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View Subscription Details: Letter ( 10/17/2006 ) - HTML Format

When to save energy?

Recently, I was told by one manager that his company paid a significant sum for electrical energy during the hot summer. He was interested in opportunity to reduce this payment. Is the summer a good time to save energy? Unfortunately, it is not.

We need 2 major conditions for good energy savings:

1.     Cool weather.

2.     Part load of refrigeration plant.

Cool weather can give us opportunity to save energy by floating head pressure down to minimum allowable pressure. What is the minimum allowable head pressure? That is a very important question, because the floating head pressure can often give us over 50 % of total energy savings. I believe that every refrigeration plant can run at head pressure lower than 100 psig as soon as wet bulb temperature of ambient air is lower than 30 °F. Different plants require different investment to reach that low head pressure. Sometimes you need a small investment to reach 100 psig and a significant investment to reduce this pressure from 100 psig to 80 psig.

Part load of refrigeration plant can give us opportunity to balance capacities of compressors, condensers and evaporators. Screw compressors should not run at capacity lower than 50 %. Capacities of condensers and compressors should be balanced to minimize total (condensers + compressors) power consumption. We should control the wet bulb approach (May, 2005 newsletter) to keep this power at minimum level. My research has shown that every refrigeration plant has its own optimum wet bulb approach and this approach will vary from 10°F to 24°F.

Capacities of compressors and evaporators should be balanced as well. Our goal is minimum power consumption by compressors and evaporators. Usually, to save energy we increase the suction pressure. I believe that every refrigeration plant should run at optimum suction pressure. Sometimes we increase suction pressure and save 5 HP of compressor power. To maintain increased suction pressure we have to run an additional evaporator with fan power 15 HP. As a result, we have lost 15 - 5 = 10HP.

We are saving energy when we have opportunity to do that. Unfortunately, the hot weather will give us very small opportunity to save energy. We can install an additional condenser and will save some energy at lower head pressure. What will be the annual operating time of this condenser? The shorter the operating time of additional condenser, the longer its payback.

Until now, I shared with you my experience in energy efficiency. However, I would like to know your preferences. What topics would you like to see in my newsletters?  Please, send me your e-mails.

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