Seals for CO2 Refrigeration
Greenhouse gas makes for a cooler world.
Carbon dioxide, long considered a “greenhouse gas” for its potential to contribute to global warming, now finds a possible future in replacing halogenated refrigerants in air conditioning and refrigeration systems. High-pressure CO2 offers interesting and unique design challenges for HVAC system development, particularly in the selection of seal materials.
Temperature
The extreme temperature range involved in the transcritical CO2 refrigeration cycle (-40° to +160°C) limits the choice of seal materials. Fluorocarbon (FKM), Hydrogenated Nitrile (HNBR), and Ethylene-Propylene (EP) compounds exhibit the most suitable thermal stability.
Chemical compatibility
When evaluating the chemical compatibility between carbon dioxide, compressor oils and rubber sealing components, it is necessary to consider several important issues that can directly affect system performance. The polarity of the carbon dioxide molecule results in lower potential for swell, permeation, and explosive decompression with EP seals than with fluorocarbon or HNBR materials.
Some Polyalpha olefin (PAO) compressor oils can cause swelling of EP compounds, but are generally compatible with fluorocarbon and HNBR. Polyalkylene glycol (PAG) and polyol ester (POE) oils are generally compatible with all three seal materials. But because rubber compounds and commercially-available PAGs and POEs vary widely, compatibility testing with specific compressor oils and seal materials should be performed to ensure acceptable seal performance.
Figures 1-3 (see reverse) document the effect of various compressor oil technologies on representative rubber compounds. Figure 4 (see reverse) demonstrates the differences in performance of compounds from within the same polymer family – the exact compound used in application is critical to success.
Pressure
Carbon dioxide refrigeration cycles must operate at significantly higher pressures than those of conventional refrigerants. Pressure-related seal failure can be caused by extrusion (gas pressure forcing the seal material into a clearance gap) permeation (pressure loss) and by explosive decompression (blisters and splits caused by expansion of gas trapped within the seal material).
Figure 5 (see reverse) displays the relationship between maximum attainable fluid pressure before extrusion occurs, the clearance gap on the low-pressure side of the rubber seal, and the hardness of a rubber seal material.
Explosive Decompression can be roughly predicted, but specific compounds should be evaluated for permeation and explosive decompression in actual operating conditions whenever possible.
This warranty comprises the sole and entire warranty pertaining to items provided hereunder. seller makes no other warranty, guarantee, or representation of any kind whatsoever. All other warranties, including but not limited to, merchant ability and fitness for purpose, whether expressed, implied, or arising by operation of law, trade usage, or course of dealing are hereby disclaimed. Notwithstanding the foregoing, there are no warranties whatsoever on items built or acquired wholly or partially, to buyer’s designs or specifications.
Limitation of remedy: Seller’s Liability arising from or in any way connected with the items sold or this contract shall be limited exclusively to repair or replacement of the items sold or refund of the purchase price paid by buyer, at seller’s sole option. In no event shall seller be liable for any incidental, consequential or special damages of any kind or nature whatsoever, including but not limited to lost profits arising from or in any way connected with this agreement or items sold hereunder, whether alleged to arise from breach of contract, expressed or implied warranty, or in tort, including without limitation, negligence, failure to warn or strict liability.
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Temperature
The extreme temperature range involved in the transcritical CO2 refrigeration cycle (-40° to +160°C) limits the choice of seal materials. Fluorocarbon (FKM), Hydrogenated Nitrile (HNBR), and Ethylene-Propylene (EP) compounds exhibit the most suitable thermal stability.
Chemical compatibility
When evaluating the chemical compatibility between carbon dioxide, compressor oils and rubber sealing components, it is necessary to consider several important issues that can directly affect system performance. The polarity of the carbon dioxide molecule results in lower potential for swell, permeation, and explosive decompression with EP seals than with fluorocarbon or HNBR materials.
Some Polyalpha olefin (PAO) compressor oils can cause swelling of EP compounds, but are generally compatible with fluorocarbon and HNBR. Polyalkylene glycol (PAG) and polyol ester (POE) oils are generally compatible with all three seal materials. But because rubber compounds and commercially-available PAGs and POEs vary widely, compatibility testing with specific compressor oils and seal materials should be performed to ensure acceptable seal performance.
Figures 1-3 (see reverse) document the effect of various compressor oil technologies on representative rubber compounds. Figure 4 (see reverse) demonstrates the differences in performance of compounds from within the same polymer family – the exact compound used in application is critical to success.
Pressure
Carbon dioxide refrigeration cycles must operate at significantly higher pressures than those of conventional refrigerants. Pressure-related seal failure can be caused by extrusion (gas pressure forcing the seal material into a clearance gap) permeation (pressure loss) and by explosive decompression (blisters and splits caused by expansion of gas trapped within the seal material).
Figure 5 (see reverse) displays the relationship between maximum attainable fluid pressure before extrusion occurs, the clearance gap on the low-pressure side of the rubber seal, and the hardness of a rubber seal material.
Explosive Decompression can be roughly predicted, but specific compounds should be evaluated for permeation and explosive decompression in actual operating conditions whenever possible.
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- Warranty
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This warranty comprises the sole and entire warranty pertaining to items provided hereunder. seller makes no other warranty, guarantee, or representation of any kind whatsoever. All other warranties, including but not limited to, merchant ability and fitness for purpose, whether expressed, implied, or arising by operation of law, trade usage, or course of dealing are hereby disclaimed. Notwithstanding the foregoing, there are no warranties whatsoever on items built or acquired wholly or partially, to buyer’s designs or specifications.
Limitation of remedy: Seller’s Liability arising from or in any way connected with the items sold or this contract shall be limited exclusively to repair or replacement of the items sold or refund of the purchase price paid by buyer, at seller’s sole option. In no event shall seller be liable for any incidental, consequential or special damages of any kind or nature whatsoever, including but not limited to lost profits arising from or in any way connected with this agreement or items sold hereunder, whether alleged to arise from breach of contract, expressed or implied warranty, or in tort, including without limitation, negligence, failure to warn or strict liability.
