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CO2 as a Refrigerant — Introduction to Transcritical Operation

This is post number 4 of a series.

Many R744 systems operate above the critical point some or all of the time. This is not a problem; the system merely works differently and is designed with these needs in mind.

  • R744 systems work subcritical when the condensing temperature is below 31 °C (88 °F).
  • R744 systems work transcritical when the gas cooler exit temperature is above 31 °C (88 °F).
  • HFC systems always work subcritical because the condensing temperature never exceeds the critical temperature (e.g., 101 °C / 214 °F in the case of R134a).

The pressure enthalpy chart in Figure 1 shows an example of a simple R744 system operating subcritically at a low ambient temperature and transcritically at a higher ambient temperature. The chart shows that the cooling capacity at the evaporator is significantly less for transcritical operation.

Figure 1: R744 pressure enthalpy chart showing subcritical and transcritical systems

Figure 1: R744 pressure enthalpy chart showing subcritical and transcritical systems

An efficiency drop also occurs with HFC systems when the ambient temperature increases, but the change is not as great as it is with R744 when the change is from sub- to transcritical.

It is important that appropriate control of the high side (gas cooler) pressure is used to optimize the cooling capacity and efficiency when transcritical. For example, increasing the high side pressure will increase the cooling capacity when operating above the critical point.

Behavior in the Reference Cycle

Simple comparisons between R744 and other refrigerants can be misleading because its low critical temperature either leads to differences in system design, such as the use of cascade systems, or to transcritical operation. As a result, like-for-like comparisons are not easy to make.

Theoretical comparisons between R744 and common HFC refrigerants are outlined in the list below.

  • R744 compares reasonably well with HFC systems when subcritical and at low condensing temperatures. But the comparison is less favorable at higher condensing temperatures and when transcritical.
  • The high suction pressure and high gas density of R744 results in very good evaporator performance. In like-for-like systems the evaporator temperature of an R744 system would, in reality, be higher than for an HFC equivalent.
  • The index of compression is very high for R744, so the discharge temperature is higher than for the HFCs. This can improve heat reclamation potential in retail systems, although the requirement for heat in the summer when the system is transcritical is limited.
  • The density of R744 results in very high volumetric capacity. This reduces the required compressor displacement, but not the motor size, which would be similar to that required for HFC refrigerants.
  • The required suction pipe cross-section area is in proportion to the volumetric capacity. For R744 the diameter of the suction line is approximately half that required for R404A.
  • The compression ratio for R744 is less than for HFCs. This can result in higher isentropic efficiency.

Upcoming CO2 as a Refrigerant series topics will cover the potential hazards of R744, compare it to other refrigerants (both traditional and new), and weigh its advantages and disadvantages as a refrigerant.

Andre Patenaude
Director – CO2 Business Development, Emerson Climate Technologies

Visit our website for additional information on CO2 Solutions from Emerson. 
Excerpt from original document; Commercial CO2 Refrigeration Systems, Guide for Subcritical and Transcritical CO2 Applications.


To read all posts in our series on CO2 as a Refrigerant, click on the links below:

  1. Series Introduction
  2. Criteria for Choosing Refrigerants
  3. Properties of R744
  4. Introduction to Trancritical Operation
  5. Five Potential Hazards of R744
  6. Comparison of R744 with Other Refrigerants
  7. R744 Advantages / Disadvantages
  8. Introduction to R744 Systems
  9. Introduction to Retail Transcritical Systems
  10. Retail Booster Systems
  11. Introduction to Retail Cascade Systems
  12. Introduction to Secondary Systems
  13. Selecting the Best System

 

CO2 as a Refrigerant — Series Introduction

CO2 offers refrigeration with low global warming impact, but with new application and handling considerations

Weighing up natural refrigerant alternatives

This is post number 1 of a series.

Commercial refrigeration has been in the environmental spotlight for more than a decade, especially as leakage studies have revealed the true effects of hydrofluorocarbon (HFC) emissions. Considerable reductions in emissions are possible, but implementing them is having a major impact on the refrigeration industry. In response, many new refrigerant options and system architectures have appeared — both on paper and in practice — creating tricky choices for decision makers in commercial refrigeration.

The significant environmental advantages of R744 (CO2) have guaranteed its position as a leading option for future refrigeration systems. It has demonstrated favorable results in different system configurations over many years, particularly in Europe, Australia and Canada. Initially high investment costs are now on a downward trend, while innovations in component technology and application methods continue to reveal potential performance gains. These results have ensured that CO2 will be a long-term option in the foreseeable future.

