When it comes to evaporators and condensers required for heat exchange in industrial refrigeration, you have options. Here, we start with system basics and move on to the advantages of the various evaporator types and particulars of three condenser options. The overview of principles and approaches will arm designers to specify the right equipment for the right circumstance.

The basic vapor-compression refrigeration cycle consists of four components. These are the compressor, condenser, expansion device, and evaporator which are shown in Figure 1. The evaporator absorbs thermal energy or heat, from the space or fluid being cooled by boiling, or "evaporating," the refrigerant inside.

[FIGURE 1 OMITTED]

At the compressor, the vapor is compressed to a pressure sufficiently high for condensation of the refrigerant in the condenser to occur. The expansion device, a special valve or capillary tube, regulates the flow of refrigerant to the evaporator so that only vapor exits, thus preventing liquid refrigerant from entering the compressor. Since the entire refrigeration cycle or system must satisfy an energy balance, the heat absorbed at the evaporator, plus the power input to the compressor, must equal the heat rejected at the condenser to the surroundings.

The condenser and evaporator are both classified as heat exchangers that transfer heat from a warm fluid to a cold fluid. Mixing of warm and cold fluids may occur in some types of heat exchangers, but not in evaporators and condensers, because the refrigerant must be contained within the system.

Industrial refrigeration systems generally utilize the vapor-compression cycle but usually with added complexity in order to meet all the requirements for which they were designed. For example, it is common for industrial systems to have multiple evaporators and condensers so as to meet large refrigerating loads, several application temperature requirements, and operating flexibility. Regardless of the number of evaporators and condensers an industrial refrigeration system may have, all function as heat exchangers. This article explains the operation of heat exchangers and then presents useful information concerning evaporators and condensers for their selection in refrigeration systems.

HEAT EXCHANGERS

Heat transfer occurs by three methods: conduction, convection, and radiation. For heat exchangers applied to industrial refrigeration, heat transfer by radiation may be ignored. Conduction is the heat transfer that occurs through a material by virtue of a temperature difference. For a plain wall consisting of a single material, conduction is given by

1. Q = kA([T.sub.1] - [T.sub.2])/t

where Q is the heat transfer rate (Btuh), k is the thermal conductivity of the material (Btu/hr-[ft.sup.2]-[degrees]F or Btu/hr-[ft.sup.2]-[degrees]F), A is the area through which the heat transfer occurs ([ft.sup.2]), [T.sub.1] is the surface temperature on the warmer side ([degrees]F), [T.sub.2] is the surface temperature on the cooler side ([degrees]F), and t is the thickness of the material. Note that the units for the thermal conductivity may be expressed in two ways, which differ by a factor of 12. The temperature difference [T.sub.1], and [T.sub.2] is sometimes called the "driving potential" and the larger it is, the larger the heat transfer rate. Heat transfer always moves in a direction toward the cooler surface. Equation 1 may be re-arranged in terms of the driving potential and transfer rate to give

2. (Driving potential)/(Transfer rate) = ([T.sub.1] - [T.sub.2])/Q = t/kA = [R.sub.Cond]

where [R.sub.Cond] is the resistance to conductive heat transfer (hr-[ft.sup.2]- [degrees]F/Btu) and is analogous to electrical resistance.

Convection heat transfer is also driven by a temperature difference and occurs by virtue of the fluid motion near the surface in a region called a "boundary layer." A fluid flowing adjacent to a solid surface will transfer heat with that surface according to

3. Q = hA([T.sub.1] - [T.sub.0])

where Q is the heat transfer rate (Btu/hr), h is the convective heat transfer coefficient (Btu/hr-[ft.sup.2]-[degrees]F), A is the surface area through which the heat transfer occurs ([ft.sup.2]), [T.sub.1] is the surface temperature ([degrees]F), and [T.sub.0], is the bulk fluid temperature outside of the boundary layer ([degrees]F). Equation 3 may also be re-arranged in terms of driving potential to yield

4. (Driving potential)/(Transfer rate) = ([T.sub.1] - [T.sub.0])/Q = 1/hA = [R.sub.Conv]

where [R.sub.Conv] is the resistance to convective heat transfer (hr-[ft.sup.2]- [degrees]F/Btu) and also analogous to electrical resistance.

Figure 2 shows a simple heat exchanger where the warm and cold fluid streams flow in counterflow (opposite) directions. In Figure 2, the cold fluid is inside the tube and the warm fluid is in the chamber or shell; the position of the warm and cold fluids could also he switched. Also shown in Figure 2 is an enlargement of the tube cross-section showing three regions: the fluid inside the tube, the tube wall material, and the fluid outside the tube. The heat transferred by the warm fluid must pass through the tube wall and into the cold fluid.