2017 Tech Report Nov-Dec

TECHNICAL REPORT

THE

REFRIGERATING ENGINEERS & TECHNICIANS ASSOCIATION | NOVEMBER/DECEMBER 2017

COMING TO A NEIGHBORHOOD NEAR YOU Over the past several years CO2 has begun to reemerge as an alternative refrigerant to both traditional halocarbons and is considered for use in a particular application: • its normal operating “There are several draw-backs to CO2 that must be taken into

pressures are higher than most common refrigerants; • it has a low critical point compared to other common refrigerants—87.76ºF (31ºC); • it has a high triple point compared to other common refrigerants— -69.8ºF (-56.6ºC); and • it is heavier than air, so it tends to concentrate in occupied areas and displace air, so it is an inhalation hazard. As the CO2 refrigeration system technology has evolved, a number of different system types have developed. The most common in industrial applications is the cascade

ammonia. CO2, like ammonia, is a natural—green—refrigerant. Being a natural refrigerant means that it occurs in, and is extracted from air or other naturally occurring sources. CO2 and ammonia are both components of air, they both are part of organic decay, and they are both found in living plants and animals. CO2 or R744, has some advantages as a refrigerant. It is available at a reasonable cost. Because it is more dense than other refrigerants, less volume has to be moved through the system for an equivalent mass flow—less cubic feet per minute (CFM) for the same pounds per minute flow rate.

account when CO2 is considered for use in a particular application.”

CO2 has a better BTU capacity per pound than the halocarbon refrigerants. Only ammonia has a lower ozone depletion potential (ODP) and global warming potential (GWP) numbers. R744 has no fire hazard potential, is non-toxic, and non-reactive. While there is no perfect refrigerant, CO2 is a good refrigerant by most measures. There are several draw-backs to CO2 that must be taken into account when CO2

system. Others that are becoming popular are: the hybrid cascade system, the

SIMPLE CO2 CASCADE SYSTEM

volatile brine cascade system, transcritical system, transcritical parallel compression system, and parallel compression ejector system. Some of these are now in use in small beverage cooler and grocery store applications in a neighborhood near you. As the technology matures it is likely that they will all find their way into large systems. THE CASCADE SYSTEM The cascade system has been used with refrigerants that have a low critical point—the point at the top of the Mollier Diagram, above which gas cannot be condensed—which for CO2 is 87.76ºF (31.0ºC) and 1055.3 psig (72.8 bar). Refrigerants other than CO2, such as R23 and R503—no longer available— have been used in the low stage of cascade systems to operate at temperatures down to -150ºF (-101ºC). CO2 is limited by its triple point to temperatures above -69.8ºF (-56.6ºC). The cascade system is often confused with two stage systems. Two stage systems use one refrigerant and compress the refrigerant vapor through two stages from low—evaporator—pressure,

Figure 1 Simple Cascade System

to intermediate pressure, and to high—condensing— pressure. This is done to reduce the compression ratio of the individual low stage and high stage compressors. The two- stage system achieves improved efficiency as the volumetric efficiency of the compressors increases with the lowering of the compression ratio.

Cascade systems use two systems interconnected by a heat-exchanger that is the evaporator for the high stage system, and the condenser for the low stage system. This type system uses two different refrigerants. One refrigerant that has a low critical point in the low stage system. The other is a common refrigerant,

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not guarantee there will not be an accident, but guarantees everything possible has been done to mitigate an accident and possible injuries. A variation on the simple cascade system is the hybrid cascade system . The hybrid system is a typical cascade systemwith a water or glycol heat exchanger added to handle cooler or high temperature loads. A brine solution, like a propylene glycol/water solution, is cooled in the heat exchanger and pumped to the cooler or high temperature load. Heat is transferred to the brine as sensible heat and the solution returned to the heat exchanger where the heat is removed by the high stage refrigerant of the cascade system. The use of the hybrid system allows a refrigerant like ammonia to be used and still be isolated to a location outside the conditioned or occupied space. Yet another variant of the cascade system is the volatile brine system . In this type system liquid CO2 is circulated using a pump in the low stage of the system. The difference between a volatile brine system and a typical secondary refrigerant system is that a portion of the CO2 liquid flashes off as it

absorbs heat. The advantage of using this system is the circulated CO2 absorbs latent heat to boil the liquid. By doing this it picks up more heat per pound circulated, fewer pounds have to be circulated, and this requires less pump horsepower. In addition, there is no low stage compressor. The liquid and vapor from the evaporator is returned to an accumulator/ pump receiver, the liquid and gas separated, the liquid drops to the bottom to be pumped back to the evaporator, and the gas flows by natural convection to the evaporator/condenser. TRANSCRITICAL SYSTEMS Transcritical systems are those that operate in the area above the critical point. Traditionally, they been limited to areas that have moderate ambient conditions. However, as the technology develops, the use of transcritical systems will continue to move toward the equator. The refrigeration cycle in the transcritical cycle takes place both above and below the critical point. Below the critical

“Even used in a cascade system, CO2 has normal operating pressures higher than those encountered with most common refrigerants.”

either a halocarbon or a natural refrigerant such as ammonia,

in the high stage system. Cascade systems are used in both commercial and industrial applications.

