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View Subscription Details: Letter ( 05/17/2005 ) - HTML Format

Floating Condensing Pressure

Let me show you an example of floating condensing (head) pressure for ammonia refrigeration plant.


Assume that we have a condensing pressure of 152 psig, a corresponding condensing temperature of  85 °F, a wet bulb temperature of 60 °F, the compressor power is 1000 HP, and the condenser power is 60 HP. Every step will increase condenser power by 20 HP. This means that an additional fan (20 HP) will be turned on. Wet bulb approach is Tcond – Twet bulb.

Step

Cond. Pressure

Cond. Tem-re

Wet bulb Approach

Comp. Power

Cond. Power

Total Power

Psig

°F

°F

HP

HP

HP

1

152

85

25

1000

60

1060

2

138

80

20

950

80

1030

3

131

77

17

920

100

1020

4

125

75

15

895

120

1015

5

121

73

13

880

140

1020

6

119

72

12

870

160

1030

From this table we can see that step 4 has a minimum total power of 1015 HP at a condensing pressure of 125 psig and a corresponding condensing temperature of 75 °F. Wet bulb approach for this step is 15 °F. This is an optimum for this plant and for a wet bulb temperature of 60 °F.


My research has shown that optimum wet bulb approach can very from 10 °F to 24 °F for different plants and for different ambient conditions.


For the mentioned plant, we are overspending energy for compressors at steps 1, 2, 3, and we are overspending energy for condensers at steps 5, 6, 7.

Assume that during the night the wet bulb temperature has changed to 55 °F. Our optimum condensing temperature would be 55 + 15 = 70 °F and the condensing pressure is 114 psig. On the next day, the wet bulb temperature has increased to 62 °F. Our optimum condensing temperature would be 77 °F and the condensing pressure is 131 psig. Therefore, the optimum condensing temperature and the condensing pressure will follow wet bulb temperature.


A refrigeration plant with fixed condensing pressure control will always overspend energy for compressors or for condensers. If you have that type of condensing pressure control, you have several options to improve efficiency of your plant:

  1. Change condensing pressure setting several times per day, per week, per month, per year.
  2. Reprogram yourPLC.
  3. Replace PLC.

If you do not have PLC, you still have opportunity to save energy by changing settings of condensing pressure controls several times per year.


Let me remind you, that floating condensing pressure feature will only give you opportunity to save energy.

Regrettably, people very often do not use this opportunity. I have visited several plants with floating condensing feature, but operating engineers prefer to run these plans at fixed condensing pressure.

Some plants float condensing pressure, but they do not have the optimum wet bulb approach.


How can we apply cost-effective energy saving approach to condensing pressure control?


Step 1. Housekeeping. Operating engineers, usually, know the direction of energy saving, but they do not know optimum set points.

Step 2. Low cost investment. Consultant knows direction and set points, but sometimes he does not have tools to reach these set points.

Step 3. Retrofitting. We have direction, set points and tools to maximize efficiency of refrigeration plant.

Optimum condensing pressure can float up and down, but every refrigeration plant has minimum allowable condensing pressure. This pressure is very important, because every refrigeration plant operates at minimum allowable pressure thousands of hours per year.

During periods of cool weather, floating condensing pressure will give us opportunity of significant energy saving. Very often we do not use this opportunity, because of minimum condensing pressure. To maximize efficiency of a refrigeration plant we need to have this pressure as low as possible. There are some real and imagined barriers to reducing condensing pressure. Typical reasons for maintaining a higher condensing pressure include hot gas defrosting, liquid supply, liquid injection oil cooling, medium heating, and compressor lubrication.

Fortunately, every barrier has a solution. In the next several newsletters we will have a look at these barriers.

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