Loop Testing

A closed-loop test probably will be necessary to performance tests within the code. There is more accuracy in the loop test than in the air test because volume ratio matching can be more closely achieved. The loop test has several limitations that may not be obvious. Loop tests are generally expensive and time-consuming. They may become complicated with complex compressor configurations and become impractical to set up. Finally, the number of gases available for shop-loop testing is quite limited. It should be noted that most gases must be recovered after the test because of environmental regulations. Air, which is quite available for normal testing, is too dangerous for use in loop testing. In a loop test, air and oil may come into contact causing the danger of explo where sion. Figure 10-2 shows a typical shop test schematic of a loop arrangement. A two-body loop test arrangement is shown in Figure 10-3.

Because of safety concerns, all combustible and/or toxic gases must be used in outdoor test loops or in a special indoor test building with the required safety monitoring equipment. The gas cost factor makes the problem even more difficult. The problem of known gas properties adds another complication. Despite all the negative aspects just mentioned, most performance tests are closed-loop tested.

Test Loop Design

After calculating the test point conditions at the equivalent design rating, similar calculations should be carried out at surge and overload. These points can then be checked for significant deviation from equivalence. The operating envelope, so defined, can now be drawn on a Mol-lier diagram. This is especially important when the test gas used is a refrigerant in order to establish if condensing can occur in any part of the loop. Although the gas discharged is hot, any of this gas in static lines will, of course, be at ambient temperature and the possibility of condensation must be checked. When oil film seals are used, condensation in the

Figure 10-3. Compressors set up for both a closed-loop and string test on the test stand. (Courtesy ofA-C Compressor Corporation)

overhead tank will cause serious problems. On the other hand, the use of mechanical contact seals on compressors wiil avoid this problem except for die effect on static instrument lines because there is no overhead tank. When manometers or transducers are used on the discharge side and pressure lines are arranged to be short and self-draining, it is still possible for an acceptable test to be carried out.

In order to avoid the need to measure velocity head, the loop piping must be sized to have a velocity pressure less than 5% of the static pressure. Flow conditions at the required overload capacity should be checked for critical pressure drop to ensure that valves are adequately sized. For ease of control, the loop gas cooler is usually placed downstream of the discharge throttle valve. Care should be taken to check that choke flow will not occur in the cooler tubes. Another cause of concern is cooler heat capacity and/or cooling water approach temperature. A check of these items, especially with regard to expected ambient condi-

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tions, should be carried out. It is surprising how often tests are abandoned because of these problems, causing delays in shipment.

The code requires the submission of an instrument and piping flow sheet. All instruments should be numbered for identification and the actual instruments labeled. The code is very helpful in recommending the range and sensitivity required, and close adherence is recommended.

Currently, most manufacturers use automatic data gathering. Here, pressure signals will be obtained by the use of transducers. Under these circumstances, the transducers should be calibrated per code and certified

If data gathering is also computer-linked, then the problem is to check out the correctness of the program. Hand calculation of one rated point for flow head, efficiency, and horsepower will serve as verification.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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