HEAT EXCHANGERS COMMONLY USED FOR EVAPORATORS AND CONDENSERS

The most common styles of evaporator and condenser heat exchanges used in Water Cooled DX and Water Chiller applications are Shell & Tube and Brazed plate. In the evaporator, refrigerant changes from the liquid to the gaseous state while removing heat from the cooling fluid. In the condenser, refrigerant changes from the vapor to the liquid state giving heat to the heat rejection fluid. Choosing the right heat exchanger depends on the situation at hand.

Brazed Plate Heat Exchangers

Their typical construction materials consist of stainless steel plates held together with a copper based brazing material. Water and refrigerant circulate in alternating plates. This style of heat exchanger can be used for both evaporators and condensers.

Figure 1: Brazed plate heat exchanger installed in chiller as evaporator.

 Advantages:

1. Their space efficient shape (rectangular allows for compact unit design.

2. The non-ferrous construction eliminates rusting.

3. The alternating plate design makes the brazed plate evaporators less susceptible to freeze damage when compared with shell and tube evaporators.

4. The small passages encourage turbulent flow, which can benefit heat transfer.

5. They are less expensive when compared with a shell & tube design.

Disadvantages:

1. They cannot be serviced (cleaned, leaked repaired, etc.…)

2. The smaller passages lead to a higher water side pressure drop.

3. They are subject to plugging/fouling, also due to the smaller passages.

4. Although they are typically constructed of stainless steel, the brazing material is copper based and can sacrificially corrode.

Figure 2: Passages on a brazed plate heat exchanger. Each passage is approximately 1/8” wide

Shell & Tube Heat Exchangers

Their construction materials usually consist of a carbon steel shell with end plates and copper tubes. With a DX Evaporator, water circulates in the shell side while the refrigerant passes through the inside of the tubes. In a condenser, the water flows through the tubes while the refrigerant remains inside the shell.

Figure 3: Two shell & tube heat exchangers being used as condensers in a water chiller.

Advantages:

1. Less water side pressure drop due to the larger tube size.

2. Tube leaks are easily located and plugged.

3. They can act as a liquid refrigerant receiver helping with pump-down and low ambient situations.

4. Much easier to service (clean and repair leaks).

5. A better solution for water derived from open cooling towers, rivers, lakes, sea coolant, and other fluids at risk of clogging in narrow spaces.

6. Rugged mechanical construction due to its thicker tube walls.

7. Tubes are available in a wide variety of construction types (Copper, Cupro-Nickel, Stainless steel, and Titanium)

8. The shell is also available in stainless steel, for sea water and other highly corrosive applications.

 Disadvantages:

1. Less thermally efficient due to less total surface area.

2. Requires a larger space.

3. More expensive when compared to brazed plate.

4. In evaporators, the water is passing over a carbon steel surface.

  

Figure 4: Shell & tube heat exchanger with end cap removed, tube passages are typically 3/4″.

Conclusion

For heat exchangers that use cooling tower water, river, lake or a similar source, it is highly recommended to use a Shell & Tube condenser because of its larger passages and lower probability of fouling and scaling. Shell & Tube heat exchangers can be easily cleaned just by removing the end plates and brushing the tubes. However, for situations that use a closed loop water source, or coolant such as glycol, a Brazed Plate condenser may be used to lower cost and achieve better thermal performance in compact design. When using a brazed plate heat exchanger, a strainer with an appropriate mesh size should always be used.