This post is the first in a series that will introduce CO2 as a refrigerant. The blog series will summarize the properties of R744 and examine how well it meets traditional and emerging needs for refrigerants. The series will also covers some of the reasons why CO2 refrigeration systems differ from conventional systems, notably the design considerations created by the need for transcritical operation under certain conditions. Other topics concerning R744 will also be examined, including the general aspects of R744 systems; more detailed information about the design of R744 cascade, transcritical booster and secondary systems; and key points about their commissioning, operation and service.

In the next post I will discuss the basics and considerations in criteria for choosing refrigerants. Multiple factors need to be carefully considered and we’ll take a look at how well R744 meets key criteria.

Andre Patenaude
Director – CO2 Business Development, Emerson Climate Technologies

Visit our website for additional information on CO2 Solutions from Emerson. 
Excerpt from original document; Commercial CO2 Refrigeration Systems, Guide for Subcritical and Transcritical CO2 Applications.


To read all posts in our series on CO2 as a Refrigerant, click on the links below:

  1. Series Introduction
  2. Criteria for Choosing Refrigerants
  3. Properties of R744
  4. Introduction to Trancritical Operation
  5. Five Potential Hazards of R744
  6. Comparison of R744 with Other Refrigerants
  7. R744 Advantages / Disadvantages
  8. Introduction to R744 Systems
  9. Introduction to Retail Transcritical Systems
  10. Retail Booster Systems
  11. Introduction to Retail Cascade Systems
  12. Introduction to Secondary Systems
  13. Selecting the Best System

 

Making Sense of the Recent Rulemaking Proposals on Acceptable Refrigerants

Global phase-downs and bans of hydrofluorocarbon (HFC) refrigerants in an effort to lower the global warming potential (GWP) of refrigeration and air conditioning systems are an everyday reality for equipment and component manufacturers, OEMs and end users alike.

The most recent development in this arena is the U.S. EPA’s Notice of Proposed Rulemaking (NOPR) for its Significant New Alternatives Policy (SNAP) wherein numerous common HFCs and HFC-containing blends will be listed as unacceptable for some uses. The NOPR is currently going through its mandatory comment period, but regardless of the exact, final outcome, real and significant changes are imminent. Understanding the likely implications of these changes is essential.

Join our next webinar on Tuesday, August 26 at 2 p.m. EDT, 11 a.m. PDT.

Our next Making Sense webinar, Staying Ahead of Recent EPA Rulemaking on Acceptable Refrigerants, will help explain the current status of the NOPR, explore compliant, low-GWP refrigerant alternatives, and discuss the challenges these changes will pose to our industry. Whether you manufacture refrigeration systems, operate supermarkets or other food retail operations, or supply stand-alone equipment or condensing units, you’ll need a clear understanding as to where this latest update to SNAP is headed and when. During this webinar, we’ll cover many topics, including:

  • The current status of the EPA’s most recent NOPR
  • Applications targeted by the new proposal
  • Viable, acceptable refrigerant alternatives
  • Equipment and system considerations when applying low-GWP refrigerants

This educational webinar will be presented by Dr. Rajan Rajendran, Emerson Climate Technologies’ vice president of engineering services and sustainability. He is one of the most respected, global authorities on alternate refrigerants and their applications across a variety of industries. He will be joined by a panel of experts to help answer any questions you may have.

Please join Dr. Rajendran and his panel on Tuesday, August 26 at 2 p.m. EDT, 11 a.m. PDT, for the latest information about the pending NOPR and its potential implications. Register now by visiting our website at www.emersonclimate.com/makingsensewebinars. We’re helping the industry Make Sense of the issues that matter most.

Craig Raney
Director of Marketing, Refrigeration
Emerson Climate Technologies

EPA Releases Proposed Timeline for HFC Status Changes

Earlier this year I reported the EPA’s intention to move on the delisting of certain hydroflourocarbons (HFCs) and HFC blends this summer. That process is now officially underway. On July 9, the EPA released the prepublication version of a Notice of Public Rule (NOPR) detailing proposed changes in listing status, tentatively scheduled to take effect as early as January 1, 2016.

The proposed rule is part of the Significant New Alternatives Policy (SNAP) program, which evaluates and revises acceptable alternatives to ozone-depleting substances (ODS) on an ongoing basis. A 60-day comment period for industry stakeholders begins once this NOPR is published in the Federal Register.

Key points in the rule include bans on R-404A and R-507A for new and retrofitted retail food refrigeration, including direct and indirect supermarket systems and walk-in cooler/freezer condensing units. It also proposes bans on R-404A, R-507A and HFC-134a in new, stand-alone refrigeration units and vending machines. Other blends to be delisted as unacceptable for direct and indirect supermarket systems include: HFC-227ea, R-407B, R-421B, R-422A, R-422C, R-422D, R-428A and R-434A.

