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CO2 Evaporator Solutions

From The Heat Transfer Experts 

CO2 Evaporator Solutions for Industrial Refrigeration Systems

Author: Jeremy Olberding, Vice President
  |  Categories: Industrial Refrigeration

CO2 as a refrigerant is becoming an increasingly popular option for a wider range of industrial refrigeration applications. Colmac Coil has developed tools and a comprehensive  product offering to provide the market with the evaporators needed for any type of CO2 Industrial refrigeration system being installed today. Watch the following training to learn about Colmac Coil's CO2 evaporator solutions for the industrial refrigeration market.


I am Jeremy Olberding vice president at Colmac Coil and today I would like to introduce everyone to our growing line of CO2 evaporators for the industrial refrigeration market.

With the increased interest in using CO2 for a wider range of industrial refrigeration applications we at Colmac Coil have been investing significantly in our design tools and product offering to provide the market with the evaporators needed for any type of CO2 system being installed today.

I want to start with providing an overview of reasons we are seeing CO2 becoming more common, and the decisions required to finalize an overall design including system type, design pressure, certifications required and method of defrost.  My intention is not to make the case that CO2 is the best refrigerant, but that Co2 is a good option when the goal is to remove ammonia from the refrigerated space.  A two-stage ammonia system will continue to be the most efficient option but there are many cases where a CO2 system can provide a compelling value to an end user when all factors are taken in account.

I will follow with a review of the capabilities Colmac Coil provides with the evaporators to meet the need of any CO2 project.

As part of the product capabilities, I will also demonstrate our online selection software program that allows users to design CO2 coils with all the features discussed.  Included with the selection software are several design tools to allow for example users to design the glycol defrost for an evaporator coil with flow rates, glycol temps and defrost times.  Access to this software and any tools covered can be requested from the contact us page at

Why use C02 as a refrigerant

Carbon dioxide (CO2) is a non-toxic and non-flammable natural refrigerant but is dangerous in high concentrations. CO2 is different from ammonia in that it is heavier than air and so will tend to collect in higher concentrations at low points in confined spaces like engine rooms. As a result, CO2 detectors should be installed to make occupants aware of the concentration if any leak were to occur.

The A1 classification meaning CO2 is non-flammable and has low toxicity along with its unregulated status makes this refrigerant attractive to end users and with the small amounts used in most systems relative to the size of the space the OSHA 5000 PPM, 8-hour work limit would not be exceeded except in extreme release situations.

Refrigerant grade CO2 cost approximately 2 $/lb. compared to synthetic refrigerants in the 5 – 14 $/lb. range putting it comparable in price to refrigerant grade ammonia.

Equipment for CO2 even using hot gas defrost is becoming more widely available across all components especially as more manufactures enter the industry resulting in more competitively priced complete systems.  Regarding the installation cost, as stainless tubing is used instead of pipe the number of weld joints are being reduced and with orbital welding becoming more common the cost should continue to drop over the next several years relative to other more mature options.

The picture in the background is a CO2 project with insulated penthouse units being installed at a distribution center for a major food retailer and uses two coils per penthouse each isolated to allow a single coil to be hot gas defrosted at one time.

Removing ammonia from the occupied space is becoming of interest to many end users where doing so is feasible.

Good efficiencies at subcritical pressures make CO2 an excellent option for the low side of cascade systems whether that be with ammonia for higher efficiency or synthetics for possibly a lower cost.

While efficiencies are lower, transcritical CO2 is compelling in climates where trans critical operation hours will be minimal.  Also, recent improvements in eductors, parallel compression along with heat recovery have led to a surge in new transcritical systems being installed in recent years.

CO2  Design Decisions

Some of the decisions that need made early in the design process include:

What refrigerated space temperature levels are required and how this new space may interact with an existing part of the refrigeration system.  This an area where adding a complete CO2 DX package to provide a standalone system for a specific temperature level could be of value.  This could be for example a smaller size low temp -30F ice cream storage or larger medium temp load the existing system may not currently have spare capacity for.

The seasonal range in ambient temperatures will be an important factor in deciding what type of CO2 system to consider.  An area with only a few months of warm weather may be a good candidate for a trans critical system whereas in warmer climates a cascade system may have a lower overall operating cost that can justify the additional initial cost of a more complex system.

The type of defrost options available will depend on the type of system being installed, the room temperatures, and the cost of energy in that location.

Colmac Coil CO2 Solutions

The design code the evaporator must meet at the Maximum allowable working pressure (MAWP) is an important aspect of selecting equipment and needs to be established at the being of the project to make sure the equipment being proposed meets the operating pressures intended using a given standard.  We build a CO2 evaporator differently depending on what codes are required for a given project.

