wet gas meter and hot wire anemometer.ppt

wet gas meter and hot wire anemometer

1. 1

2. Schematic of wet gas
meter
2
The gas fills a rotating segment
and an equal volume of gas is
expelled from another segment.

This is a positive displacement
type meter.
These are usually bulky in size
because the linear velocity of a
gas in a pipe is normally very high
compared to that of a liquid, and
the large volume is needed so that
the speed of moving parts can be
reduced and wear minimised.
Compartment
inlet port
Sight box

3. Operation
It consists of a horizontally disposed drum
divided into compartments by means of an
impeller with curved blades rotating freely about
its axis.
The drum is partly filled with water / oil to a level
just above the axis of rotation.
The gas enters the drum at the centre and as it
fills the compartment, the gas pressure causes
the drum to rotate.
When the compartment is filled, the inlet port is
sealed by liquid.
The inlet port then opens to the next
compartment and the drum continues to rotate.
3

4. Operation
As it rotates, the liquid enters the first compartment
and the gas contained is expelled through the outlet.
As the drum revolves, each compartment fills
alternately with gas and with liquid, so that it is
completely filled with gas and emptied once per
revolution.
During one revolution, an accurately known volume
of gas is discharged.
Discharge of gas from a compartment cannot begin
before charging is complete so that for every
rotation of the drum an accurately defined volume
of gas is delivered.
4

5. Cautions and Precautions
The liquid level should be accurately maintained
because it controls the effective volume of each
compartment.
The gas passing through the meter should be
saturated so that humidity variations are taken care
of (alternatively, oil may be used).
Pressures within the meter should be such that no
liquid displacements are caused.
Speed of the drum should be kept very low.
WGM’s need calibration because the effective volume
of the compartment cannot be accurately
determined.
5

6. Range of WGM
Used to meter flow rates between 2.5 X 10
-6
m
3
/s and 4 X 10
-
3
m
3
/s.
Pressure and temperature are normally close to ambient.
The pressure differential across the meter is usually less
than 2.5 kPa.
Needs carefully controlled conditions and skillful operation.
Uncertainties of about 0.25 % of reading can be achieved
and have range abilities up to 10:1.
Large size for moderate flow rates also.
Maximum delivery rated to be 9.5 X 10
-3
m
3
/s and the
corresponding size of the meter is 1.2m long and 1.05m
high.
A head of 5 – 10 mm H2O is required to operate even a
small meter.
6

7. Hot-wire anemometer
If a heated wire is immersed in a fluid, the rate of
loss of heat will be a function of the flow rate.
In the hot-wire anemometer a fine wire whose
electrical resistance has a high temperature
coefficient is heated electrically.
Under equilibrium conditions the rate of loss of
heat is then proportional to I
2
, where . is the
resistance of the wire and I is the current flowing.
Either the current or the resistance (and hence
the temperature) of the wire is maintained
constant.
7

8. Typical circuit
8

9. Operation ( is
constant)
The wire is incorporated as one of the resistances of a
Wheatstone network in which the other three resistances
have low temperature coefficients.
The circuit is balanced when the wire is immersed in the
stationary fluid but, when the fluid is set in motion, the rate
of loss of heat increases and the temperature of the wire
falls.
Its resistance therefore changes and the bridge is thrown out
of balance.
The balance can be restored by increasing the current so
that the temperature and resistance of the wire are brought
back to their original values; the other three resistances will
be unaffected by the change in current because of their low
temperature coefficients. The current flowing in the wire is
then measured using either an ammeter or a voltmeter.
9

10. Calculation of rate of
flow
10
The rate of loss of heat is found to be approximately
proportional
to (u + b’)
1/2
Under equilibrium conditions a’ (u + b’)
1/2
= I
2

where u is the velocity of the fluid,  the density, and a' and b'
are constants for a given meter. The resistance of the wire is
maintained constant ,  u = (I
4

2
/ a’
2
) – b’ = a’’ I
4
– b’
where a" = 
2
/a‘
2
remains constant.
The mass rate of flow per unit area is a function of the fourth
power of the current, which can be accurately measured.

11. Comment
11
The hot-wire anemometer is very accurate even for
very low rates of flow, it is one of the most convenient
instruments for the measurement of the flow of gases
at low velocities; accurate readings are obtained for
velocities down to about 0.03 m/s. If the ammeter has a
high natural frequency, pulsating flows can be
measured. Platinum wire is commonly used.