Energy and Exergy Assessments

2.1 Introduction

Psychrometrics involves the use of thermodynamics to analyze conditions and processes involving moist air. A thorough understanding of psychrometrics is important in the heating, ventilating, air conditioning, and refrigeration (HVACR) community. Psychrometrics is used not only in assessing and designing heating and cooling processes and ensuring the comfort of building occupants but also in constructing building materials (e.g., insulation and roofing) and in assessing their stability and fire resistance (Dincer and Rosen, 2013).

Numerous researchers in their related publications and books (e.g., Dincer et al., 2007; Wepfer et al., 1979; Stecco and Manfrida, 1986; Dincer and Rosen, 2011; Dincer and Rosen, 2013; Kanoglu et al., 2007; Ratlamwala and Dincer, 2012) illustrate the application of exergy analysis to a variety of heating, ventilating, and air conditioning (HVAC) processes.

This chapter describes energy and exergy assessments of the components and psychrometric processes in HVAC systems and illustrates this material by assessing a novel integrated system for HVACR applications. The basic components in HVACR systems include heat exchangers, pumps, compressors, throttles, and turbines, and these are introduced, classified, and thermodynamically analyzed. This chapter also describes the energy and exergy assessments of psychrometric processes. Mass, energy, entropy, and exergy balances for all components and processes are provided.

In this chapter, kinetic and potential energy changes are considered to be negligible and all processes are assumed to be steady-flow and steady-state. Of course, transient processes can be assessed if required.

For a proposed integrated system involving psychrometric processes, thermodynamic analyses are performed. The energy and exergy efficiencies for individual components and the integrated system are calculated and parametric studies are performed that determine the impact on system performance of varying dead-state properties and system operating conditions.

2.2 Heat Exchangers (Heating/Cooling)

Closed heat exchangers (see Fig. 2.1) transfer heat from one fluid to another without the fluids coming in direct contact with each other. Heat transfer in a heat exchanger can occur without the fluid undergoing phase change or with phase change (e.g., from a liquid to a vapor, as in an evaporator, or from a vapor to a liquid, as in a condenser). The transfer of heat is driven by a temperature difference. In most HVACR applications, heat exchangers are selected to transfer either sensible or latent heat. Sensible heat applications involve heat transfer that results in a temperature change without phase change. Latent heat transfer involves a phase change of one of the liquids, for example, transferring heat to a liquid by condensing steam.

Heat exchanger performance is commonly evaluated with one of two methods, which are described in the next two subsections.

2.2.1 Log Mean Temperature Difference Method

One method of evaluating heat exchanger performance is the logarithmic mean temperature difference method. When heat is exchanged between two fluids flowing through a heat exchanger, the rate of heat transfer may be expressed as

=UAΔtm

where U is the overall heat transfer coefficient from fluid to fluid, A is the heat transfer surface area of the heat exchanger associated with U, and Δtm is the log mean temperature difference (LMTD or Δtm).

For a heat exchanger with a constant U, the LMTD can be calculated as

tm=CfT1−T4−T2−T3lnT1−T4/T2−T3

where Cf is a correction factor (less than 1.0) that is applied to heat exchanger configurations that are not truly counterflow. Figure 2.1 illustrates a temperature cross, where the outlet temperature of the heating fluid is less than the outlet temperature of the fluid while heated (T2 < T4). A temperature cross is only possible with a heat exchanger with a counterflow arrangement. The physical arrangement of the surface area affects the overall coefficient UA. Not every heat exchanger with identical surface area carry out equally for a given load. Henceforth, for specific applications, defining load conditions when selecting a heat exchanger is critical.

The load for each fluid stream can be calculated as

˙=m˙cpTin−Tout

value of Δtm is an significant factor in selection of heat exchanger. For a given load, if Δtm has a high value, a comparatively minor heat exchanger surface area is necessary. The commercial effect is that the design of the heat exchanger must accommodate the forces and actions convoying with huge difference in temperatures. When the approach temperature is small i.e. the change in T2 and T4 is minor, Δtm is also insignificant and a fairly large A is obligatory.

2.2.2 ε-NTU (Effectiveness Analysis)

An substitute method of assessing heat exchanger performance includes the calculation of exchanger heat transfer effectiveness ε and number of exchanger transfer units (NTU). This method is grounded on the identical assumptions as the logarithmic mean temperature difference technique designated earlier.

Equations (2.1) and (2.2) for Δtm are more conveniently applied when inlet and outlet temperatures are known for both fluids. Though during most times, the temperatures of fluids leaving the heat exchanger are...