S K Energy Consulting Ltd.

View Subscription Details: Letter ( 10/31/2014 ) - Plain Text Format Do you have right consultant?

Recently, I found that some engineers who save energy for buildings try to save energy in industrial refrigeration as well. I read a couple of reports and I was not impressed. First report is absolutely useless because it suggests close monitoring (not controlling) of the temperature in the refrigerated room. Second report suggests using VFDs for compressors and condenser fans as well as use of free cooling for the cooler when weather permits. Free cooling requires significant investments and it can be used for a limited period of time. Every refrigeration plant will benefit from VFDs, but they are expensive and the payback will be long. Payback for energy savings measures mentioned in the second report will be 10 years. Most likely many companies will not invest in these measures. Probably, the consultants who have done these reports are good engineers, but approaches to energy savings for the buildings and for the industrial refrigeration are fundamentally different.
Let me show the difference between energy savings for the buildings and for industrial refrigeration.

ENERGY USE = EFFICIENCY x REFRIGERATION

When engineers save energy for the buildings they try to reduce refrigeration load. Less cooling when rooms are not occupied, avoid simultaneous cooling and heating, use of free cooling by ambient air when weather permits and so on. I think that many good engineers can do these energy saving projects.
Look at the energy savings in industrial refrigeration. Very limited savings can be achieved by reducing refrigeration load. Sometimes free cooling can be used but it is very limited and expensive. Improving energy efficiency of the refrigeration plant is the major focus of the energy savings in industrial refrigeration. However, it is complicated to achieve these savings because one component of the refrigeration plant influences energy use of another component. System performance, at different refrigeration loads and at different ambient conditions, should be optimized. This is the reason why only experts can do full optimization. What an expert should know?
1. Design of the industrial refrigeration plant. He should determine deficiencies of the refrigeration plant and he should find the cost effective way to overcome these deficiencies.
2. Operation of the refrigeration plant. This is very important, because 70-80% of total energy savings are operating savings.
3. Service of the refrigeration plant. To maximize efficiency of the plant it should be tuned up properly. For example. To save energy, hot gas defrosting should be done at minimum condensing pressure. Some people do defrost at 150 psig, others at 120 psig. However, every evaporator can be defrosted at condensing pressure below 100 psig. 2 psig of lower condensing pressure saves 1% of energy.
4. An expert should do his own research to determine optimum set points and operating strategies for the refrigeration plant. What is the optimum wet bulb approach of the condensing pressure? How this approach should be changed based on variation of the refrigeration loads and ambient conditions? What capacity of the evaporator should be to initiate hot gas defrost? 90%, 80%, 70%, 50%... How to determine this capacity? These are just a few questions that should be answered.
As far as I know, only a limited number of people have done this research because thousands of tests should be done for different refrigeration plants at different ambient conditions. Only these people can provide full optimization of the refrigeration plant operation.
Ask the right questions when you choose a consultant for your next energy efficiency project.

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