One phenomenon on site is regularly taken for a fault: the heater is clearly switched off, yet the outlet oil is a dozen degrees or so warmer than the oil coming in. There is no oil leak, and the filter elements have just been changed. In most cases this is not a fault but the pump and the filter turning the mechanical power that goes in into heat, which the oil then carries away.
Where the heat comes from: two energy conversions
The first source is the oil pump. The ZH screw oil pump moves the oil from the oil inlet to the degassing chamber. There is friction between the impellers, the rotors and the bearings, and there is leakage at the seals; all of this mechanical work ends up as heat. Part of the shaft power does not become pressure but becomes a rise in temperature of the oil and of the pump body itself. This heat is produced continuously and in proportion to the oil flow.
The second source is the filtration section. The oil has to pass through the filter elements to reach the outlet, and there is friction resistance between the filter media and the oil. The larger the pressure difference, the larger that dissipation, and the more heat it turns into. While the element pressure difference is within its normal range this share of the heat is small; when an element is blocked and the differential pressure has risen clearly, the temperature rise goes up with it.
The total power of the ZYD-100 is 72 + 5 kW. These two figures are the input power on the motor side; only part of it becomes heat, the rest becomes mechanical work and fluid pressure energy. The mechanical work still ends up as heat, it is simply distributed across the pump, the pipework and the filter elements.
How large a temperature rise is normal
There is no single figure that can be applied everywhere, because the temperature rise depends on three variables at once: the flow rate, the pressure difference across the machine and the time the oil spends inside it. The higher the flow, the more mechanical work passes through per unit time; the higher the differential pressure, the more the filtration section dissipates; and the shorter the residence time, the more effectively the oil carries heat away, which can even lower the measured outlet temperature.
The more reliable approach is to establish your own baseline: with the same machine, the same oil and one fixed flow and temperature setting, record several sets of inlet and outlet temperatures and obtain a stable range of differences. Only a temperature rise that clearly deviates from that range is worth chasing.
| Change in the inlet to outlet difference | More likely cause | What to do first |
|---|---|---|
| The difference is steady and matches the baseline | The machine is in a steady condition | Record the baseline and keep watching |
| The difference grows slowly | The element differential pressure has risen, the filtration section dissipates more | Check the element differential pressure and the replacement record |
| The difference jumps suddenly | The flow rate has been raised, or the pump has a mechanical fault | Check the flow setting and the noise the pump makes |
| The outlet oil is cooler than the inlet oil | The heating section is still working, or the ambient loss is obvious | Check the temperature setting and the state of the heater |
How the temperature rise relates to the temperature control
The heating temperature control range is 45–80 °C, and the upper limit is reduced to 70 °C when the water content is high. As the temperature rises, the viscosity of the oil falls and the flow resistance decreases, so the same machine passes a larger flow at the same setting; the differential pressure across the filtration section falls as well, which shows that the temperature is taking part of the resistance away from the filter. The two are coupled, and the output of the heater cannot be looked at in isolation.
The same holds in reverse: when the inlet oil is cold the viscosity is high, the differential pressure is large, the filtration section dissipates more and the temperature rise is clearer. If the outlet temperature is found to be high on site, look first at whether the inlet oil is still in the preheating phase rather than assuming that the heater has run away.
When to suspect the machine
- The temperature rise clearly exceeds the baseline and the pump body is also unusually hot. That is a sign of growing mechanical friction, and the machine should be stopped to check the bearings and the lubrication of the pump.
- The temperature rise grows together with the current and with a heavier running note: look at the filter resistance first and confirm that the pump is not labouring against a high back pressure.
- The temperature rise comes with a darker colour and a changed smell in the oil. The heat source may not be the machine at all, but the oil oxidising because it is heated repeatedly inside the machine.
- The longer the machine runs continuously, the larger the temperature rise. Writing this figure into the operating record lets it be read side by side with the oil test results.
The temperature rise is not an acceptance figure; it is an indicator of the working condition. Once its origin is understood, it can be used to judge the state of the filtration section and the oil pump instead of checking the heating system every time.
Outlet oil warmer than inlet oil means mechanical work has become heat; heat beyond your own baseline is the problem.
