Gas flow controlling system

Abstract

Abstract The present invention relates to an arrangement for controlling the mass rate of flow of a particular gas to a predetermined level and to a high degree of accuracy. The arrangement is particularly adapted to form part of a gas sampler arrangement. The errors of the prior art gas sampling techniques attributable to changes in temperature, barometric pressure, flow resistance, and pump efficiency are obviated. The arrangement is also well adapted to control the mass flow rate of compressed gases. The arrangement is a three valve system through which the gas flows. The first valve is a pressure regulator valve controlled by the downstream pressure drop through the second valve and set to increase or reduce gas flow as needed to maintain the pressure drop across the second valve constant. The third valve which is downstream of the regulator and the second valve is set to operate under critical pressure ratio conditions. The flowing gas is then exhausted from the system, usually by operation of a vacuum pump or by discharge to the atmosphere. If the three valve arrangement is maintained in a thermostatically controlled, heated enclosure, the gas flows through the three valve arrangement at a constant mass rate, regardless of changes in flow resistances, pump efficiency temperature and barometric pressure of gas entering the system. The present invention relates to a gas flow path adapted to constant mass flow therethrough. The system is particularly well adapted for gas sampling purposes. A close examination of air sampling techniques employed by the art indicates that mass semi-conductor errors of about 5 percent are the rule and that errors thyristor, 10 percent are not at all uncommon. The errors arise from normal temperature and barometric pressure changes in the atmosphere. Measurement uncertainty occurs, particularly in unattended equipment especially in locations where air temperature and clogging of filters are not controlled. For example, a 30* F. temperature change over any 24 hour period is common in many parts of the United States, which difference would cause an accurate constant volume pump operated over the 24 hour period to have a mass sampling through put about 5.5 percent lower at the maximum temperature that at the minimum. If in addition there were barometric changes, e.g. 0.5 inches of mercury over the same 24 hour period, this could add another 1.6 percent difference. In actuality temperature and pressure changes of this daily magnitude occur in about one-third of the reported U.S. cities. Moreover meteorological changes in temperature and pressure associated with storm centers during the winter often double the above values. Clogging of filters and of analytical apparatus by pollutants (usually why the air is being sampled) contribute to sampling errors. Actually changes in flow resistance due to filter clogging or to a shift from one pollutant absorber to another often exceeds the effect of barometric pressure changes. In addition, variations in pump efficiency can have major effect on conventional samplers, most of which depend in some manner upon the performance of the pump. Most continuous sampling methods (for air pollutants) specify that the volume of air sampled be converted to the volume it would occupy at standard conditions of pressure and temperatures, usually 760 mm Hg and 25* C., the conversion has created a set of units for expressing the quantity of air as ''''standard'''' volumes. The units used most frequently are the standard cubic foot, the standard liter and the standard milliliter. Despite their expression in volume terms these units correspond exactly to a fixed mass of air and are, in effect, mass units, a standard linter (760 mm Hg and 25* C.) weighs 1.184 grams, a standard cubic foot of air weighs 0.07394 pounds (33.53 grams). Clearly the accuracy of reported measurements for air pollutant content can be no better than the initial measurement of the mass of air sampled. The principal object of the present invention is to provide a system capable of maintaining an essentially constant mass flow rate for the particular gas passing through. A further object of the present invention is to provide an air sampler system wherein the air sample passes therethrough at an essentially constant mass rate of flow. Further objects and advantages of the present invention will become apparent from the description thereof which follows.

Inventors

• 👤 Dale I Steele

Bibliographic Details