Knowledge base
A radiator or dry cooler is sold by its cooling capacity in kilowatts, and that number is the most misread figure in the trade, because it means nothing without the conditions it was calculated for. This guide explains the arithmetic behind it - the heat balance on the liquid side, the air side and across the coil - and the three or four figures that must accompany a capacity for it to be comparable.
The Liquid Side: Flow, Specific Heat, Temperature Difference
The heat a liquid gives up is its mass flow times its specific heat times the temperature drop. For water that is roughly 1.16 kW for every cubic metre per hour cooled by one kelvin: a flow of 100 m³/h cooled from 90 to 82 °C rejects about 930 kW. A glycol mixture has a lower specific heat and a higher density; at 50 % ethylene glycol the figure is around 1.0 kW per m³/h and kelvin, so the same flow and temperatures reject about 15 % less heat - or, turned round, the same heat needs a larger temperature drop or more flow.
The engine data sheet fixes two of the three: the flow and the heat. The temperature drop follows, and the radiator has to deliver it at the stated inlet temperature.
The Air Side: Mass Flow And Temperature Rise
The same heat arrives in the air: mass flow of air times its specific heat times its temperature rise. Air carries about 0.34 W per cubic metre per hour and kelvin at sea level and 20 °C, less when it is hot or thin; rejecting 930 kW with a 15 K air temperature rise needs roughly 180,000 m³/h. That is why radiators are large and why altitude - 10 % less density per 1,000 m - and the design ambient change the airflow the fans must deliver.
Across The Coil: Approach And Initial Temperature Difference
The coil moves the heat from the liquid to the air through its surface, driven by the temperature difference between the two. Two figures describe how hard that is. The initial temperature difference (ITD) is the liquid inlet minus the air inlet: 90 °C coolant at 40 °C ambient gives 50 K, the same coolant at 50 °C ambient gives 40 K - a fifth less driving force and, roughly, a fifth more surface for the same heat. The approach is the liquid outlet minus the air inlet: the smaller it is, the larger the coil, and a dry cooler cannot make it zero.
For a given coil the capacity is close to proportional to the ITD, which is the quickest way to see what a change of ambient does: a unit rated 500 kW at 40 °C ambient with 90 °C coolant has about 400 kW left at 50 °C.
Why A Capacity Needs Its Conditions
The same radiator is a 500 kW unit at one set of conditions and a 400 kW unit at another. A capacity is therefore only complete with the liquid inlet and outlet temperatures, the flow, the glycol percentage, the air inlet temperature and the altitude - and, for exact comparisons, the humidity basis. Two offers are compared only after they are brought to the same conditions; a cheaper offer at a lower ambient is an offer for another site. TECS prints every one of these conditions on the datasheet next to the required and the calculated capacity, so that the number can be checked by anyone with the engine data sheet in hand.
In Short
- Water: about 1.16 kW per m³/h and kelvin; 50 % glycol about 1.0
- Air: about 0.34 W per m³/h and kelvin at sea level and 20 °C
- ITD drives the coil; a hotter ambient means less ITD and more surface
- The approach is the limit a dry cooler cannot cross
- A capacity is complete only with its temperatures, flow, glycol, ambient and altitude
- TECS states all of them on the datasheet beside required and calculated capacity
What To Send With An Enquiry
- Heat to be rejected, or the flow and the temperatures that define it
- Liquid: water or the glycol mixture and percentage
- Design air inlet temperature and altitude
- Required liquid outlet temperature or the allowed approach
- Pressure drop limit on the liquid side
- Site, sound limit and space, for the design that follows
Questions We Are Often Asked
How do I convert kW to kcal/h or BTU/h?
1 kW = 860 kcal/h = 3,412 BTU/h. Engine data sheets from different makers use all three; the radiator design uses kW.
Why does my 500 kW radiator only reject 420 kW?
Check the conditions: a hotter ambient than designed, a lower coolant inlet, more glycol, a fouled coil or an altitude that was not in the enquiry each take a share. The datasheet says what 500 kW was calculated for.
Can the capacity be raised by more flow?
More flow at the same inlet temperature raises the capacity a little and the pressure drop a lot; the engine maker's flow and pressure drop limits bound it. More surface or more air is the usual way.
What is a good approach for a dry cooler?
Five to ten kelvin is usual for engine radiators, where the coolant is hot; smaller approaches are possible but each kelvin costs surface. Below about three kelvin the unit becomes very large.
Does humidity count in the capacity?
Humid air is slightly lighter and carries vapour that is heated too; the effects are a few percent on a hot, humid site. The datasheet states the basis so that figures can be compared.
Contact Us
Send us the heat duty, the fluid temperatures and flow rates, the ambient conditions and the installation limits. We answer with a design proposal - or write to us on WhatsApp.
