A Scroll Economiser Cycle for use in Multi-temperature Indirect Transport

Refrigeration Systems.

Shane Smyth, Donal P. Finn, Barry Brophy and David J. Timoney

School of Electrical, Electronic and Mechanical Engineering

University College Dublin, Ireland

Presentation Outline

1. Context

2. Objective and Approach

3. Experimental Test Facility

4. Experimental Programme and Test Matrix

5. Results – Influence of Injection Ratio on Economiser Pressure

6. Algorithm Development and Evaluation

7. Algorithm Evaluation Results

8. Conclusions

Context

Direct Expansion System Economised Indirect System

Compressor/ Condenser Unit

Primary Circuit Multiple Pumps

Economiser Cycle

Approach

Objective

• The capacity and COP of an economised multi-temperature indirect can be maximised by control of the economiser injection ratio.

• This method of control necessitates mass-flow instrumentation

which is impracticable for field applications.

• An alternative method of control is described based on the more

easily-measured economiser pressure as the primary control

parameter, thereby eliminating the requirement of mass-flow

instrumentation.

Approach

• Experimental Test Rig and ATP Test Matrix

• Influence of Injection Ratio on Economiser Pressure

• Identification of Key Trends and Development of Control Algorithm

• Control Algorithm Implementation and Evaluation

• Conclusions

Experimental Testing Facility

Two Chambered

Insulated Cold Room

Side-by-Side testing of

Direct Expansion and

Secondary Refrigeration

Systems

U Values of chamber

walls experimentally

measured

Solenoid Valves to allow

different flow

configurations

Experimental Test Facility

Secondary

Coolant Pumps

Compressor-

Condenser

UnitScroll Compressor IDX Cooling Coil Chamber 2

Multi-

Chambered

Insulated Cold

Rooms

S&T Condenser

(Water cooled)

• Vapour Injected

• Variable Speed

• Hermetic

DX Remote

EvaporatorsChamber 1

Test Matrix

• Secondary Refrigerant: 50% V/V Aqueous Ethylene Glycol.

• Standard: UNECE ATP Procedure (2003) – Class C Multi-Compartment.

• Mechanically Refrigerated.

• Normally insulated compartment.

• Independent setpoint control between adjacent compartments.

• Test Matrix:

• 3 ATP Chamber Setpoints: -20,-20ºC; -10,-10ºC and 0,0ºC.

• 3 Condensing Temperatures (for each setpoint): +18ºC, +22ºC, +25ºC.

Results: Economiser Pressure Economiser Pressure

0

1

2

3

4

5

6

7

8

9

0 10 20 30 40 50 60

Injection Ratio (INJR) [%]

Pre

ss

ure

[b

ar]

Box 0,0C Box -10,-10C Box -20,-20C

• Strong INJR-Economiser Pressure Relationship.

• Economiser pressure increases continuously with increasing INJR.

• Minimum economiser pressure (SIP) when INJR is zero (no economiser action).

• Different SIP for different ATP set-points.

Examination of Saturated Injection Pressure

SIP v Compressor Suction Pressure

y = 1.64x + 0.475

R2 = 0.99

3

3.5

4

4.5

5

5.5

6

6.5

7

1.5 2 2.5 3 3.5 4

Compressor Suction Pressure (bar)

Satu

rate

d I

nje

cti

on

Pre

ssu

re (

bar)

• SIP strongly dependent on compressor suction pressure.

• Linear Relationship between SIP and compressor suction pressure.

• Strong relationship due to close proximity of injection port to suction port on scroll involute.

Capacity Maximisation

Evaporator Capacity

0

1

2

3

4

5

6

7

8

9

3 4 5 6 7 8 9

Economiser Pressure [bar]

Evap

ora

tor

Cap

acit

y [

kW

]

Box -20, -20C Box -10, -10C Box 0, 0C

• Degree of capacity augmentation depends on the economiser pressure

• Optimum pressure for capacity maximisation.

• Optimum pressure depends on ATP setpoint temperature.

• After optimum economiser pressure, capacity drops due to increased saturation temperature of refrigerant in economiser

COP MaximisationEvaporator COP

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

3 4 5 6 7 8 9

Economiser Pressure [bar]

CO

P

Box 0,0C Box -10,-10C Box -20,-20C

• Bi-polar maxima for optimisation of COP

• Optimised COP under high capacity or low capacity conditions.

• The “low-pressure” optima maximises COP by minimising compressor power.

• The “high-pressure”optima maximises COP at high capacity.

• Choice of optima will depend on power availability and cooling requirements.

Algorithm Development

• Optimum economiserpressure for maximisation of capacity and COP

• SIP determined for each ATP condition by measurement of economiser pressure when no INJR = 0%

• Fitted trendline allows determination of optimum pressure for maximisation of capacity or COP

Optimisation of Capacity

y = 0.84x + 2.87

R2 = 0.85

0

1

2

3

4

5

6

7

8

9

10

3 3.5 4 4.5 5 5.5 6 6.5 7

Saturated Injection Pressure (bar)

Eco

no

mis

er

Pre

ssu

re (

bar)

3 4 5 6 7

Optimisation of COP

y = 1.03x + 0.75

R2 = 0.96

0

1

2

3

4

5

6

7

8

9

3 3.5 4 4.5 5 5.5 6 6.5 7

Saturated Injection Pressure (bar)

Eco

no

mis

er

Pre

ssu

re (

bar)

