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View Subscription Details: Letter ( 02/15/2005 ) - HTML Format

Before we start a journey in energy efficiency of industrial refrigeration, it is better to know our goal. The goal is an ideal (energy efficient) refrigeration plant.

Control of this plant consists of 3 parts:

  1. Software to determine optimum operating strategies and optimum set    points at any given moment.
  2. SophisticatedPLC to implement these strategies and set points.
  3. VFD (variable frequency drive) for compressors, condenser fans and evaporator fans.

Unfortunately, mentioned software doesn’t exist because it is a very complicated and expensive issue. Fortunately, if we change operating strategies and operating set points several times per year, we will be very close to optimum (the most efficient) operation.

What is the most important part of this control?

I believe that optimum strategies and optimum set points are the most important part of control for every refrigeration plant. VFD can save energy, but it less effective than right strategies and right set points.

Several chief engineers told me that their companies have bought very expensive (sometimes more than $100,000) PLC, but they haven’t received energy savings that were expected. Major reason of this is wrong set points and operating strategies.

Sophisticated PLC doesn’t save energy itself; it just helps us to implement right operating strategies and right set points. These PLC are good tools, but you have to know how to use them.

I’ll give you a real life example of optimum (flexible) operation against traditional (fixed) operation.

One company has a cooler. Temperature in this cooler is 50 °F. Three walls have constant exterior temperature of 35 °F. One wall and roof have ambient exterior temperature. This cooler requires heating all year around. Two evaporator coils are located inside this cooler and supplied by discharge hot gas (ammonia) for heating. Set point for minimum condensing pressure is 130 psig and back pressure regulators (BPR) for these coils are set to 110 psig. Requirement to keep minimum condensing pressure at 130 psig is a major barrier to reduce this pressure and save energy. This setting \was unchanged for the last several years. This is a traditional approach.

Does this cooler require the same amount of heat all year around? No.

Maximum heat requirement is at winter time, less heat is required at fall and spring time, and minimum heat requirement is at summer time.

During fall and spring we can change settings for minimum condensing pressure and BPR to 120 psig and 105 psig accordingly.

During summer we can change these settings to 110 psig and 100 psig accordingly. Not every summer day we can reach condensing pressure of 110 psig, but this setting will give us opportunity to float condensing pressure according to refrigeration load and ambient condition.

New settings will reduce heating capacity of these coils to match heat requirements and they will give us opportunities to save energy. To optimize performance of mentioned coils we also have to balance hot gas supply. This is an efficient (flexible) operation.

As I mentioned, sophisticated PLC is the good tool. This refrigeration plant has a good PLC, which gives opportunity to control operating time of these coils. I suggest keeping this time at 90-95 % level by reducing minimum set point for condensing pressure.

My estimation has shown that optimization of minimum set point of condensing pressure will reduce annual energy bill for this company by $20,000.

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