. THESESEMINARS SHOULD NOT BE CONSIDERED PEER-REVIEWED (OR

THE FINAL WORD ON ANY SUBJECT). ASHRAE HAS NOT

INVESTIGATED, AND ASHRAE EXPRESSLY DISCLAIMS ANY DUTY

TO INVESTIGATE ANY PRODUCT, SERVICE, PROCESS, PROCEDURE,

DESIGN, OR THE LIKE WHICH MAY BE DESCRIBED HEREIN. THE

APPEARANCE OF ANY TECHNICAL DATA OR EDITORIALMATERIAL IN THIS PRESENTATION DOES NOT CONSTITUTE

ENDORSEMENT, WARRANTY, OR GUARANTY BY ASHRAE OF ANY

PRODUCT, SERVICE, PROCESS, PROCEDURE, DESIGN, OR THE

LIKE. NEITHER ASHRAE, THE AUTHORS OR THEIR EMPLOYERS

WARRANT THAT THE INFORMATION IN THIS PRESENTATION IS

FREE OF ERRORS. THE ENTIRE RISK OF THE USE OF ANYINFORMATION IN THIS PRESENTATION IS ASSUMED BY THE USER.

BEFORE MAKING ANY DECISION OR TAKING ANY ACTION ON THIS

SUBJECT, YOU SHOULD CONSULT A QUALIFIED PROFESSIONAL

ADVISOR.


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Economizers

This presentation will cover thefundamentals of economizers as usedwith twin rotary screw compressors.

port location and function

characteristics

sideloads economizers

cycle efficiency

refrigerant comparisons


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Suction process


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Maximum Suction VolumeTrapped

Vs

Vs


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Compression


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Continuation ofCompression


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Beginning of Discharge

Vd


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PrimarySuction


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PrimarySuction


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PrimarySuction


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PrimarySuction


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PrimarySuction


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PrimarySuction


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PrimarySuction


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PrimarySuction


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PrimarySuction


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Discharge

PrimarySuction


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PrimarySuction

Discharge


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Discharge

PrimarySuction

Econo.Inlet


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Economizer Port Location


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The Economizer Port A port placed in the rotor housing can

locate any pressure between suction and

discharge. This port acts as a secondary suction

port.

The additional flow does not reduce theprimary suction gas flow.

The gas mixes in the screw threads and is

carried on to discharge. Compression efficiency is high as

additional frictional power is small.

Like a frictionless compressor


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Uses of the Econo. Port

This secondary suction port is usedin two ways.

Sideloads are generally gas flows

coming from a secondary, (highertemp.) evaporator.

Economizers are used to subcoolcondenser liquid.


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Condenser

Evaporator

Sideload Evaporators

Comp.Secondary Evaporator

Vapor from a secondary evaporator can be drawn intothe compressor economizer port, providing the

secondary evaporator is at a pressure higher

than the econo. pressure.


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96 F

200 psia

-17 F20 psia

Econo.? psia

NH3 Sideload Example -What pressure is the port ?


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Sideload port location for refrigeration application

62% volume = volume reduction ratio 1.6 : 1

100 %Volume

62% Volume


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96 F

200 psia

-17 F20 psia

Econo.? psia

Port pressure ~=(Volume ratio)^k * (P suction)NH3 k ~= 1.29

Port pressure = (1.6^1.29) * 20 psia = 37 psia

20 psia


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96 F

200 psia

-17 F20 psia

8 F37 psia




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With a -17 F primary evaporator temp. a secondaryevaporator can be used as long as the temperature isabove ~ 8 F.

Secondary Evaporator8 F Secondary Evaporator

96 F Condenser

-17 F Evaporator

-17 F Evaporator

Comp.


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R-717

1

10

100

1000

10000

0 100 200 300 400 500 600 700 800 900

Enthalpy (B.T.U. per LB)

AbsolutePressure(PS

IA)


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R-717

1

10

100

1000

10000

0 100 200 300 400 500 600 700 800 900


AbsolutePressure(PS

IA)

Evaporator

Sideport


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96 F

200 psia

- 40 F10 psia

-20 F18 psia




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With a -40 F primary evaporator temp. a secondaryevaporator can be used as long as the temp. is above - 20 F

Secondary Evaporator-20 F Secondary Evaporator

96 F Condenser

-40 F Evaporator

-40 F Evaporator

Comp.


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96 F

200 psia

40 F73 psia

72 F133 psia




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With a 40 F primary evaporator temp. a secondaryevaporator can be used as long as the temp. is above 72 F.

Port pressure is too high to be useful !!!!!!!!!!

Secondary Evaporator72 F Secondary Evaporator

96 F Condenser

40 F Evaporator

40 F Evaporator

Comp.


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96 F

200 psia

-17 F20 psia

8 F37 psia

Effect of LowerCondensing Pressure


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75 F

140 psia

-17 F20 psia

8 F37 psia

Effect of LowerCondensing Pressure


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75 F

140 psia

-17 F20 psia

8 F37 psia




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75 F

140 psia

-17 F20 psia

8 F37 psia

Econo. Pressure is a function of suctionpressure and port location only ! Not discharge

pressure !


