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“A one-dimensional model of a closed-loop refrigeration test block for centrifugal compressors”

Literature review

 

 

 

As discussed in Chapter 1, thermodynamic models of each component in the hot gas bypass test cycle have already been created and are widely used. This literature review surveys the existing test methods and models and summarizes information about the JCI test block cycle, including equipment details. A general description of each source is contained here, while any mathematical models used are presented (with citations) in the complete model development of Chapter 3.

2.1 General Compressor Testing

One component of the research effort was to study different methods of testing compressors. Therefore, this section provides information regarding compressor performance metrics and testing methods commonly used in the HVAC industry. The research was done to better understand the testing methodology, the data collected during a typical test, and the expectations for a mathematical model of the test block refrigeration cycle.

2.1.1 Compressor Testing Methodology—ASME PTC 10

The AMSE PTC 10-1972 standard (Gerber, 1998) prescribes test conditions, procedures, and measurement locations for a compressor test. This allows different manufacturers’ test results to be compared. The standard also presents dimensionless coefficients used by different industries to characterize the operating point of the compressor with associated conversions. The relevant coefficients are described in greater detail as part of the compressor model development in Section 3.2. These coefficients serve as inputs to the thermodynamic model that fully characterize the performance of the compressor. Because the compressor operating point constrains many variables in the model, these coefficients are critical to the model’s performance.

Significant portions of PTC 10 focus on multistage compressor testing and sideload calculations. These are not needed for the present modeling effort, since the vast majority of recent compressor tests conducted on the 1500 hp gas test block use single-stage compressor setups (Trevino, 2012).

Finally, ASME PTC 10 contains several sets of sample calculations which are of use in verifying the implementation of the model, particularly the calculation of the prime mover power requirements. These calculations are similar in form to those used in the existing compressor test block data acquisition program and the compressor module of the present thermodynamic model.

2.1.2 Compressor Performance Metrics

Although ASME PTC 10 prescribes many parts of the test methodology, it is also important to understand other recommendations for compressor testing. Wilcox (2007) advocates the use of ASME PTC 10 (Section 2.1.1) and lists suction and discharge pressures, suction and discharge temperatures, mass flow rate, fluid (refrigerant) composition, rotational speed, and driver load as critical field data for any compressor test. These critical field data also appear as inputs or outputs of the thermodynamic model of the compressor, presented in Section 3.2.

Furthermore, Wilcox outlines some general guidelines for instrumentation on the test block to ensure representative data. He notes that pressure and temperature sensors should be located at least 10 pipe diameters from potential disturbances or obstructions such as tees or elbows. All sensors should be calibrated prior to the test run, and data should only be collected at steady-state conditions. For a typical compressor test, Wilcox defines steady-state conditions to be achieved once the discharge temperature remains constant (within sensing accuracy) over a 15 min interval. According to Wilcox, resistance temperature detectors (RTDs) should be used instead of thermocouples wherever possible for improved accuracy. Finally, he stresses the importance of recording test data at several different operating points to allow recognition of a bad measurement in any one set of data. All of these guidelines are met by the instrumentation and testing procedures in use on the 1500 hp gas test block at JCI. This ensures that validation data provide a good representation of the actual operation of the system.

Under the framework of this order dissertation online other compressor performance metrics was studied in details.

2.2 Existing Component Thermodynamic Models

The 1500 hp gas test block contains a number of components which must be modeled. The compressor is not directly modeled in this work, which instead uses external compressor maps or other means to specify the discharge conditions given a set of suction conditions as discussed in Sections 2.1.1 and 3.2. The flow measurement orifice, hot gas bypass (HGBP) flow split, condenser, cooling towers, throttling expansion devices, and a mixing chamber must be modeled. This section summarizes a number of texts and papers that were consulted to model these devices.

2.2.1 Flow Measurement Orifice

The flow measurement orifice is modeled according to correlations presented in ASME PTC 19.5 (American Society of Mechanical Engineers, 1972) and following the general form of Munson, Young, Okiishi, and Huebsch (2009). The ASME PTC 19.5 standard presents relationships between the differential pressure across the orifice and the flow rate, as shown in Section 3.3. Compressible effects are included since the refrigerant will be in a superheated vapor state at the orifice. According to Trevino (2012) and J.N.O. (1984), flange taps are used on the test block, so the correlations in ASME PTC 19.5 for flange tap pressure measurements are used.

2.2.2 Condenser

Incropera, DeWitt, Bergman, and Lavine (2007), Incropera and DeWitt (1985), and Kays and London (1984) developed equations describing the performance of the condenser, which in this case is a shell-and-tube unit. The NTU-effectiveness method is used to calculate the heat transfer rate in terms of inlet and outlet temperatures since internal temperature measurements are not available. Engineering drawings from JCI (J.N.O., 1983) and a technical manual for the condenser (York Division, Borg-Warner Corporation, n.d.) were used, along with experimental data, to determine the effectiveness and number of transfer units for the condenser.

2.2.3 Cooling Tower

Braun, Klein, and Mitchell (1989) and Mitchell and Braun (2013) developed equations for modeling cooling towers using an effectiveness approach analogous to the heat exchanger model. This is useful as it avoids iteration wherever possible and does not require detailed cooling tower data. An example useful for verification of the cooling tower model is presented by Mitchell and Braun (2013). The model requires two characteristic performance parameters, which affect the number of transfer units (NTU) of the cooling tower. Therefore, a linear regression process is performed on experimental data from the Baltimore Aircoil Company (BAC) cooling towers (Baltimore Aircoil Company, Inc., 2001) to determine these parameters. Braun et al. (1989) outlined this process and provided several values for typical cooling towers. These parameters are discussed in Section 3.6. Qureshi and Zubair (2006) extended the model to include fill fouling. The effect of fill fouling is not included currently, but could be added in the future if desired.

2.2.4 Minor Components

The flow split, expansion devices, and mixing chamber are modeled using basic extensive property balances (mass and energy) as detailed in undergraduate thermodynamics texts such as Çengel and Boles (2011). The equations are developed from the general balance equations in Sections 3.4, 3.7, and 3.8.

2.3 Test Block Design and Construction

In addition to the York documentation already cited, further details regarding the test facility are found in the Applied Systems Operating Limits report (Graham, 2006). This includes pipe diameters, orifice diameters, and operating instructions. The condenser water pump is a fixed-speed unit, Aurora Type 410, size 6 × 8 × 15, with pump curves given by Aurora Pentair Water (2007).

The US Patent by Sahs and Mould (Apparatus for Testing Refrigeration Compressors, 1956), Dirlea et al. (1996), and McGovern (1984) describe the general configuration of the cycle, and illustrate how the different components interact with one another. However, no holistic modeling effort is attempted in any of these sources.

2.4 Summary

To the best of the author’s knowledge, a complete model of the hot gas bypass test block cycle is a novel undertaking. The completion of this model will have significant impact on the testing process used every day by test engineers at JCI (Sommer, 2013). The survey of literature contained in Chapter 2 provides the theoretical basis for each component-level model in the complete cycle. Chapter 3 will discuss each component model in greater detail.

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