2020 Refrigeration Seminar

”All About Refrigerants”

Design & Performance Validation of CO2 Chiller with Parallel Compression & Adiabatic Cooling in Ice Rink Application

02/11/2020

Adam Ciesielski Engineering Manager

Zero Zone, Inc. 6151 140th Avenue NW

Ramsey, MN 55303

Adam.Ciesielski@zero-zone.com 763-398-1996

1) Transcritical CO2 Refrigeration Cycle

2) Chiller design with CO2 as refrigerant

3) System characterization and operating parameters

4) Energy performance and ranking of design options

Agenda

CO2- Characteristics & Properties

Types of CO2 Refrigeration Processes

2 Basic Categories of CO2 systems

Subcritical

Condensing below the critical point of 87.9°F

Cascade system with primary refrigeration to condense the CO2

System pressures generally do not exceed 650 psig

Transcritical

Operates above the critical point of CO2 – 87.9 °F

High side pressures above 1100 psig

Subcritical Refrigeration Cycle

condensing

evaporation

compression

expansion

Transcritical Refrigeration Cycle

compression expansion

evaporation

|----flash----|

Transcritical Refrigeration Cycle

Transcritical Refrigeration Cycle

Subcritical

operation

COP = 3.4

Condenser

20%

Standard Transcritical System

• Standard booster, the flash gas pressure reduced to MT Suction Pressure and pumped by MT Compressors

• FGT Pressure of 500 psig reduced to MT Suction Pressure 350 psig

• Increased compressor work due to higher compression ratio of 2.8.

• Warmer the ambient the higher the flash gas flow (about 45% of total)

Simple Schematic for Parallel Compression

Show Benefits of Parallel Compression

Standard Transcritical with FGB

Disadvantage : Lower Efficiency

Transcritical Parallel Compression

• In parallel compression, flash gas is pumped directly by parallel compressor

• FGT Pressure and Parallel Compressor suction is equal @ 500 Psi

• Reduced compressor work as compression ratio is 1.9 (versus 2.8 with FGB)

• Higher Efficiency...Warmer the climate the more the efficiency

Simple Schematic for Parallel Compression

Show Benefits of Parallel Compression

Transcritical System with Parallel Compression

Disadvantage : Complex controls, Added Suction Group

Adiabatic Gas Cooler

• Operates in dry mode and wet mode – ambient temperature dependent • Conserve water

• Lower gas cooler pressures • Mostly subcritical operation

• Minimize transcritical operation

• Less compressor work • Low peak energy use

• Most efficient for dry areas

Disadvantage : Higher First Cost, Reduced Heat Recovery

Background Information

• STMA Ice Rink

• Owned and operated by cities of St. Michael and Albertville and St. Michael-Albertville School District

• Ice rink operational since Aug 2018

System Overview

• Two Ice Rinks designed for 12 months operating season • Rink 1 : 201’ x 86’ • Rink 2 : 200’ x 85’

• Refrigeration System Design: • Primary : CO2 Transcritical System • Secondary : 40% Ethylene Glycol

• Refrigeration System Capacity: • 160 TR

Design Highlights of CO2 Chiller

• Refrigeration Cycle • 160 TR Chiller with direct

expansion, two-stage expansion on high side, and parallel compressors for flash gas recovery

• Capacity Control • Main and parallel reciprocating

compressors with VFD on lead compressor and compressor cycling

• Energy-Efficient Gas Cooler • Adiabatic gas cooler for wet

operation during warm ambient • EC Fans for speed control

• Heat Reclaim • For sub-floor and ice melt

System Design & Configuration

• Chiller Design: • Flooded Evaporator

• Shell and Plate HTX • Design Approach : 9 F • Design Suction @ 5 F / 320 psig

• Reciprocating Compressors • Main : 4 x 50 HP : 6CTE-50K • Parallel : 2 x 50 HP : 6FTE-50K

• Oil Management • Coalescing Oil Separators • Oil Recovery from Flooded Evaporator

• Refrigerant Management • Vertical Receiver for flash gas separation and liquid supply to evaporator • Receiver Operating Pressure : 500 psig • Liquid Level Control in Flooded Evaporator

• Superheat Management • LSHX for Main Compressors, exchanging heat with CO2 Liquid • FGHX for Parallel Compressors, exchanging heat with GC Outlet

System Design & Configuration

• Design Pressures • Low Side : 600 Psig • Intermediate Side : 1305 Psig • High Side : 1740 Psig

• Adiabatic Gas Cooler (or Condenser) : • Design TD : 5 F in Transcritical Mode • Design gas cooler outlet temperature : 85 F