UL 207 is the standard for Refrigerant-Containing Components and Accessories.  The code is nonelectrical and establishes safe operating limits by building and testing to failure at several times the MAWP for the published equipment.  The standard is recognized across a range of industries and is our most common listing for CO2 coils in refrigeration and HVAC applications.

The ASME U Stamp is the certification of a manufacturer's quality control system in accordance with ASME Boiler and Pressure Vessel code.  To meet these requirements we document the design, perform calculations, and have each vessel inspected.  This process is common but adds extra expense and is not typically required for CO2 coils.

The Canadian Registration Number (CRN) is a number issued by each province or territory in Canada by an authorized safety authority for any boiler, pressure vessel or fitting that operates at a pressure greater than 15 psig.

Colmac Coil has listings in place to allow us to build evaporators to meet any of the codes previously discussed for CO2 coils operating up to 120 BAR or 1740 PSIG.

DX CO2 Features

Installing the electronic expansion valves as part of the evaporator for DX CO2 coils can reduce the complexity of the piping that needs to be installed in the field and allows most of the required components to be either installed at the coils or as part of the compressor package.  The first image shows a single EEV installed on the evaporator in a 2-pipe configuration allowing both hot gas for defrost and liquid during the cooling cycle to feed into the drainpan and then into the coil with a single outlet connection to serve as both the suction outlet and hot gas condensate return.  The second image shows the same 2 pipe configuration but includes 2 valves which can be used for capacity control and may be required for large capacity CO2 evaporators. 

Along with the expansion valves it is beneficial to locate the pressure and temperature sensors for the evaporator in the piping installed on the evaporator where fittings can be welded into the piping, tested at the factory and ship installed and pre-wired.

We install venturi distributors to provide a wide operating range over a range of temps and capacities.  These are permanently installed and do not require a change in orifices or maintenance.

To provide a leak free high-quality design we used all stainless welded construction and perform hydro pressure test as required by the code and fully dry the coil before shipment.

We provide any of the 3 types of connections:

  • Pipe connections with stubs or socket weld fittings.
  • Piping connections with adapters for high pressure copper piping.
  • Pipe to tube adapters, allowing the installing contractor to only make tube to tube weld joints in the field.


Colmac Coil CO2 Defrost Solutions

Defrost is a critical process for any industrial air cooler operating at temperatures below freezing. At freezing temperatures frost builds up on the evaporator fins, causing units to lose cooling capacity and efficiency.

Air Defrost

Starting with air defrost, the only critical part of air defrost with CO2, is to design the system to never allow liquid CO2 to become trapped at any point in the system that could become isolated.  All areas of the system must have a pressure relief which typically means adding extra check and relief valves.  This prevents an extremely dangerous over pressurization event from occurring.

Hot Gas Defrost

Hot gas defrost is the preferred method of defrosting an evaporator when the design of the system allows for adequate hot gas temperatures and flow rates to be produced at all times of the year.  Hot gas defrost is fast and works well to completely clear frost from the coil in an energy efficient way. Hot gas defrost typically requires a minimum of 1.5 times the cooling capacity to adequately heat the coil during defrost but if this is not available especially seasonally at times of low load this can be supplemented with electric defrost.

Electric Defrost

Electric defrost is commonly used where hot gas defrost is not feasible.  

Electric defrost has a low initial equipment cost but the additional expense of the electrical infrastructure and controls should be included in any comparison between electric defrost and alternate options.

Electric defrost can have a high ongoing operating cost due the electricity consumed.  The cost of electricity can be reduced by using inlet air hoods as shown in the picture to the left and we typically see the pay back on adding the inlet hoods to units with electric defrost paying for themselves in 2-3 years. 

Demand defrost for both initiation and termination should also be considered to reduce energy cost.

Electric defrost does require ongoing maintenance as heater elements fail and will to be need replaced in the future.

Interlaced Glycol Defrost

Interlaced warm glycol defrosting uses a heated brine or glycol solution to introduce defrost heat into the coil block via a secondary interlaced tubing circuit. This method is particularly useful for defrosting CO2 evaporators where hot gas is not feasible.

The glycol is heated in a heat exchanger which absorbs heat from the discharge gas of one or more compressors. The heated glycol is held in an insulated tank until a defrost cycle is required. Glycol pumps deliver the warm glycol, normally stored between 70 and 120 deg Fahrenheit to the evaporator being defrosted by opening a solenoid valve in the glycol delivery pipe work at the coil block.

