Balancing cost, efficiency, reliability and risk when choosing CO₂ refrigeration.

Last month, we looked at some refrigeration applications where carbon dioxide is not a sensible choice of refrigerant. In the majority of situations this is not the case, and carbon dioxide can be used to good effect, but there are many other options. The choice of refrigerant is a trade-off between competing factors including cost of installation, cost of operation, reliability and ease of maintenance. What factors should drive that choice?

The biggest mistake you can make in this situation is to ask an engineer “Can you do this?” because the answer will always be “Of course I can!” We all love a challenge and these questions are what makes our job enjoyable. The right question is “Should you do this?” which puts a different perspective on the situation.

Applications I would describe as “borderline” for the use of carbon dioxide include supermarket refrigeration from a central pack, water chillers, large distribution warehouses in cascade with an ammonia plant, large distribution warehouses with transcritical carbon dioxide and spiral freezers. The purchasing decision is often based purely on first cost—a lower capital investment wins the day without sufficient consideration of other factors. The borderline solution might be cheaper, but it has a lower seasonal energy efficiency ratio (SEER) so it will cost more to run every year. Or perhaps it is more difficult to maintain in good condition, and the SEER will gradually drop as time goes by. The system might be more expensive than the traditional approach, but offers an “environmental” tag so is seen as worth the extra cost. However, it is unfamiliar to the technicians who are used to working on the older plants and find it difficult to adapt to these new ideas. The result could just be suboptimal, inefficient running or, in the worst case, it could result in catastrophic system failure.

The key, unfortunately often overlooked in these situations, is to do your homework when considering the adoption of new technology. Asking for a system that is the same as the old one in all respects, including the price, except that it has no fluorine in the refrigerant is like saying, “My car can go 90 mph; I want a boat that goes the same speed, but for the same price.” Just the act of asking indicates that the proposition has not been fully understood. This is where unforeseen risks can slip in unnoticed until something goes wrong. A thorough assessment of the choices to be made when on the borderline would include consideration of energy, maintenance, reliability, maintainability, capability (meaning the risk of losing temperature control under extreme conditions) and obsolescence. This thorough approach is normal in process industries, often called a “hazop” (short for hazard and operability) study. It is a giant “what if?” exercise. Often it is impossible to give a true comparison between two risks, neither of which is usually stated in financial terms, but trying to do so can be very instructive and informative.

In most cases when on the borderline the consequences of making a suboptimal choice are not fatal for a business, but they could be. More often the damage creeps in, like old age, and the gradual diminution of performance goes unnoticed until a comparison with capability from years ago is highlighted. To put it another way, a business that bases its choice of refrigeration system on a two to three year time horizon but is intending to be in business for decades to come should not be surprised if they have to spend the same again after a few years if they bought cheap the first time round.