• Secondary System : • Rink Pumps

• Variable flow system • 2 x 1050 GPM + 1 x 850 GPM

• Sub-soil Pumps • 2 x 80 GPM

• Snow Melt Pumps • 2 x 81 GPM

System Architecture

CO2 Rack

Ice Rink 2 Ice Rink 1

Adiabatic Gas Cooler

Testing Plan & Schedule

Design Mode From To

Parallel with Adiabatic GC 7/19/19 8/08/19

Flash Gas Bypass with Adiabatic GC 8/9/19 8/30/19

Parallel Compression with Dry GC 8/31/19 9/24/19

Flash Gas Bypass and Dry GC 9/25/19 10/15/19

Basis of System Performance Analysis

• 21 day period • Normalized for load variation due to use

• Data Analysis • Data recorded at 5 minutes interval

• Data Validation • Sensor calibration

• Spot check for accuracy • Actual compressor power consumption verified with

manufacturers simulation

System Characterization with Parallel Compression in Adiabatic Mode

Above data represents 24Hr Average

System Operating Data

• Very stable system operation achieved by using multiple parallel compressors and VFD’s for capacity control of compressors, gas cooler fans and glycol pumps.

• Control techniques used optimal TD during transcritical mode and target subcooling during subcritical mode

• Deployed heat exchangers for attaining proper superheat at MT and Parallel compressors that is essential for reliability

• Low pressure oil management system uses coalescing oil separator for efficient and effective oil management system that ensures optimum design life

Flash Gas Flow

• In 2 stage expansion, flash gas produced at receiver. At higher ambient, it can be as much as 45%, but in general it is about 30%

• Performance of system is improved by implementing FGB during isenthalpic process that improves evaporator efficiency

• Improves evaporator performance by feeding with liquid instead of two-phase

• Methods of dealing with flash gas : • Pumping flash gas via MT compressors (less efficient due to higher compression ratio) • Deploying parallel compressors for pumping vapor directly (most energy efficient)

• Compression ratio of Flash Gas is reduced by approx. 30 % in parallel compression mode

Energy Performance Expectation:

• Does not include gas cooler fan power, only compressor power, full design capacity

• Water precooling enabled at 70°F Dry Bulb and above

• Parallel Compression enabled 45°F Dry Bulb and above

• Constant Gas Cooler TD of 10°F • Dry Bulb Temperature for dry cooling • Wet Bulb Temperature for water precooling

Energy Performance Validation: Flash Gas Design: Parallel versus Bypass

• Main contributor is the compression ratio in parallel compression mode versus FGB

• FGB mode consumes higher compressor energy due to higher compression ratio needed for pumping flash gas

• At higher ambient, the parallel compression system far outweighs the performance of FGB • At ambient greater than 60°F, total average Efficiency gains of 15.9% is measured for test

period with dry gas cooling and 10.5% with adiabatic gas cooling. • Total annual savings is dependent on load and annual temperature bin data. Higher ambient

temperatures yield better results.

Energy Performance Validation: Gas Cooling: Adiabatic versus Dry

• Adiabatic gas cooling uses water spray to precool air during high ambient • Reduces gas cooler outlet temperatures and pressures • Reduces flash gas, and consequently lowers compression ratio and compressor power • Controlled on at 70°F ambient and 80% fan speed

• The higher the ambient conditions, the more the energy savings potential • At ambient greater than 80°F, total average Efficiency gains of 22.9% is measured for test

period with FGB and 10.5% with parallel compression.

• Adiabatic gas cooling can achieve more savings at lower ambient temperatures • Start water spray at lower ambient temperature and fan speed

• Measure and control on wet bulb temperature

Flooded Evaporator versus Dx

• Flooded evaporator design demonstrated significant gains in energy efficiency • Typical SST with Dx Evaporator : 5 F

• SST with Flooded Evaporator : 10 F

• Resulted in reducing thermal lift by 5 F

• Computed efficiency gains for flooded evaporator is an average of 10%

• Computed EER gains range from 0.8% to 2.6% per 0F as gas cooler outlet temperature falls (ambient dependent)

Approach of 3.6 F is achieved

Energy Efficient Design Options Ranking

• Ranking based on field performance measurement : 1) Parallel Compression with Adiabatic Cooling

2) Parallel Compression with Dry Gas Cooling

3) Flash Gas Bypass with Adiabatic Cooling

4) Flash Gas Cooling with Dry Gas Cooling

Above inference is a generalized representation and is best confirmed on a case-by-case evaluation which is dependent

upon application, bin data, and climate conditions.

Adam Ciesielski

Adam.Ciesielski@Zero-Zone.com

Masood Ali

Masood.Ali@Zero-Zone.com

Questions?