Heat exchanger selection and optimization
The selection process is a trade-off between the overall heat transfer coefficient (U), which has influence on the surface area, and pressure drop, which influences the pump head on the HVAC system. In general, low pressure drop increases the surface area of the heat exchanger, thereby increasing the unit’s initial cost, and the U value influences the type of chevron pattern plates to be used and pressure drop as well.
See Table 1 to calculate the surface area required for the heating and cooling load.
Using the equations described in step 1, calculate the surface area required to perform the duties using the LMTD and NTU values for summer and winter conditions. Results of the calculation are shown in Tables 1 and 2.
The key to selecting a heat exchanger is to select the smallest model with the same type of plate corrugation for both seasons (winter and summer) capable of handling the flow.
Next, simulate the heat exchanger with the largest surface area and plate corrugation type for the unit selected with the temperatures and flow profiles of the other season to obtain the duty value required for the season; see Table 4 for the heat exchanger solution.
An optimal solution has been achieved when temperature, flow, and pressure drop are within the acceptable limits.
Glycol system implications on the heat exchanger specification:
- A glycol/water system affects the heat transfer of the heat exchanger and the pump noise.
- Glycol does not carry heat as well as a water-only system.
- Engineers must pay attention to the corresponding flow increase of glycol solution to maintain the minimum flow of the chiller and the corresponding pressure increase on the pump and pipe to prevent a noise problem.
Recommendation: Do not use chromate water treatments or galvanized fittings because they might react with the glycol solution.
Jose R. Rodriguez is a technical services engineer with Wallace Eannace Assocs. where he has more than 15 years of experience in HVAC and plumbing. Mike Sammut, partner at AKF Group, has more than 30 years of experience in HVAC. Arslan Asovic is project engineer at AKF Group and has more than 10 years of experience in HVAC.