Oil coolers with shell and tube heat exchangers: sizing for viscous media
Hydraulic power units, gearboxes and stationary engines reject their losses through the oil. A shell and tube heat exchanger cools it against cooling water - reliably, but only if the sizing takes the peculiarities of viscous media seriously.
Why does oil calculate differently from water?
Three fluid properties make the difference. The heat capacity of oil, at around 2 kJ/(kg·K), is about half that of water - so for the same duty the oil side needs more mass flow or a wider temperature change. Its thermal conductivity is roughly five times lower than water’s. And its viscosity is orders of magnitude above water, depending on the ISO VG class and the temperature - it governs boundary layer thickness, heat transfer and pressure drop.
The result: the oil side dominates the overall heat transfer. A sizing that simply applies water characteristics underestimates the required area considerably.
How much does temperature change the oil?
Oil viscosity falls steeply as the temperature rises - typically by a factor of three to five between 30 °C and 60 °C. For the sizing this means the fluid properties belong evaluated at the mean temperature of the oil side, not at a nominal value from a data sheet. Our calculation core therefore stores the properties as a function of temperature and evaluates them at the operating point.
Which size suits which cooling duty?
Small power units with single-digit losses manage with compact sizes; presses, large gearboxes and test benches need correspondingly more area. Besides the duty, the decisive factor is the permissible pressure drop on the oil side: pushing viscous oil through narrow tubes costs pumping power, and a unit chosen too small buys its area with an unrealistic pressure drop. The sizing therefore checks both criteria at once against the size catalogue; with a pressure-drop limit on the oil side, our team takes this on.
Tube side or shell side - where does the oil go?
In our arrangement the tube coil carries the hot medium, which for oil cooling means the oil; the cooling water flows around the coil in the shell. The ratings in our size catalogue refer to this arrangement. Hydraulic oil usually runs filtered in a closed circuit and carries little dirt; when needed, the unit is cleaned by chemical flushing.
Frequently asked questions
Why is the oil side almost always the governing side?
Oils conduct heat poorly and form thick boundary layers because of their viscosity. The heat transfer on the oil side is therefore far below that of the water side and dominates the overall heat transfer.
Which oils are held in the Shellcore calculation core?
Hydraulic oil ISO VG 22 and ISO VG 220 as well as motor oil SAE 30; the fluid properties are stored as a function of temperature. Other oils, or an operating point outside the range shown: choose “Other medium” in the configurator and name your oil, with fluid properties if known. The enquiry then goes to our engineers, who do the sizing.
What happens on a cold start, while the oil is still thick?
As the temperature falls the viscosity rises sharply, the pressure drop grows and the heat transfer falls. The design point should therefore represent the operating condition, not the ideal case.
Request a sizing
Describe your application in the configurator - we will come back to you with an individual quote.