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Kamis, 28 Oktober 2010

Air, how hot is too hot?

/ On : 22.29/ Thank you for visiting my small blog here. If you wanted to discuss or have the question around this article, please contact me e-mail at mozabani@yahoo.com
Hello again Air,

In your last hydraulic maintenance email,
we talked about the value of the humble hydraulic
symbol and the benefit of obtaining and keeping
schematic diagrams for any hydraulic equipment
you're responsible for.

Today I want to deal with 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 efficiency 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 your next hydraulic maintenance email in a few days time,
I'll explain how to troubleshoot a hydraulic system that's

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.

Yours for better hydraulics knowledge,

Brendan Casey
Author of 'Insider Secrets to Hydraulics'; and
'Preventing Hydraulic Failures'.

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