The warm glycol is delivered into dedicated glycol tubes in the coil block, which are arranged in the coil such that heat can conduct from the tubes into the coil fins melting the frost. The drainpan is also warmed by a glycol tubing loop to ensure the pan is kept warm while melted frost is collected and drained.

The image to the right shows a stainless tube CO2 cooling circuit with a copper interlaced glycol heating circuit.

Water Defrost

Water Defrost:  This method produces a very fast and complete defrost and can be designed to work at all temperatures. Waste heat from the high side of the system can be used to heat the defrost water that is returned cold from the evaporator coil. Care must be taken to provide for complete drainage of water lines at the end of defrost as well as specifying and using high quality water supply valves.  Depending on the climate the amount of returned water from a defrost cycle will return an excess amount of water that over time can be filtered and used as makeup water for the evaporative condensers or other processes at a facility.

EC Fan Motors

EC fans are a great optional feature to include on CO2 systems to allow the fan speed control to be performed by the individual control system for a particular room or zone in the facility.  Reducing the fan speed as possible results in additional energy savings and reduced sound levels without the need for external variable frequency drives.

For the fan assembly we make the complete fan panel hinged to allow easy access for any cleaning service or replacement of fan.

The high-quality construction and electrical protection of EC fans have resulted in good reliability and in freezer environments and the addition of fan tube heaters shown in the top picture and powered during defrost prevents ice buildup issues.

A+Pro Selection Software

The A+Pro selection software allows users to enter a set of selection criteria and get a range of units that meet the requirements of the project.  We start with the design tab and for this example, we are designing an insulated penthouse unit setup for DX CO2 with a 1740 PSIG maximum allowable working pressure and hot gas defrost.  We have also included 90 deg duct extensions and additional external static pressure in the design.

Instead of using the selection method the rate method allows a user to design a penthouse unit that meets an exact set of criteria including the foot-print dimensions, fan diameter, motor horsepower, along with a completely custom coil design.  This rate method allows a user to design any unit that is within Colmac Coils manufacturing capability and the cost are all based on the exact unit designed. 

After the initial design info is entered you specify the range of sizes, dimensions, and temperatures you want the unit design based on.  For this example, we are running a 25 TR -10 F room with a -20 F SST for DX CO2 in an insulated penthouse.  By clicking on the selection button our program evaluates all possible options running through thousands of possible combinations and in about 30 seconds gives you a range of solutions depending on what features are important beyond the lowest cost selection that meets your requirements including weight, air flow, motor hp, sound, refrigerant charge.

The lowest cost option that meets all the requirements is displayed as the highlighted unit.  The list of the optional units can be sorted based on any of these features that are important for a particular project.  An example of this is we have our lowest cost option as a 4 x 5 hp unit with a slightly more expensive 3 x 5 hp unit available at a slightly higher initial cost, shown a few lines below.

Once the desired unit has been identified and chosen the optional features can be added and the additional cost of each option evaluated.  This allows the user to customize almost all unit features independently. Additional similar selections can be completed much more quickly based on using the last design as a starting point.

CO2 A+P Penthouse

Regarding the insulated penthouse unit we just designed, please download the complete brochure with all details and features for these units.

Interlaced Glycol Defrost Design Tool

Also included in the A+Pro selection software is an interlaced glycol defrost design tool.  For any evaporator you can change the type of defrost to interlaced glycol with a heat fluid pan.  You then specify your type of fluid from the drop-down list shown and the % concentration.  The program will then design this coil as part of the evaporator selection.

To fully design the interlaced glycol coil you just need to specify 1. The entering fluid temperature, 2.  The maximum allowable pressure drop through the coil and 3. A defrost time.  The flow rate required will be calculated along with a coil design.  You can also change of these 3 inputs to adjust the calculated flow rate for example a longer defrost time will result in a reduced flow rate and reduce your piping and valve sizes.

The required flow rate is then shown as part of the complete unit design in this case the 44.35 GPM will be adequate to defrost this coil in 15 min using 80 deg F glycol temperature, with an additional 6.65 GPM of glycol used to separately heat the drain pan.

After multiple units are designed and saved any combination of these documents can be created and downloaded into common files, in pdf or word format. 

An example of one of these combined files is an engineering specification document.   This engineering specification is created as an open format excel document where all unit specs and options are shown with an “Options key” and is available for the user to cut and paste data from or simply add your logo and incorporate into your PROJECT specification.

Thank you everyone for you attending today’s presentation.

For more information and to get access to the software programs you saw today, please visit or contact your local Colmac Coil sales representative. Thank you very much,

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