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Contractors and energy efficiency

Many contractors do a good job of designing, building and servicing the refrigeration plants. However, energy efficiency is a relatively new direction in industrial refrigeration. Usually, contractors do not have experience in energy efficiency. They can install new PLCs and VFDs, but new equipment will not guarantee the expected energy savings. Here is an example of poor investment in energy efficiency.

Example. A refrigeration plant has 3 evaporative condensers equal capacity.

Condenser 1. Axial fans. Fan power is 40 HP.

Condenser 2. Centrifugal fans. Fan power is 80 HP.

Condenser 3. Centrifugal fans. Fan power is 80 HP .

A refrigeration contractor has installed fan VFD for condenser 1. Is it a good investment?

Condenser fans operate at full speed until minimum head pressure is achieved. At minimum head pressure we have to reduce the condenser capacity. Two options are available.

Option 1. Reduce speed of condenser fans 1(VFD).

Option 2. Cycle condenser fans 2(3).

We can compare energy savings of using option 1 or option 2.

90 % capacity of trimming condenser.

Option 1. Fan power is 73 %. A VFD loss is 2 %. Total power is 75 % or 40 HP * 0.75 = 30 HP. Power saving is 40 - 30 = 10 HP

Option 2. Fan power is 80 HP * 0.9 = 72 HP. Power saving is 80 - 70 =8 HP

Difference of power savings between option 1 and option 2 is 10 - 8 = 2 HP.

At 90 % capacity condenser 1 VFD is saving 2 HP more than condenser 2(3) cycling.

60 % capacity of trimming condenser.

Option 1. Fan power is 22 %. A VFD loss is 2 %. Total power is 24 % or 40 * 0.24 = 9.6 HP. Power saving is 40 - 9.6 = 30.4 HP

Option 2. Fan power is 80 * 0.6 = 48 HP. Power saving is 80 - 48 = 32 HP

Difference of power savings is 30.4 - 32 = - 1.6 HP

At 60 % capacity condenser 1 VFD is saving 1.6 HP less than condenser 2(3) cycling.

30 % capacity of trimming condenser.

Option 1. Condenser power is 3 %. A VFD loss is 2 %. Total power is 5 % or 40 * 0.05 = 2 HP. Power saving is 40 - 2 = 38 HP

Option 2. Condenser power is 80 * 0.3 = 24 HP. Power saving is 80 - 24 = 56 HP

Difference of power saving is 38 - 56 = - 18 HP

At 30 % capacity condenser 1 VFD is saving 18 HP less than condenser 2(3) cycling.

Capacity of trimming condenser can change from 0 % to 100 %. In this range, total power savings from cycling condenser fans 2(3) are greater than total power savings from condenser 1 VFD. What for to buy this VFD?

In my opinion this is unnecessary investment.

Any investment in energy efficiency should be based on system (refrigeration plant) performance. Energy efficiency improvement of one element will not guarantee improvement of the whole system performance.

I would compare energy savings in industrial refrigeration with drawing a painting. Many companies are claiming that they have good paint (PLCs), good brushes (VFDs) and etc. However, only an experienced artist can draw a good painting. Find a good artist not an art supplier.

 

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