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Cool Thermodynamics Mechanochemistry of Mater ials use none none implementation toget none on none QR Code Safty 12 . HEAT EXCHANGER INTE RNAL DISSIPATION IN CHILLER ANALYSIS AND THE ESSENTIAL ROLE OF ACCURATE PROCESS AVERAGE TEMPERATURES. That is the essenc none none e of science: ask an impertinent question, and you are on the way to a pertinent answer. - Jacob Bronowski. A. PEEKING INTO THE BLACKBOX In their simplest and most approximate form, the analytic chiller models developed in s 4 5 and in 10 allow us to relate to a cooling device as a blackbox. By performing a number of completely external measurements the non-intrusive type commonly provided in manufacturer catalogs and easily conducted in-house we can use simple regression techniques to characterize a chiller by three parameters.

We needed a basic notion of what transpires inside the blackbox, but we did not require any internal measurements. We discovered that simple chiller performance formulae offered predictive tools with some diagnostic capabilities. Some of the studies covered in earlier chapters are in this spirit.

In order to develop a more comprehensive diagnostic tool, and in order to perform optimization studies, we need to peek inside that blackbox and obtain additional information about the internal components additional measurements on the heat exchangers and on the refrigerant as it enters or leaves the different chiller compartments. For simplicity, we aspire to keep the additional required information to a minimum. In this chapter, we ll open the chiller, perform a few key measurements that are suggested by the fundamental model of 5, and.

Heat Exchanger Internal Dissipation in Chiller Analysis see how to exploit that model to diagnose and optimize chillers. The fundamental model in its extended form Equation (5.3) indicates that at the very least we also need: (1) To carefully distinguish among external, internal and heat leak losses.

(2) To be careful that internal losses are not counted as external losses. For example, pressure drops in the heat exchangers, which are internal dissipation, should not enter the bookkeeping as part of the finiterate heat exchange irreversibility. If these internal heat exchanger losses are insignificant, then clearly this is a mute point; but we ll see that often this is not the case.

(3) To measure how the external losses are partitioned between the evaporator and the condenser. At the heart of these procedures lies the accurate determination of the process average temperatures (PATs). First, we ll see why accurate PATs are crucial for establishing how losses are divided not only among internal, external and heat leak losses, but also among the different sources of internal losses.

A quantitative determination of how losses divide among the key chiller components is itself an important diagnostic step. Second, we ll challenge the conventional wisdom that heat exchanger internal losses are insignificant relative to the other internal losses. With experimental data from real chillers, we ll show that overlooking internal losses in the heat exchangers can lead to non-negligible errors in diagnostic studies.

And third, optimization case studies will be presented to highlight how the precision analysis of internal losses and PATs translates into information of considerable importance to designers and manufacturers. One intriguing observation a reaffirmation of findings in earlier chapters is the extent to which commercial chillers have empirically evolved to optimal performance configurations for given technological constraints. Experimental measurements of the precision and scope required to carry out these calculations are not commonly available.

For example, they do not appear in manufacturer catalogs or most journal articles on these topics. Therefore, in order to investigate these effects and to arrive at meaningful conclusions, we needed both to perform our own measurements, and to tap the few studies available from manufacturers and in the journal literature that allow such detailed assessments to be made. Section B is a case study for a reciprocating chiller [Ng et al 1998a].

The specific implications of determining accurate PATs that we ll explore are: (a) diagnostics for the heat exchangers, with the ability to.
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