These changes are intended to drive the market to refrigerants with lower global warming potential (GWP). In the near term, the industry will move toward already-tested, available alternatives, such as R-407A, carbon dioxide (CO2) or R-290, depending on the application. The proposed rulemaking will also accelerate the exploration of emerging alternatives, such as R-448A, R-449A, R-450A and XP10, etc.

Emerson Climate has been actively participating in meetings and discussions with the EPA and industry organizations and will continue to do so throughout the commenting process. It is essential that the move to low-GWP refrigerants continues to progress. But it is also important the rulemaking strikes a balance that accounts for the infrastructure changes and resources necessary to support the transition in an effective, commercially viable manner.

We will continue to work proactively on alternatives to make sure you have access to the products and services you’ll need to remain compliant with SNAP. In many cases viable substitutes are already approved and ready for application. We continue to work on developments that will not only meet this NOPR, but keep you compliant with foreseeable future rulings.

We welcome and encourage your participation and feedback throughout the commenting period. We’ll cover the proposed changes and discuss their implications in more detail in our Making Sense webinar on August 26. Click here to register.

For specific details of the proposed changes, review the Rule and Fact Sheet published by the EPA. Check back here for further updates as new information becomes available. We believe the final rule can be expected sometime in late November or early December.

Rajan Rajendran, Ph.D
Vice President, System Innovation Center and Sustainability
Emerson Climate Technologies

LEED v4

Not all building owners decide to pursue LEED certification.  But the LEED rating system virtually guarantees that the very best environmental practices are being followed.  In 2013, a new version of the rating system called LEED v4 will be approved. USGBC will keep LEED 2009 available for three more years, but project teams can move to the new version of LEED during that period. LEED v4 focuses on increasing technical stringency from past versions and developing new requirements for project types such as data centers, warehouses & distribution centers, hotels/motels, existing schools, existing retail, and mid-rise residential. The credit requirement changes in the proposed LEED v4 rating system are the most extensive in LEED’s twelve-year history. Retail-specific requirements will be added, including the energy and refrigerant credits.

The Minimum Energy Performance prerequisite will be updated to reference to ASHRAE 90.1-2010. Retail-specific process load requirements will be added including refrigeration equipment, cooking and food preparation, clothes washing, and other major support appliances. Many industry standard baseline conditions for commercial kitchen equipment and refrigeration will be defined, meaning that no additional documentation is necessary to substantiate these predefined baseline systems as industry standard. For appliances and equipment not covered in the baseline measures, LEED project teams must indicate hourly energy use for proposed and budget equipment, along with estimated daily use hours. ENERGY STAR ratings and evaluations are a valid basis for performing this calculation. For hard-wired refrigeration loads, team must model the effect of energy performance improvements with a simulation program designed to account for refrigeration equipment.

LEED v4 will also make changes to the Enhanced Refrigerant Management credit. Stores with commercial refrigeration systems must select equipment with an average HFC refrigerant charge of no more than 1.75 pounds of refrigerant per 1,000 Btu/h total evaporator cooling load. Store must also demonstrate a predicted store-wide annual refrigerant emissions rate of no more than 15% and conduct leak testing using the procedures in GreenChill’s best practices guideline for leak tightness at installation.

Green BuildingThe LEED® Green Building Rating System is a voluntary, consensus-based, market-driven building rating system based on existing proven technology. It defines and promotes green designs, and rewards organizations that adopt some or all of its principles towards green or integrated building design. LEED credits are awarded based on criteria in six categories of performance. A building project must meet a set of prerequisites to be registered, and it must achieve the minimum number of points to earn a basic ‘Certified’ level determines the level of LEED certification (from a Certified level through Silver and Gold to the Platinum level).

The retail and foodservice industries are investing in environmentally-friendly construction, in accordance with LEED guidelines, to enhance occupant comfort and reduce environmental impact. LEED building design requires some added initial cost; however, research shows the investment becomes offset over time by a reduction in energy usage and other related expenses.

Why the interest in LEED? Concern for the environment and sustainable development is growing, and LEED is a way that businesses can prove they are good corporate citizens. LEED promotes a whole building approach to sustainability through the principles of green building and integrated building design. There is a conscious effort to systematically integrate the design of building systems, such as HVAC, refrigeration, lighting, water management, and other mechanical systems with the building design itself, so as to achieve higher levels of performance.

Rajan Rajendran, Ph.D
Vice President, Engineering Services and Sustainability
Emerson Climate Technologies

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