Hydraulics / how hot is to hot

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This is a question that I'm often
asked by hydraulic equipment users:

When it comes to the oil's operating temperature -
how hot is too hot?

Heating of hydraulic fluid in operation is caused by
inefficiencies. Inefficiencies result in losses
of input power, which are converted to heat.

A hydraulic system's heat load is equal to the
total power lost (PL) through inefficiencies
and can be expressed as:

PLtotal = PLpump + PLvalves + PLplumbing + PLactuators

If the total input power lost to heat is greater than
the heat dissipated, the hydraulic system
will eventually overheat.

Hydraulic fluid temperatures above 180??F (82??C) damage
most seal compounds and accelerate degradation of the oil.

So while the operation of any hydraulic system at temperatures
above 180??F (82??C) should be avoided, fluid temperature is
too high when viscosity falls below the optimum value for the
hydraulic system's components.

This can occur well below 180??F (82??C), depending on
the fluid's viscosity grade (weight).

To achieve stable fluid temperature, a hydraulic system's
capacity to dissipate heat must exceed its inherent heat load.

For example, a system with continuous input power of 100 kW
and an eficiency of 80% needs to be capable of dissipating
a heat load of at least 20 kW.

It's important to note that an increase in heat load or
a reduction in a hydraulic system's capacity to dissipate heat
will alter the balance between heat load and dissipation.

As you've probably gathered, there are only two ways
to solve overheating problems in hydraulic systems:

1. Decrease heat load; or
2. Increase heat dissipation.

Decreasing heat load is always the preferred option because
doing so increases the efficiency of the hydraulic system.

In the meantime, be aware that continuing to operate a
hydraulic system when the fluid is over-temperature
is similar to operating an internal combustion engine
with high coolant temperature. Damage is guaranteed.  !!!!!!


Today I want to explain a technique that is
very useful when you're troubleshooting a system
that's overheating.

This technique involves using an infrared thermometer -
sometimes called a heat gun, to measure the oil's
temperature drop across the heat exchanger.

The heat rejection of the exchanger can then be
calculated and when this is expressed as a percentage
of input power, it will reveal whether the problem
is in the cooling circuit or elsewhere in the system.

##Installed cooling capacity typically ranges between 25 and 40
percent of input power. So if a system has a continuous input
power of 100 kilowatts and the exchanger is dissipating 26
kilowatts of heat, this means the efficiency of the system
has fallen below 74 percent. If the system is overheating,
this is a good indication that there is abnormal heat load
somewhere in the system.

On the other hand, if a system has a continuous input power
of 100 kilowatts and the exchanger is dissipating 10 kilowatts
of heat and the system is overheating, this means that there's
a problem somewhere in the cooling circuit or the system does
not have enough installed cooling capacity

Hope this sheads some light on some probs.

Anthony Campbell

AC Service & Parts



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