Fluid Conditioning My Notes and Calculations

CHAPTER 8

Fluid Conditioning
Introduction
Fluid conditioning is critical in maintaining proper operation of a hydraulic system. In this section,
you will learn about different types of filters, their location, and how they keep hydraulic fluid clean.
You will also learn about the importance of regulating the temperature of hydraulic fluid with devices
like heat exchangers, because fluid that is too hot or too cold can have a negative impact on
system performance.

Filtration
Cleanliness of hydraulic fluid has become critical
in the design and operation of fluid power
components. With pumps and valves designed
to closer tolerances and finer finishes, fluid
systems operate at ever increasing pressures
and efficiencies. These components will perform
as designed as long as the fluid is clean. Oil
cleanliness results in increased system reliability
and reduced maintenance.

As particles are induced or ingressed into a

hydraulic system, they are often ground into
thousands of fine particles. These tiny particles
are tightly packed between valve spools and Figure 8.1 –engineered filtration in a hydraulic system
their bores, causing the valve to stick. This is
known as silting.
To prevent silting, early component wear, and
eventual system failure, engineered filtration
is required. Engineered filtration includes
understanding the required micron rating,
application of the beta ratio, maintaining
proper ISO code cleanliness levels, and filter
location specific to the system design and
environment.

Figure 8.2 –silt that has formed between hydraulic components

DID YOU KNOW?

Ingression is defined as the rate at which external contaminates enter the system
from the cylinder rods, air breathers, shaft seals, and other possible points of entry.

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Micron (µm)
Micron (µm) is the designation used to describe
particle sizes or clearances in hydraulic components.
A micron is equal to 39 millionths of an inch. To put
this into perspective, the smallest dot that can be
seen by the naked eye is 40 µm.

Consider Figure 8.3. If we looked at a human hair

magnified 100 times, the particles you see next to the
hair are about 10 µm. Industrial hydraulic systems
usually filter in the 10 µm range. This means that
filters are filtering particles that cannot be seen by
the naked eye.
Figure 8.3 –a human hair compared to particles of 10 µm in size magnified 100X

Beta Ratio
Filtration devices are used to filter
particles out of the system’s fluid. A
filter’s efficiency is rated with a beta
ratio. The beta ratio is the number of
particles upstream from the filter that
are larger than the filter’s micron
rating, divided by the number of
particles downstream that are larger
than the filter’s micron rating. In
Figure 8.4 there are 200 particles
upstream which are larger than 3 µm.
These flow up to and through the
Figure 8.4 –a beta ratio of 2 compared to a beta ratio of 200
filters. A filter that allows more particles
through, or in other words, one that
is less efficient, has a low beta ratio. The filter at the top allowed 100 particles through. The filter on the
bottom allowed only 1 particle through. By applying these numbers to the beta ratio formula, it becomes
clear that the filter at the top has a lower or less efficient beta ratio, and the filter at the bottom has a
higher or more efficient beta ratio.

ISO Code
To specify the cleanliness level of a given volume of fluid, we refer to what is known as an ISO
(International Standards Office) code, or ISO solid contamination code. This code, which applies to all
types of fluid, provides a universal expression of relative cleanliness between suppliers and users of
hydraulic fluid.

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Based on 1 milliliter of fluid, a

particle count is analyzed using
specific sizes of particles, 4 µm, 6
µm, and 14 µm. These three sizes
were selected to give an accurate
assessment of the amount of silt
from 4 µm 0particles and 6 µm
particles, while the number of
particles above 14 µm reflects the
amount of wear type particles in
the fluid.
Figure 8.5 –particle count is analyzed in 1 milliliter of fluid

To interpret the meaning of these results, a graph like the one in Figure 8.6 is consulted. Figure 8.6 shows
an example of a rating of 22/18/13. The first number, 22, indicates that the number of particles greater
than or equal to 4 µm in size is more than 20,000 and less than or equal to 40,000 per milliliter.
The second number, 18, indicates that the number of particles greater than or equal to 6 mm in
size is more than 1,300 and less than or equal to 2,500 per milliliter. The third number, 13, indicates that
the number of particles greater than or equal to 14 µm in size is more than 40 and less than or equal to
80 per milliliter.

This ISO code is meaningful

only if it can be related to the
required cleanliness level of
the hydraulic system. This is
usually based on a manufac-
turer’s requirement for
cleanliness levels in which a
component may operate. For
example: Most servo valves
require a ISO code of 15/13/12
or better, while gear pumps
may operate adequately in
fluids with 18/16/15 ISO.

Figure 8.6 –a chart, such as the one shown here, is used to analyze the particle count

Filter Placement
Filter placement is critical for maintaining acceptable fluid cleanliness levels, adequate component
protection, and reducing machine downtime. Filter breathers are critical in prevention of airborne
particulate ingression. As the system operates, the fluid level in the reservoir changes. This draws in
outside air, and with it, airborne particulates. The breather filters the air entering the reservoir.