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96 F

200 psia

-17 F20 psia

8 F37 psia

What is the Sideload capacity?on NH3

Li id f d


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-17 F20 psia

Secondary

Evaporator37 psia (8F)

P=37 psia

Liquid from cond

100 lbm/min

Zero flow

Li id f d


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-17 F20 psia

Secondary


P=37 psia

Liquid from cond

100 lbm/min

16 lbm/min

Liquid from cond


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-17 F20 psia

Secondary


P=38 psia

Liquid from cond

100 lbm/min

21 lbm/min

Liquid from cond


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-17 F20 psia

Secondary


P=40 psia

Liquid from cond

100 lbm/min

28 lbm/min

Thread packing brings up the pressurein the screw thread


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-40 -20 0 20

0%

100%200%

300%

400%

Sat. Suct Temp. F%

ofSuct.

TonsRefr.

40F

20F

0F

-10F

Sideload Capacityas a % of suction capacity

Sideload Temp.

NH3 Refrig.

Sid l d E Pi i


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Sideload Evaporator Piping

Economizer PipingCondenser

Evaporator

Flash

Economizer

Secondary EvaporatorSecondary Evaporator

Condenser

Evaporator


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Condenser

Evaporator -17F

(20 psia)

Flash

Economizer

An Economizer is a special case of a sideloadevaporator, where part of the condensed liquid is

evaporated to cool the remainder of the condensedliquid.

96F liq

17 F liq(45psia)

18 lbm/min

100 lbm/min

NH3

Comp.


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R-717

1

10

100

1000

10000

0 100 200 300 400 500 600 700 800 900


A

bsolutePressure(PSIA)

Single stage compression

(hg-hf)=458b/lbm100lb

m/min

229 TonsRefrigeration

-17F/96F NH3


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R-717

1

10

100

1000

10000

0 100 200 300 400 500 600 700 800 900


A

bsolutePr

essure(PSIA)

Economizer cycle

(hg-hf)=458b/lbm(hg-hf)=545b/lbm

19 lbm/min

273 TonsRefrigeration

19% increase in

evaporator tons@ same evaporator

massflow

100lb

m/min


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R-717

1

10

100

1000

10000

0 100 200 300 400 500 600 700 800 900


A

bsolutePr

essure(PSIA)

Economizer cycle

19 lbm/min119lb

m/min

100 lbm/min

19% More Tons19% More massflow

Only

9% More Power

100 lbm/min


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119 lbm

96 F200 psia

100 lbm/min

-17 F20 psia

19 lbm/min45 psia

The economizer vapor is at a lowercompression ratio, and adds no

additional friction to the compression


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NH3 Cycle efficiency

-17 F evaporator96 F condensing

Capacity Power HP/TR

229 494 2.16 non-Econ

273 538 1.97 Econo.

19% 9% 9.6% improved


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R-22 Cycle efficiency

-10 F evaporator95 F condensing

Capacity Power HP/TR

106 212 2.0 non-Econ

136 230 1.7 Econo.

28% 8% 17% improved


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Capacity and Power Non-E

0

100

200

300

400

500

600

700

800

-60 -40 -20 0 20 40 60

Evap Temp (F)

TR

&BHP NH3

R507

R22

NH3 BHP

R507 BHP

R22 BHPCapacity

Power

Capacity and Power- Economized


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Capacity and Power Economized

0

100

200

300

400

500

600

700

800

-60 -40 -20 0 20 40 60

Evap Temp (F)

HP

andTR

NH3 Cap

R507 Cap

R22 Cap

NH3 HPR507 HP

R22 HPCapacity

Power


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BHP/TR (E and non-E)

0

1

2

3

4

5

6

-60 -40 -20 0 20 40 60

Evap Temp (F)

NH3 E

R507 E

R22 E

NH3 non

R22 non

R507 non


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BHP/TR (E and non-E)

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

6

-55 -50 -45 -40 -35 -30 -25 -20

Evap Temp (F)

NH3 E

R507 E

R22 E

NH3 non

R22 non

R507 non

Econo. Flash Gas Flow


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Econo. Flash Gas Flow

0

50

100

150

200

250

300

350

400

450

500

-60 -40 -20 0 20 40 60

Evap Temp (F)

Lbm/min

NH3

R507

R22

TR increase E/Non-E


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TR increase E/Non E

0.0%

10.0%

20.0%

30.0%

40.0%

50.0%

60.0%

70.0%

80.0%

90.0%

-60 -40 -20 0 20 40 60

Evap Temp (F)

NH3 Cap

R507 Cap

R22 Cap


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Efficiency Gain w/ Economizing

0.0%

5.0%

10.0%

15.0%

20.0%

25.0%

30.0%

35.0%

40.0%

45.0%

50.0%

-60 -40 -20 0 20 40 60

Evap Temp (F)

NH3

R507

R22


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Economizer Piping

Open flash vessel Flash vessel with HX

Direct Expansion

Open Flash Vessel -


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p

One Compressor.

DX - Shell and Tube


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S e a d ube

Level Controlled Shell and Coil


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If load varies widely andecono is used below 90% SV

use BPR controllingcompressor side

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