Apparatus for measuring the flow rate of water vapor in a process gas including steam

An apparatus for measuring the flow rate of water vapor in a process gas including steam includes a sample system having a dew point meter and a pressure meter, and an inert gas supply device for mixing an inert gas with a process gas in the sample system. The process gas comprises a first gas, such as natural gas, and steam. A partial pressure calculating unit calculates the partial pressure of the water vapor contained in the gas mixture on the basis of the dew point of the mixture measured by the dew point meter and the pressure measured by the pressure meter. A flow rate calculating unit calculates the flow rate of the steam contained in the process gas on the basis of the water vapor partial pressure, the inert gas flow rate, and the flow rate of the first gas in the process gas.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an apparatus for measuring the flow rate 
of water vapor in a process gas including steam and, more specifically, to 
an apparatus which is capable of measuring, with a high level of 
precision, the flow rate of the steam contained in a gas with a high dew 
point supplied to, a process reactor such as a fuel cell power generating 
system when the power generating system is being operated, adjusted or 
checked. 
2. Description of the Related Art 
FIG. 2 is a schematic diagram for explaining the principles of a 
conventional method of measuring a dew point by means of a 
thermoelectrically cooled, photoelectric dew point meter, the method being 
known from, e.g., "Handbook of Process Measurement and Control" (FIG. 
3.324, published in 1970 by Nikkan Kogyo Shinbun). As shown in FIG. 2, a 
photoelectric dew point meter for use in process control has a dew point 
detecting section 10 into which a sample gas is drawn by suction. A mirror 
3 made of a metal is disposed inside the section 10. The temperature of 
the mirror 3, which is automatically held at the dew point of the gas, is 
continuously measured to indicate the dew point, and the measured dew 
point is used to perform automatic control. The mirror 3 is cooled by a 
thermoelectric cooling device 4 employing the Peltier effect and 
comprising semi-conductor cooling elements. The cooling and the heating of 
the device 4 are controlled in such a manner as to maintain the 
temperature of the mirror 3 at the dew point. Specifically, when the 
humidity of the gas flowing into section 10 changes, the amount of dew on 
the surface of the mirror 3 increases or decreases. This increase or 
decrease causes a corresponding increase or decrease in the quantity of 
light projected from a lamp 1 through a slit 2. This light is then 
reflected from the mirror 3 and then enters a cadmium sulfide member 6. On 
the basis of the change in the light quantity, a mirror surface 
temperature control section 11 comprising an adjuster 8 and a controller 9 
operates to either raise or lower the temperature of the mirror 3 so that 
whenever the dew point changes, the temperature of the mirror 3 is updated 
to become equal to the new dew point. The temperature of the mirror 3, 
which is thus held at the dew point, is indicated by a thermometer 5. On 
the basis of the indicated dew point and on the basis of a certain 
relation between a gas temperature and saturated water vapor pressure, 
shown in the following Table 1 (which is the same as Table 3.53 shown in 
the above-mentioned document), the partial pressure of the water vapor is 
obtained. 
TABLE 1 
______________________________________ 
SATURATED WATER-VAPOR PRESSURE (mb) 
(WHEN COEXISTENT WITH WATER) 
TEMP- 
ERATURE 
.degree.C. 
0 1 2 8 9 
______________________________________ 
. . . . . . 
. . . . . . 
. . . . . . 
0 6.1078 6.5662 7.0547 10.722 11.474 
. . . . . . 
. . . . . . 
. . . . . . 
50 123.40 129.65 136.17 181.53 190.22 
. . . . . . 
. . . . . . 
. . . . . . 
90 701.13 728.19 756.11 943.02 977.61 
______________________________________ 
The above-described conventional method of measuring the dew point entails 
the following problems when dew point measurement is performed in a 
system, such as a fuel cell power generating system, where a gas with a 
high dew point exists and where the rated temperature is often above 
90.degree. C. The dew point measurement performed can be considerably 
inaccurate because, as will be clearly understood from Table 1, a change 
of the dew point by 0.1.degree. C. corresponds to a change of the water 
vapor partial pressure by about 3%. In such systems, therefore, it is 
difficult to accurately measure the actual flow rate of steam. In 
addition, when the measured flow rate of the water vapor is used in a 
feedback arrangement to achieve the correct flow rate of the steam, a 
manual operation is performed. As a result, the feedback requires a great 
amount of labor. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the present invention is to overcome the 
above-described problems and provide an apparatus capable of determining, 
with a high level of precision, the flow rate of steam in a system, such 
as a fuel cell power generating system, to which a high dew point process 
gas is supplied, the apparatus being also capable of supplying, with a 
high level of precision, the correct flow rate of steam to the system. 
In an apparatus for measuring the flow rate of steam according to the 
present invention, an inert gas is mixed with a process gas comprising a 
first gas (such as natural gas) and steam to obtain a gas mixture. The dew 
point and the pressure of the gas mixture are measured, and the partial 
pressure of water vapor in the gas mixture is calculated based on the 
measured dew point and pressure. Based on the calculated partial pressure 
and the flow rates of the first gas and the inert gas, the flow rate of 
water vapor in the process gas is calculated and is used to control the 
flow rate of steam. 
