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Do I need a Hydraulic Cooler for my power pack? ben lee


Do I need a Hydraulic Cooler for my powerpack?

A design decision to fit a hydraulic oil cooler like this can only be made with a full assessment of how much heat is going to be put into the oil and how much of that heat can the system dissipate without the oil overheating.

The heat comes from inefficiencies or losses. The greater the inefficiencies the greater more power input will be required to overcome them. The inefficiencies will manifest themselves as heat. So the amount of extra input power required can be assessed as:-

Input Power due to inefficiencies = Power Loss (pump) + Power Loss (valves) + Power loss (pipework)+….

This is often described as “heat Load” and if this is greater than the system heat dissipation the system will overheat without a hydraulic cooler.

Where does the heat come from?

The most common cause of heat is pressure drop, this being a decrease in pressure across a device as the oil flows through it.

Sizing of control valves and pipework is critical to reducing heat sources, for example a Cetop03 Directional Valve that is rated at 80lpm with 50lpm of flow from the A to T ports may have a pressure drop of 10 bar would generate (10*50/550) 0.9kW of heat.

Selection of high efficiency pumps and motors is also important, for example a Gear pump that is driven by a 11kW motor would typically have an efficiency between 90 and 95%. This will mean that the output power of the pump will only be 10 to 10.5kW, so 0.5 to 1kW will be heat.

From the initial design of the whole system it is possible to make an estimation of the total heat load. The heat load of an existing system can simply be estimated by measuring the oil temperature rise over time.

Will my powerpack dissipate enough heat without a hydraulic cooler?

Typically the oil reservoir will be sized with heat dissipation in mind. Lets not forget that a system that operates for 1 minute every hour will put a lot less heat into the oil and has much longer to dissipate that heat.

Tank heat loss can be expressed as :- kW= DT * A * 0.016

Where

DT = temperatute difference between oil and ambient (oC)

A = Surface area of tank, generally excluding base (m2)

For example a 55Litre tank could have a surface are of 1m2 and if there is an ambient temperature of 20oC, and the oil temperature is 60oC, then the tank will dissipate 0.7kW.

The maximum oil temperature allowable will depend a little on the type of components and oil used, but in general temperatures above 80oC will damage most seals and degrade performance of many commonly used hydraulic oils.

All of the system will dissipate heat, pipes, fittings and valve bodies etc. So for a full system heat loss figure the total surface area of the system could be considered. In systems where pipework and hydraulic cylinders are large this surface area could be worth consideration.

If the system heat loss is less than the heat load then, yes the hydraulic system will require hydraulic oil coolers if it is to be 100% duty rated.

Keep cool and improve the design

Sources of heat can be designed out of systems by careful selection of the correct valves and by setting them appropriately.

Some of the simple design errors to avoid:-

• Ensure the pump is not operating at full pressure while it is not in use. This will cause the system relief valve to by-pass to tank at full pressure drop. This will cause a heat input equal to the full input power. It is always recommended to use an loading valve so the pump by-passes to tank at the lowest possible pressure drop.

• Slowing cylinder or motors down using a throttle or flow control valve. If a system has a fixed displacement pump then the use of a flow restriction like this cause the pump pressure to increase to the pressure setting of the system relief valve. So the cylinder or motor is only slowed because some of the pump output is by-passing to the tank via the pressure relief valve. Correct pump selection is vital if this situation occurs.

• Load control valves such as overcentre or counterbalance valves that are set too high will cause the system to require excessive pressure when lowering. The pump will have to work harder than necessary.

• Pipework sizing, can be misunderstood, particularly when long hoses are used. For example a 30metre long 3/8” hose with 50lpm flow will have a 15bar pressure drop.  

If your system is to run continuously then firstly ensure that system design is optimised and then consider the heat load. A mixture of system heat dissipation and the use of hydraulic coolers will always ensure your powerpack doesn’t overheat.




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