3 4 5 6 7

Algorithm Implementation and Evaluation

• Algorithm Implementation

• Developed in LabVIEW

• Single Input, Single Output Control Loop

• Initial Determination of SIP from Compressor Suction Pressure

• Determination of Optimum Pressure from Appropriate Regression Equation

• Evaluation: UNECE ATP Standard (2003) – Class C Multi-Compartment

• 3 ATP Chamber Setpoints: -20,-20ºC; -10,-10ºC and 0,0ºC

• 3 Condensing Temperatures (for each setpoint): +18ºC, +22ºC, +25ºC

• Test Matrix:

Box Capacity

0

2

4

6

8

10

12

Ca

pa

cit

y (

kW

)

IDX IDX IDX DX IDX IDX IDX DX IDX IDX IDX DX

(Cond) 18ºC 22ºC 25ºC 22ºC 18ºC 22ºC 25ºC 22ºC 18ºC 22ºC 25ºC 22ºC

(Box) (-20, -20ºC) (-10, -10ºC) (0, 0ºC)

4.17 4.07 3.81

5.34

5.93 5.9 5.79

7.11

8.58.17 7.83

9.66

Evaporator Capacity

0

2

4

6

8

10

12

Cap

acit

y (

kW

)

IDX IDX IDX DX IDX IDX IDX DX IDX IDX IDX DX

(Cond) 18ºC 22ºC 25ºC 22ºC 18ºC 22ºC 25ºC 22ºC 18ºC 22ºC 25ºC 22ºC

(Box) (-20, -20ºC) (-10, -10ºC) (0, 0ºC)

4.8 4.71 4.43

5.346.45 6.45 6.35

7.11

9.1 8.57 8.23

9.66

• IDX Box capacity varied between 76-84.5% of DX baseline

• IDX Chamber Capacity varied between 88-90% of DX baseline

•Capacity decreases as as condensing temperature is increased

Algorithm Evaluation: Capacity

Algorithm Evaluation: COP

• IDX Box COP varied between 80-86% of DX baseline

• IDX Chamber COP varied between 65-74% of baseline

• Considerable reduction in IDX COP due to Additional HX, Temperature Glide and Liquid Secondary Pumps.

Box COP

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

CO

P

IDX IDX IDX DX IDX IDX IDX DX IDX IDX IDX DX

(Cond) 18ºC 22ºC 25ºC 22ºC 18ºC 22ºC 25ºC 22ºC 18ºC 22ºC 25ºC 22ºC

(Box) (-20, -20ºC) (-10, -10ºC) (0, 0ºC)

0.85 0.77

0.68

1.18 1.14

1.04 0.96

1.48 1.471.35 1.31

1.82

Evaporator COP

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

CO

P

IDX IDX IDX DX IDX IDX IDX DX IDX IDX IDX DX

(Cond) 18ºC 22ºC 25ºC 22ºC 18ºC 22ºC 25ºC 22ºC 18ºC 22ºC 25ºC 22ºC

(Box) (-20, -20ºC) (-10, -10ºC) (0, 0ºC)

1.050.94

0.86

1.184

1.37

1.27 1.16

1.48

1.75

1.53

1.44

1.82

Comparison with Injection Ratio Strategy

• At -20,-20C pressure controlled evaporator capacity was within 4.5% of INJR controlled capacity

• At-10-10C and 0,0C the pressure controlled evaproator capacity was within 1.7% and 1.6% of the INJR controlled system.

• Similar increases in evaporator capacity.

• Increases in COP also evident for COP algorithm

Capacity

0

1

2

3

4

5

6

7

8

9

10

Cap

acit

y (

kW

)

IDX IDX IDX IDX IDX IDX IDX IDX IDX IDX IDX IDX

-20,-20ºC (cond +22C) -10,-10ºC (cond +22C) 0, 0ºC (cond +22C)

Box Box Evap Evap Box Box Evap Evap Box Box Evap Evap

Set. Alg. Alg. Set. Set. Alg. Alg. Set. Set. Alg. Alg. Set.

4.27 4.07

4.71 4.93

6.00 5.90 6.45

6.58

8.30 8.17

8.57 8.71

Coefficient of Performance

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

CO

P

IDX IDX IDX IDX IDX IDX IDX IDX IDX IDX IDX IDX

-20,-20ºC (cond +22C) -10,-10ºC (cond +22C) 0, 0ºC (cond +22C)

Box Box Evap Evap Box Box Evap Evap Box Box Evap Evap

Set. Alg. Alg. Set. Set. Alg. Alg. Set. Set. Alg. Alg. Set.

0.737 0.768

0.94 1.04

0.96 1.04

1.27 1.25 1.26

1.35

1.53 1.51

5. Conclusions

Optimum Injection Pressures Exist for control and maximisation of:

Cooling CapacityCOPCompressor Power

Optimum injection pressures are dependent on setpoint conditions.

Two Optima exist for optimisation of COP

Optimisation under high capacity conditions

Optimisation under low compressor power conditions.

Pressure Control Algorithm provides comparable performance to INJR controlled system.

Pressure control algorithm can be used to maximise either capacity or COP, depending on the requirements

This control algorithm can be used as part of an economiser cycle to compensate for parasitic losses associated with increased pressure lift on indirect systems.

A Scroll Economiser Cycle for use in Multi-temperature Indirect Transport

Refrigeration Systems.

Shane Smyth, Donal P. Finn, Barry Brophy and David J. Timoney

School of Electrical, Electronic and Mechanical Engineering

University College Dublin, Ireland