Plate Heat Exchanger Selection for Chilled Water: Approach Temperature and Pumping Trade-Offs
The temperature penalty belongs in the plant design
A plate heat exchanger can separate a district cooling network from a building, isolate a glycol circuit, or separate open cooling-tower water from a closed chilled-water loop. In each case, the selection creates a temperature difference between the source and the load. That difference must appear in the upstream plant duty from the beginning.
A schedule that specifies only “500 kW plate heat exchanger” leaves most of the design unresolved. The same heat duty can require very different equipment depending on the four water temperatures, the permitted pressure drop on each side and the condition of the fluids. The useful starting point is a thermal diagram with clearly named connections.
Define the four temperatures before choosing an approach
Call the warmer stream the hot side, even when every pipe contains chilled water. Its entering temperature is Th,in and its leaving temperature is Th,out. The colder source stream enters at Tc,in and leaves at Tc,out. In counterflow, the two terminal differences are:
ΔT1 = Th,in − Tc,out
ΔT2 = Th,out − Tc,in
The smaller terminal difference is commonly called the approach. For a building chilled-water interface, designers often emphasize Th,out − Tc,in because it links building supply temperature to source supply temperature. Name the terminals explicitly in the specification; a bare “2 K approach” can be ambiguous when the stream temperature changes differ.
The inlet and outlet temperatures must also satisfy the heat balance on both sides. Alfa Laval's calculation method identifies duty, temperatures, flows, pressure-drop allowances and operating limits as core selection inputs. Its counterflow temperature definitions provide the basis for the checks below.
Worked example 1: what a closer approach costs thermally
Assume a 500 kW building duty with water entering the exchanger at 12°C and leaving at 7°C. The source water enters at 5°C and leaves at 10°C. Assume steady operation, negligible heat loss and water properties of 1,000 kg/m³ and 4.18 kJ/(kg·K).
Each stream changes by 5 K, so each requires:
V̇ = 500 ÷ (1,000 × 4.18 × 5) = 0.0239 m³/s = 23.9 L/s
The terminal differences are 12 − 10 = 2 K and 7 − 5 = 2 K. For unequal terminal differences, the counterflow log-mean temperature difference is:
LMTD = (ΔT1 − ΔT2) ÷ ln(ΔT1/ΔT2)
When both differences are equal, use their common value rather than evaluating the indeterminate expression. Here LMTD = 2 K. For an ideal counterflow calculation with correction factor F = 1:
Q = UA × LMTD
Required UA = 500 ÷ 2 = 250 kW/K
UA is the overall thermal conductance. This result is not a plate count, because the overall heat-transfer coefficient U depends on the selected plate geometry, flow distribution, fluid properties and allowance for fouling.
Now retain the same duty and building temperatures, but consider source water at 4°C entering and 9°C leaving. Both terminal differences become 3 K, so required UA falls to 500/3 = 166.7 kW/K. The 2 K approach requires 50% more UA than the 3 K case. It does not necessarily require exactly 50% more plate area, because U can change with the design.
The broader trade-off is now visible. A tighter approach permits a warmer source supply for the same building supply temperature, but asks more of the exchanger. Compare the actual upstream plant selections and exchanger quotations. Do not attach a universal chiller-energy saving to a one-degree change.
One feature of the example can look surprising: source water leaves at 10°C while building water leaves at 7°C. This is a legitimate counterflow temperature cross. Those two outlets are at opposite ends. At each local end, the warmer stream remains warmer than the colder stream.
Illustrative 500 kW counterflow duty with 2 K terminal differences at both ends. LMTD is 2 K, so required UA is 250 kW/K.
Treat pressure drop as a design variable
The allowance should be stated separately for the building and source sides. Clarify whether the quoted loss includes ports and connections, and identify the fluid temperature and flow at which it applies. Keep external strainers, control valves and connecting pipework visible as separate allowances.
Channel velocity and plate pattern influence both heat transfer and resistance. Selecting the lowest possible pressure drop can increase exchanger size and alter low-load behavior. Conversely, accepting a high pressure drop simply to obtain a smaller unit can create a recurring pumping cost. Alfa Laval's pressure-drop guidance for HVAC exchangers discusses the interaction between pressure drop, channel turbulence, fouling and capital cost. Its application examples are guidance, not a universal project limit.
Ask the supplier for at least two feasible selections using the same thermal duty. One might use a lower pressure allowance and more surface; another may use a different plate pattern or frame. Comparing different duties or different fouling assumptions makes the quotations misleading.
Worked example 2: price the pumping difference
Continue with 23.9 L/s on both sides. Assume two hypothetical selections provide the same 500 kW duty and specified temperatures. Selection A loses 35 kPa per side; selection B loses 65 kPa per side. For this comparison, assume each pump-and-drive combination has a constant 70% wire-to-water efficiency.
The electrical input attributable to exchanger resistance is:
P = Δp × V̇ ÷ η
With pressure in Pa and flow in m³/s, the result is watts. For the two separate circuits combined:
PA = 2 × 35,000 × 0.0239 ÷ 0.70 = 2.39 kW
PB = 2 × 65,000 × 0.0239 ÷ 0.70 = 4.44 kW
The difference is 2.05 kW at this operating point. If, only for screening, both circuits remained at those flows for 3,000 hours, the difference would be approximately 6,150 kWh. Multiply by the applicable electricity tariff to compare against the installed-cost difference.
This is an incremental exchanger comparison, not a whole-pump prediction. The actual pump efficiencies may differ, and a variable-flow plant needs an operating-bin calculation rather than 3,000 full-flow hours. Include any change in chiller or cooling-tower energy caused by a different approach temperature when comparing alternatives that do not have identical thermal duties.
Specify the operating range and maintenance access
Peak duty is only one selection point. Give the supplier a second condition representing the lowest sustained operating load, with actual inlet temperatures and intended flows. Ask for resulting outlet temperatures and pressure drops. If units are arranged in parallel, review how many will operate at low load instead of assuming every plate pack should always receive a share of the flow.
Provide the fluid chemistry, glycol concentration if present, and anticipated particulate contamination. Request documented compatibility of plates and gaskets with both fluids and the intended cleaning chemicals. “Stainless steel” alone is not a complete material specification. Reserve physical space to open the frame, remove plates, service strainers and connect cleaning equipment.
Alfa Laval's service recommendations emphasize upstream filtration, venting, controlled valve operation at startup and monitoring temperatures and pressures. Put the required service provisions on the drawings; they are much harder to add after installation.
Acceptance requires a thermal and hydraulic record
Trend all four temperatures and both flow rates at a stable test point. Calculate heat transfer independently on each side and compare the results within an agreed measurement uncertainty. Record the pressure drop on each side, with pressure taps that distinguish the exchanger from external accessories.
An increased approach alone does not prove fouling. First establish whether inlet conditions, flow, valve position and load match the reference condition. A clean baseline is especially valuable when later readings need to distinguish an exchanger problem from a changed operating duty.
For the wider plant coordination, the Chilled Water System Design Handbook is a related resource. The final exchanger submittal should still contain the four temperatures, both flows, both pressure drops, design pressure and temperature limits for each side, materials and verified off-design performance. Those details make the selection reviewable long after the model number has been chosen.
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