Pressure filters are often required to protect the component immediately downstream of the filter, such as a

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sensitive servo valve, from

accelerated wear, silting, or
sticking. Pressure filters must
be able to withstand the
operating pressure of the
system as well as any pump
pulsations. Return line filters
are neccessary to provide for
total system cleanliness. They
can trap very small particles
before they return to the
reservoir. They must be sized
to handle the full return flow
from the system. A kidney
loop or off-line filtration is
Figure 8.7 –a hydraulic system demonstrating different filter locations
often required when fluid
circulation through a return filter is minimal. Being independent of the main hydraulic system, off-line
filters can be placed where they are most convenient to service or change. It is normal for off-line
filtration to run continuously.

Heat Exchangers
Temperature control is critical in hydraulic
systems. Even with the best circuit design,
there are always power losses in converting
mechanical energy into fluid power. Heat is
generated whenever fluid flows from high to
low pressure without producing mechanical
work. Heat exchangers may be required when
operating temperatures are critical or when
the system cannot dissipate all the heat that
is generated.

Figure 8.8 –a schematic of a heat exchanger in a hydraulic system

There are two basic types of heat exchangers.

Each is based on a different cooling medium:
water cooled heat exchangers and air-cooled
heat exchangers. If cooling water is available, a
shell and tube heat exchanger may be pre-
ferred. Cooling water is circulated through a
bundle of bronze tubes from one end cap to
the other. Hydraulic fluid is circulated through
the unit and around the tubes containing the
Figure 8.9 –an example of a water cooled heat exchanger

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water. The heat is removed from the hydraulic

fluid by the water. There are advantages to this
type of cooler. It is the least expensive,
it is very compact, it does not make noise, it
provides consistent heat removal year round, and
it is good in dirty environments. The
disadvantages are that water costs can be expen-
sive, with rupture, oil and water may mix, and it
usually requires regular maintenance to remove
mineral buildup.

Air-cooled heat exchangers consist of a steel

radiator core through which fluid flows
Figure 8.10 –a fan cooled heat exchanger
while a strong blast of air passes across the
core. In industrial applications the air is pushed by an electric motor driven fan. The advantages of an
air-cooled heat exchanger over a water-cooled heat exchanger are that an air-cooled heat exchanger
eliminates the problems associated with water, and it allows dissipated heat to be reclaimed. The
disadvantages are that the installation cost is higher, noise levels range from 60 to 90 decibles, and its size
is larger than a comparable water-cooled heat exchanger.

Reservoirs
In addition to holding the
system’s fluid supply, the
reservoir serves several other
important functions. It cools
the hydraulic fluid. This is
accomplished by dissipating
excess heat through its walls.
It conditions the fluid. As
fluid waits to leave the
reservoir, solid contaminants
settle while air rises and
escapes. The reservoir
may provide mounting
Figure 8.11 –a hydraulic reservoir
support for the pump or
other components. A well designed hydraulic system always includes a properly designed reservoir. An
industrial reservoir should include the following components: a baffle plate to prevent returning fluid
from entering the pump inlet, a clean out cover for maintenance access, a filter breather assembly to
allow air exchange, a filler opening well protected from contaminant ingression, a level indicator allow-
ing upper and lower levels of fluid to be monitored, and adequate connections and fittings for suction
lines, return lines, and drain lines. It is often stated that the hydraulic fluid is the heart of the system or
the most important component. The reservoir serves a critical role in maintaining the efficiency of fluid
transfer and conditioning.

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QUESTIONS
Filtration
DID YOU KNOW?

1. The beta ratio of 75 _____.

Reservoirs may be classified as vented or pressurized.
a. is less efficient than beta 100
b. is more efficient than beta 100
Vented reservoirs are open to the atmosphere.
c. indicates the micron size
d. None of the above.
Pressurized reservoirs offer several advantages over vented reservoirs:
contaminants and condensation are reduced, and pressurized reservoirs
2. Filter breathers are critical in the prevention of airborne particulate ingression.
help force fluid into the pump inlet.
a. True
b. False

Reservoirs

3. Reservoirs help to condition and store hydraulic fluid.

a. True
b. False

4. Hydraulic fluid that is returning to the reservoir may contain entrained air and solid contaminants.
SUMMARY a. True
b. False
Micron (µm) is the designation used to describe particle sizes or clearances in hydraulic components.

5. All fluid conductor lines entering the reservoir terminate below the fluid level.
The beta ratio is the number of particles upstream from the filter that are larger than the filter’s micron
a. True
rating, divided by the number of particles downstream that are larger than the filter’s micron rating.
b. False

An ISO code provides a universal expression of relative cleanliness between suppliers and users of
hydraulic fluid.

Heat exchangers may be required when operating temperatures are critical or when the system cannot
dissipate all the heat that is generated.

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