Even used in a cascade system, CO2 has normal operating pressures higher than those encountered with most common refrigerants. At a temperature of -30ºF (-34.4ºC) CO2 has a saturation pressure of 163.1 psig (11.2 bar). And at a condensing temperature of +20ºF (-6.7ºC) CO2 has saturated condensing pressure of 407.2 psig (28.1 bar). But even with the high pressures there should be no concern about safely handling CO2 or servicing a system. The same precautions taken with other refrigerants have to be taken. Use the standard operating procedures (SOP) and personal protection equipment (PPE) the same as is done with any other system. Doing so does

point, the system operates much like a conventional system. Liquid is metered into the evaporator, the pressure and temperature

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Figure 2 Diagramed

Subcritical System and Transcritical System. Courtesy of Sporlan Valve Company a Division of Parker Hannifin Corp.

drop, heat is transferred from the surroundings to the liquid CO2 refrigerant, and it boils. The vapor is drawn off by the compressor, compressed to a higher pressure—and temperature—and pushed out to the discharge line. As the gas exits the compressor it is now above the critical point and the dynamics change. The refrigerant is now in a state that does not strictly behave as a gas or a liquid, and is called a supercritical fluid . The fluid cannot be condensed while in this state. A conventional condenser is not used at this point, instead a gas cooler is used. Although

a gas cooler may be similar in appearance and mechanical function to a condenser, the heat transfer done in a gas cooler is sensible—at a constant pressure—rather than latent. As the name indicates, the gas temperature is reduced. During low ambient conditions, air temperatures below 80ºF, the gas cooler may operate as a condenser. This is because the temperatures are below the critical point of the CO2. When the supercritical fluid exits the gas cooler it is expanded— the pressure is dropped—at a constant temperature and enthalpy to below the critical point into an expansion tank.

The CO2 will now be a mixture of gas and liquid at normal state conditions. The liquid can now be used to supply the evaporator, and the gas is drawn off by the compressor. The gas cooler and the expansion tank are maintained at a constant pressure using two pressure control valves. The compressor can be a single stage compression system with one or multiple compressors. The single stage system has one suction pressure and one load temperature. Other load temperatures can be achieved by using an evaporator pressure regulator. The single stage system is graphed on

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TRANSCRITICAL SYSTEM

Figure 3 Simple Transcritical System

point B to point C and some of the gas flashes to a liquid as it drops below the critical point. See a simple single stage system in Figure 3. It is typical to use air cooled gas coolers in the more northern and cooler regions. By using adiabatic gas coolers, the use of transcritical systems can be moved south. Because air- cooled gas coolers depend on the dry bulb temperature of the air, these systems are limited to cooler climate areas. Adiabatic gas coolers use

evaporation of water to cool the air stream before it passes over the refrigerant coils. This process is dependent on the wet bulb temperature of the air. During most normal conditions, the wet bulb temperature will be lower than the dry bulb temperature. As an example: with an outside air temperature of 95ºF dry bulb a transcritical systemwould not be practical using an air- cooled gas cooler. However, if an adiabatic gas cooler is used, with a 95ºF dry bulb, at 50% relative humidity, the wet

Figure 2. From point C to point D, the system acts like a sub- critical system. The refrigerant absorbs heat in an evaporator, boils, and the vapor is drawn to the compressor. At point D, the superheated refrigerant vapor enters the compressor or compressors. From point D to point A the CO2 vapor is compressed. At point A the gas, now above the critical point, is discharged into the gas cooler. The gas is cooled in the gas cooler from point A to point B. The cooled gas is expanded from

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bulb temperature is 79ºF. The adiabatic gas cooler can deliver air that is between 81ºF and 82ºF to the gas cooling coil. It is now practical to use a transcritical CO2 system in this location. By using some of the alternative transcritical systems, such as parallel compression and parallel compression with ejectors, operating efficiencies competitive with or better than comparable halocarbon systems can be achieved. CO2 systems in general offer some advantages, in particular

the reduction of refrigerants with high ODP and GWP factors. When used in cascade system with refrigerants like ammonia, that is seeing increased regulatory demands, the ammonia charge and leak potential can be reduced, and the ammonia confined to the machinery room. The transcritical systems offers even more advantages:

• a low-cost refrigerant, • simple systems,

• reduced installation cost, • reduced refrigerant charge, and • comparable or better operating costs. CO2 may not have moved into your neighborhood, but there is a strong likelihood it will be sometime in the future. It never hurts to learn more, so why not expand your knowledge base and learn more about a new— really old—refrigerant.

• no halocarbon, • no ammonia, • a single refrigerant,

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The Technical Report is an official publication of the Refrigerating Engineers & Technicians Association (RETA). RETA is an international not-for-profit association whose mission is to enhance the professional development of industrial refrigeration operating and technical engineers. Don Chason Executive Editor Jim Barron Executive Director TECHNICAL REPORT THE

704-455-3551 Sara Louber Senior Director, Office Operations sara@reta.com Mary Hendrickx

jim@reta.com Dan Reisinger Certification Manager dan@reta.com Dan Denton Chapter Relations Manager ddenton@reta.com

Conference Manager mhendrickx@reta.com Jim Price Education Manager jprice@reta.com

The information in this publication is based on the collective experience of industry engineers and technicians. Although the information is intended to be comprehensive and thorough, it is subject to change based on particular applications, field experience, and technological developments. The Refrigerating Engineers & Technicians Association expressly disclaims any warranty of fitness for aparticular application, as well as all claims for compensatory, consequential, or other damages arising out of or related to the uses

of this publication. Copyright © 2017

REFRIGERATING ENGINEERS & TECHNICIANS ASSOCIATION 1035 2nd Avenue SE, Albany, OR 97321 Telephone: 541.497.2955 | Fax: 541.497.2966 RETA.com

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