Views: 0 Author: Site Editor Publish Time: 2026-10-08 Origin: Site
The approach temperature of a dry cooler is the difference between the fluid outlet temperature and the outdoor air inlet (dry-bulb) temperature. If the air is 30 °C and the dry cooler delivers 36 °C fluid, the approach is 6 K. A dry cooler can never deliver fluid colder than the air, so the approach is always positive, and the smaller it is, the larger and more expensive the coil. Typical design approaches run from about 4 K for very large, generously sized units to 10–15 K for compact, economical ones. CSTHEATEXCHANGER sizes every dry cooler from the approach the process can accept at the site's design ambient — and tells the customer honestly when that approach is not achievable with a dry cooler alone.
Term | Definition | Example |
|---|---|---|
Design ambient | Outdoor dry-bulb temperature the unit must perform at (usually a high-percentile summer value) | 35 °C |
Range (ΔT) | Fluid inlet minus fluid outlet | 45 → 40 °C = 5 K |
Approach | Fluid outlet minus air inlet | 40 − 35 = 5 K |
The heat load fixes the product of flow and range. The process fixes the required outlet temperature. The site fixes the ambient. The approach is what is left — and it is the single input that most changes the size and price of the dry cooler.
Heat transfer in the coil is driven by the temperature difference between fluid and air. As the approach shrinks, that driving force shrinks toward zero at the fluid outlet, and the coil needs much more surface to move the same heat. The relationship is not linear:
Approach | Relative coil size (same load, same ambient) | Comment |
|---|---|---|
15 K | ~1.0 | Compact, low cost; fluid runs warm |
10 K | ~1.4 | Common industrial compromise |
6 K | ~2.3 | Large coil; typical for data center and process duties that need cooler fluid |
4 K | ~3.5+ | Very large; often cheaper to add adiabatic assist instead |
0 K | Infinite | Not possible — dry cooler cannot reach ambient |
(Ratios are illustrative of the trend, not catalogue values.)
CSTHEATEXCHANGER achieves smaller approaches by adding rows (deeper coil), increasing face area (more or larger modules, V-type layout), using counter-flow circuiting so the coldest air meets the coldest fluid, and raising airflow — our 2.5 mm fin spacing and corrugated fins are chosen for air-side turbulence at reasonable fan power.
When the process needs fluid cooler than "ambient + economical approach" on the hottest days:
Accept a larger dry cooler — more coil area, smaller approach. Simple, no water, higher capital cost.
Adiabatic or spray assist — wet the inlet air or coil in peak hours so the coil sees air near wet-bulb instead of dry-bulb. Our Free Cooler spray system lowers water temperature by 5–8 °C compared with a conventional free cooler; the coil stays dry the rest of the year. CSTHEATEXCHANGER also builds hybrid and adiabatic coolers.
Chiller trim — a chiller in series takes the last few degrees only when the dry cooler cannot; free cooling the rest of the year.
In most climates option 2 is the cheapest way to shave 5–8 K off the effective approach for a few hundred hours a year.
Application | Fluid outlet needed | Economical approach | Consequence at a 35 °C design day |
|---|---|---|---|
Genset jacket water | High (engine runs hot) | 10–15 K | Dry cooler alone works easily |
Injection molding hydraulics | Warm | 8–12 K | Dry cooler alone, all year |
Chiller free cooling | Cold-loop temperature | n/a in summer | Dry cooler works only in cool weather; chiller in summer |
Data center liquid cooling | Warm facility water | 5–8 K | Dry cooler most of the year; adiabatic assist on peak days |
Battery storage | Narrow band | 5–8 K | Dry cooler with EC control; adiabatic in hot climates |
Transformer / lube oil | Warm | 10–15 K | Dry cooler alone |
No. The approach is always positive. Only evaporative assist or a chiller gets below dry-bulb.
As large as your process can tolerate. Every kelvin of approach you give up makes the cooler smaller and cheaper. CSTHEATEXCHANGER will show you the size at two or three approach values.
Because the approach was specified at a lower design ambient than the site actually sees, or the coil is fouled. Check the design ambient on the data sheet against the local climate.
Slightly — glycol reduces heat transfer, so for the same approach the coil must be a little larger. We size for the actual glycol concentration.
Higher airflow lowers the approach but raises fan power and noise. EC fans let the controller trade these off automatically against the daily ambient.
Send CSTHEATEXCHANGER your heat load, fluid, required outlet temperature, site design ambient (and wet-bulb if adiabatic is an option) and footprint. We will return a dry cooler selection at the achievable approach, show the alternatives, and provide a drawing for approval.
Email: info@cstheatexchanger.com
International business: +86 0519 8878 2189
Website: https://www.cstheatexchanger.com/
CSTHEATEXCHANGER — we size dry coolers from the approach, not from a catalogue page.
