Tech Briefs Magazine - December 2022 - BET8

Leak Testing
considered, but with a mixture of water
and glycol. Two typical automotive applications
here are the cooling of the engine
block in an internal combustion engine
and the cooling circuit in a traction
battery box for cooling the battery cells.
The requirement for leak tightness of
the coolant circuit in the engine block is
defined as less critical than the requirement
in the cooling circuit of traction a
battery enclosure. In the application case
of the engine block to be cooled, coolant
loss must not exceed specific limits prior
to any required replenishment. In the
case of the application for cooling a battery
enclosure, the requirements are defined
much more critically. Here, damage
or short circuits to the battery cells must
be prevented. Leakage coolant from the
cooling circuit can cause a battery fire.
0.80
0.60
0.40
0.20
0.00
2.0
Ø blocked leak channel water
3.5
Overpressure cooling system [bar]
Ø blocked leak channel ethylene glycol
Figure 1: Calculated leak channel diameter of blocked glass capillaries for water and ethylene
glycol at different overpressures of the cooling system. (Image: Inficon)
Pressure gauge
Inspection glass
(paddle wheel)
Pressure regulator
Cooling circuit
5.0
Independent of the leak tightness required
in each respective application
regarding the loss of liquid, a requirement
must be defined for the leak tightness
during leak testing with test gas. In
this paper, the smallest acceptable
cross-section or diameter of a leakage
channel for the coolant glycol is derived
and the leakage rate value to be assigned
for the test gas leakage test is
given. Compared to the requirements
regarding IP67, three essential differences
must be considered in the application
case of a coolant circuit.
In the coolant circuit, overpressure
of up to 5 bar prevails during operating
conditions, whereas the requirements
for IP67 generally consider an effective
force at the leakage channel corresponding
to a pressure difference of
1100 mbar against 1000 mbar. With
increasing pressure difference at the
leakage channel, the leakage rate increases
correspondingly with the same
leakage channel geometry and significantly
more medium leaks out than
under the IP67 test conditions and thus
requirements for test criteria for testing
a coolant circuit must be defined
more strictly.
Furthermore, the temperature in the
coolant circuit is significantly increased
during operation, which in turn affects
the viscosity of the medium. As the temperature
increases the viscosity decreases,
which in turn increases the
leakage rate.
The difference in temperature from
room temperature to the operating temperature
in the coolant circuit changes
the viscosity by up to an order of magnitude,
which correspondingly increases
the leakage rate.
Third, the property of surface tension
or wetting angle of the liquid in a leakage
channel and its wall affects the
channel geometry of both the leakage
flow, which may be prevented due to
blockage of the leakage channel. Thus,
when setting
rejection limits
for leak
testing, the property of the liquid medium
used must be considered.
Theory of Leak-Channel
Behavior
The blocking of a leak channel with
a liquid, e.g., a water-glycol mixture,
depends mainly on the surface tension
(σ), the contact angle (θ) between the
solid-state material and the fluid, and
the maximum overpressure (p).
The leakage channel radius at which a
liquid can no longer escape from a leakage
channel is described in the appendix.
Using equation (1), the leakage channel
radius at which the leak channel is
blocked by the liquid due to capillary
forces and prevents the liquid escaping
the tube may be calculated.
r = 2*σ*sinӨ
p
Where:
Holder with glass capillaries
Figure 2: An experimental setup. (Image: Inficon)
8
p = pressure inside the drop of liquid
σ = surface tension of the liquid
θ = contact angle Theta
r = radius of the leak channel
Battery & Electrification Technology, December 2022
Ø Leak channel [μm]

Tech Briefs Magazine - December 2022

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