Refrigeration: Back to the Basics

Seminar 32Sponsor: TC10.1, and Refrig.

Committee

Economizing for Industrial Refrigeration Screw Compressors

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

Waynesboro, PAAll Rights Reserved

Copyright 2001 by YORK International Corporation

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. THESE SEMINARS 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 EDITORIAL MATERIAL 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 ANY INFORMATION 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.

Economizers This presentation will cover the

fundamentals of economizers as used with twin rotary screw compressors. port location and function characteristics sideloads economizers cycle efficiency refrigerant comparisons

Suction process

Maximum Suction Volume Trapped

Vs

Vs

Compression

Continuation of Compression

Beginning of Discharge

Vd

PrimarySuction

PrimarySuction

PrimarySuction

PrimarySuction

PrimarySuction

PrimarySuction

PrimarySuction

PrimarySuction

PrimarySuction

Discharge

PrimarySuction

PrimarySuction

Discharge

Discharge

PrimarySuction

Econo.Inlet

Economizer Port Location

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 the primary 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

Uses of the Econo. Port This secondary suction port is used

in two ways. Sideloads are generally gas flows

coming from a secondary, (higher temp.) evaporator.

Economizers are used to subcoolcondenser liquid.

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 higherthan the econo. pressure.

96 F200 psia

-17 F20 psia

Econo.? psia

NH3 Sideload Example -What pressure is the port ?

Sideload port location for refrigeration application

62% volume = volume reduction ratio 1.6 : 1

100 %Volume

62% Volume

96 F200 psia

-17 F20 psia

Econo.? psia

Port pressure ~=(Volume ratio)^k * (P suction)NH3 k ~= 1.29Port pressure = (1.6^1.29) * 20 psia = 37 psia

20 psia

96 F200 psia

-17 F20 psia

8 F37 psia

Port pressure ~=(Volume ratio)^k * (P suction)NH3 k ~= 1.29Port pressure = (1.6^1.29) * 20 psia = 37 psia

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

Secondary Evaporator8 F Secondary Evaporator

96 F Condenser

-17 F Evaporator

-17 F Evaporator

Comp.

R-717

1

10

100

1000

10000

0 100 200 300 400 500 600 700 800 900

Enthalpy (B.T.U. per LB)

A

b

s

o

l

u

t

e

P

r

e

s

s

u

r

e

(

P

S

I

A

)

R-717

1

10

100

1000

10000

0 100 200 300 400 500 600 700 800 900

Enthalpy (B.T.U. per LB)

A

b

s

o

l

u

t

e

P

r

e

s

s

u

r

e

(

P

S

I

A

)

Evaporator

Sideport

96 F200 psia

- 40 F10 psia

-20 F18 psia

Port pressure ~=(Volume ratio)^k * (P suction)NH3 k ~= 1.29Port pressure = (1.6^1.29) * 10 psia = 18 psia

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.

96 F200 psia

40 F73 psia

72 F133 psia

Port pressure ~=(Volume ratio)^k * (P suction)NH3 k ~= 1.29Port pressure = (1.6^1.29) * 73 psia = 133 psia

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.

96 F200 psia

-17 F20 psia

8 F37 psia

Effect of LowerCondensing Pressure

75 F140 psia

-17 F20 psia

8 F37 psia

Effect of LowerCondensing Pressure

75 F140 psia

-17 F20 psia

8 F37 psia

Port pressure ~=(Volume ratio)^k * (P suction)NH3 k ~= 1.29Port pressure = (1.6^1.29) * 20 psia = 37 psia

75 F140 psia

-17 F20 psia

8 F37 psia

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

96 F200 psia

-17 F20 psia

8 F37 psia

What is the Sideload capacity?on NH3

-17 F20 psia

Secondary Evaporator37 psia (8F)

P=37 psia

Liquid from cond

100 lbm/min

Zero flow

-17 F20 psia

Secondary Evaporator

38 psia (10F)

P=37 psia

Liquid from cond

100 lbm/min

16 lbm/min

-17 F20 psia

Secondary Evaporator

41 psia (13F)

P=38 psia

Liquid from cond

100 lbm/min

21 lbm/min

-17 F20 psia

Secondary Evaporator

43 psia (15F)

P=40 psia

Liquid from cond

100 lbm/min

28 lbm/min

Thread packing brings up the pressurein the screw thread

-40 -20 0 200%

100%

200%

300%

400%

Sat. Suct Temp. F

%

o

f

S

u

c

t

.

T

o

n

s

R

e

f

r

.

40F20F0F

-10F

Sideload Capacityas a % of suction capacity

Sideload Temp.

NH3 Refrig.

Sideload Evaporator Piping

Economizer PipingCondenser

Evaporator

FlashEconomizer

Secondary EvaporatorSecondary Evaporator

Condenser

Evaporator

Condenser

Evaporator -17F

(20 psia)

FlashEconomizer

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.

R-717

1

10

100

1000

10000

0 100 200 300 400 500 600 700 800 900

Enthalpy (B.T.U. per LB)

A

b

s

o

l

u

t

e

P

r

e

s

s

u

r

e

(

P

S

I

A

)

Single stage compression

(hg-hf)=458b/lbm

100 l

bm/m

in

229 TonsRefrigeration

-17F/96F NH3

R-717

1

10

100

1000

10000

0 100 200 300 400 500 600 700 800 900

Enthalpy (B.T.U. per LB)

A

b

s

o

l

u

t

e

P

r

e

s

s

u

r

e

(

P

S

I

A

)

Economizer cycle

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

19 lbm/min

273 TonsRefrigeration

19% increase inevaporator tons

@ same evaporatormassflow

100 l

bm/m

in

R-717

1

10

100

1000

10000

0 100 200 300 400 500 600 700 800 900

Enthalpy (B.T.U. per LB)

A

b

s

o

l

u

t

e

P

r

e

s

s

u

r

e

(

P

S

I

A

)

Economizer cycle

19 lbm/min

119 l

bm/m

in

100 lbm/min

19% More Tons 19% More massflow Only9% More Power

119 lbm96 F

200 psia

100 lbm/min-17 F

20 psia

19 lbm/min45 psia

The economizer vapor is at a lower compression ratio, and adds no

additional friction to the compression

NH3 Cycle efficiency-17 F evaporator96 F condensingCapacity Power HP/TR229 494 2.16 non-Econ273 538 1.97 Econo.19% 9% 9.6% improved

R-22 Cycle efficiency-10 F evaporator95 F condensingCapacity Power HP/TR106 212 2.0 non-Econ136 230 1.7 Econo.28% 8% 17% improved

Capacity and Power Non-E

0

100

200

300

400

500

600

700

800

-60 -40 -20 0 20 40 60

Evap Temp (F)

T

R

&

B

H

P

NH3R507R22NH3 BHPR507 BHPR22 BHPCapacity

Power

Capacity and Power- Economized

0

100

200

300

400

500

600

700

800

-60 -40 -20 0 20 40 60

Evap Temp (F)

H

P

a

n

d

T

R

NH3 CapR507 CapR22 CapNH3 HPR507 HPR22 HPCapacity

Power

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 ER22 ENH3 nonR22 nonR507 non

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 ER22 ENH3 nonR22 nonR507 non

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)

L

b

m

/

m

i

n

NH3R507R22

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 CapR507 CapR22 Cap

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)

NH3R507R22

Economizer Piping Open flash vessel Flash vessel with HX Direct Expansion

Open Flash Vessel -One Compressor.

DX - Shell and Tube

Level Controlled Shell and Coil

If load varies widely and econo is used below 90% SV use BPR controlling compressor side