In the apparatus for measuring the flow rate of steam according to the 
present invention, introducing an inert gas at a known flow rate allows, 
the partial pressure of the water vapor to be reduced without changing the 
flow rate of steam so that the dew point at which the partial pressure is 
measured is lowered. The advantage provided by introducing an inert gas 
will be understood by considering the case where the process gas has a 
high dew point of, for instance, above 90.degree. C., and where, as shown 
in Table 1, a change in the dew point by 0.1.degree. C. corresponds to 
about 3% change in the partial pressure of the water vapor. According to 
the present invention, if the dew point at which the water vapor partial 
pressure is measured is around 50.degree. C., the change in the water 
vapor partial pressure corresponding to a 0.1.degree. C. change in the dew 
point is only about 0.5%.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A preferred embodiment of the present invention will now be described with 
reference to the drawings. FIG. 1 is a block diagram of a steam flow rate 
measuring apparatus according to one embodiment of the present invention 
which is incorporated into a fuel cell power generating system. Referring 
to FIG. 1, a process gas supply system 14 is connected with a process 
reactor comprising reformer 12 and fuel cell 13 connected to the reformer 
12. The process gas supply system 14 comprises a sub-system 14a for 
supplying natural gas (not shown) to the reformer 12, and a sub-system 14b 
for supplying steam (not shown) to the reformer 12. A sample system 15 
comprises a dew point meter 16 provided in the vicinity of the entrance of 
the reformer 12 and a pressure meter 17. The sample system 15 is connected 
with an inert gas supply means 18 for mixing of an inert gas (nitrogen 
gas, not shown, in this embodiment) having a known flow rate with the 
process gas in the sample system 15. A partial pressure calculating means 
19 comprising a partial pressure calculating unit 19a calculates the 
partial pressure of the water vapor contained in a gas mixture of the 
process gas and the inert gas on the basis of the measurements by the dew 
point meter 16 and the pressure meter 19. A flow rate calculating means 20 
comprising, by a steam flow rate calculating unit 20a calculates the flow 
rate of the water vapor contained in the process gas on the basis of data 
including the value calculated by the partial pressure calculating means 
19. A correct flow rate setting means 21 comprising a flow rate setting 
computing element 21a calculates the correct amount of the water vapor 
flow rate, the actual amount of which has been calculated by the flow rate 
calculating means 20, and then sets the calculated correct flow rate. A 
flow controller 22 receives a signal from the correct flow rate setting 
means 21 which indicates the correct flow rate, and controls the flow rate 
of steam on the basis of the received signal. The natural gas supply 
sub-system 14a, the steam supply sub-system 14b and the inert gas supply 
means 18 include of flow control valves 23a, 23b and 23c, and of flow 
meters 24a, 24b and 24c, respectively. 
Next, the operation of the apparatus will be described. 
A certain amount of an inert gas is mixed with the process gas in the 
sample system 15 provided in the process gas supply system 14 so as to 
obtain a gas mixture in the sample system 15. The inert gas is supplied at 
a temperature and flow rate which do not cause the condensation of the 
water vapor in the sample system 15. The flow rate of the inert gas is set 
by the inert gas flow meter 24c at a value corresponding to the range 
within which the dew point meter 16 performs measurement. The dew point of 
the gas mixture in the sample system 15 is measured by the dew point meter 
16 and the pressure of the gas mixture is simultaneously measured by the 
pressure meter 17. The dew point and the pressure measured by meters 16 
and 17, respectively are processed by the partial pressure calculating 
means 19 so as to obtain the partial pressure of the water vapor contained 
in the gas mixture. 
Next, the flow rate of the inert gas mixed with the process gas in the 
sample system 15, the flow rate of the natural gas measured by the natural 
gas flow meter 24a, and the water vapor partial pressure calculated by the 
partial pressure calculating means 19 are all input to the steam flow rate 
calculating means 20. In this way, the flow rate of the water vapor is 
measured at a dew point lower than the dew point which would exist if an 
inert gas were not mixed with the process gas. Thus, it is possible to 
accurately control the flow rate of the steam. 
The steam flow rate which has thus been obtained is input to the flow rate 
setting means 21 which sets the correct steam flow rate by causing, 
through the flow controller 22, the steam flow control valve 24b to 
generate a feedback signal. 
In the above-described embodiment, the system to which a process gas with a 
high dew-point is supplied is a fuel cell power generating system. 
However, the present invention may be applied to any system supplied with 
a high dew point water vapor. 
Further, the present invention is not intended to be limited by the 
arrangement in which an inert gas is mixed with the process gas in the 
sample system. An inert gas may be mixed directly with the process gas in 
the process gas system in such a manner that this mixing causes no adverse 
influence on the entire process.