M. Theodore Gresh
Elliott Co.
Jeannette, Pa.
A field procedure is available to monitor and verify the performance of refrigeration compressors equipped with side ports.
Side ports are installed on a compressor in addition to the main suction port so that the compressor can pick up loads at intermediate pressures between suction and discharge pressure. Using side ports allows higher efficiency operation of a refrigeration system where several loads are encountered at different temperature levels.
Field verification of compressor performance is a difficult task, even for singlestage units. The task becomes more difficult if the compressor is equipped with side ports.
Verification of a compressor's performance is necessary to ensure that a compressor performs as the vendor claims it will. Monitoring a compressor's performance is valuable if the compressor is to be used at operating conditions different from its original design operating conditions, and for predictive maintenance measures.
For this field procedure the compressor must be broken down into two (or three depending on the number of nozzles) separate sections. Each section must be treated as a totally separate compressor and the appropriate head and efficiency equations used to calculate performance. Determining the end points of the sections (pressure, temperature, and flow rate at the mixing point) can be difficult. The following provides this required data:
Preferred instrument locations are as shown in Fig. 1. Sideload and extraction line, if applicable, are to be treated as inlet and discharge lines, respectively. If existing instrument tapping points must be used, care must be taken that those used are reasonably close to the compressor flanges. There must be no valves, strainers, silencers, or other sources of significant pressure drop between the pressure tap points and the compressor flanges.
Evaluation of sideload and extrication compressor performance is difficult and subject to significant inaccuracies unless internal pressure and temperature probes at the sideload or extraction are available.
Pressure and temperature taps can be relatively easily added to a horizontally split compressor as shown in Fig. 1 by drilling and tapping the casing in the return channel crossover area. A minimum of two each pressure and temperature taps should be used.
For barrel-type compressors, use the casing drain in this area if available.
Although neither of these methods will provide precision data, they will obtain good relative data which one can use to track the relative performance of a compressor.
These data can become more realistic if a total mass balance and power balance is done on the system to determine accuracy of the test.
If more precise data are required, such as for the acceptance test, it may be necessary to run temperature and pressure probes to the end of the return channel as shown in Fig. 1.
Once pressures and temperatures are known at the discharge of Section 1, a mixing calculation is required to establish suction conditions for the next section. Fig. 1 shows the locations, whose numbered designation serve as subscripts in the following equations:
P1 = P2 = P3
WHERE:
P1 = Discharge pressure of Section I, psia
P2 Sideload pressure, psia
P3 Mixed suction pressure to Section II, psia
M1h1 + M2h2 = M3
WHERE:
M = Mass flow rate, lb/min; T=Temperature R., h = Enthalpy, BTU/lb and
M1h1 + M2h2 = M3
T3 is then found by working back through the gas properties or Mollier diagram knowing h3 and P3.
T3 may be very accurately approximated by:
T3 = M1T1 + M2T2
------------
M1 + M2
SPECIAL DATA REDUCTION FOR SIDELOADS
Special data-reduction techniques can be used on sideload and extraction compressors where internal pressures and temperatures are not available. Internal pressures can be estimated from flange pressures, gas velocity through the compressor nozzle, and standard pressure drop loss coefficients for a given sideload or extraction-nozzle design.
Internal gas temperature at the discharge of each section is also required to determine sectional performance. This can be accomplished through an iterative process that makes use of predicted work curves for each section.
The procedure begins for a given test point by establishing the inlet volume flow for Section 1. From the predicted work curves, the estimated work input is obtained. These data, along with the internal pressure determined above, are used to establish the estimated discharge temperature for Section 1.
A Mollier diagram, or data with the enthalpy as a function of pressure and temperature, is used. The sideload flow, as measured on site, will then be mixed with the calculated discharge flow from Section I to establish the inlet flow to Section 11. This procedure is then repeated for each following compressor section using its respective work input plot.
The test on the validity of the work input curves is made by comparing the calculated final discharge temperature with the measured final discharge temperature. If these two temperatures agree, the assumption is made that the correct work input has been used. If, however, the two temperatures do not duplicate one another within 2, the work input curves for each section are varied by the same percentage, and the process is repeated. Once the sectional inlet and discharge conditions are determined, the sectional head and efficiencies can be calculated.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.