An Improved Pressure-Regulating Device B. R. WARNER, Central Experiment Station, Bureau of Mines, Pittsburgh, Penna.
A manostat capable of controlling a large range of pressures over a large range of flows is described. The method of operation is given and the limiting, conditions for its use are stated in terms of mathematical equations and graphs.
E
XPERIMENTS in progress necessitated the use of a manostat capable of controlling a large range of pressures over a large range of flows. The problem was to pass a gas through a reaction chamber a t definite pressures and a t varying rates of gas flow. As the gaseous products must be undiluted with other gases when collected on the low-pressure side, a foreign gas or air-leak devire could not be used. A simple and arcurate devire for such a purpose has not been described i n the literature. Caldwell and Barham ( 1 ) describe a float-valve manostat for high pressures, while Lewis (8) depicts an accurate device for lowpressure control based upon the throttling action of a sintcredglass disk in rontac% with mercury. Both these systems are self-contained hut have their limitations in pressure and flow ranges. In the present method, a sintered-glass disk, a by-pass controllable needle valve, and a mer w y column which also serves as a manometer are used on the high-pressure side, and a throttling needle valve on the low-pressure side. In Figure 1the system is schematically shown. is mercury connected by three-way stopcock either to the mercury reservoir i, or to the system whose pressure is to be controlled. d is a sintered-glass disk that allows gas,
but not mercury, to pass. This disk is slightly inclined to the mercury column in contact with it, so that it may be only partly covered. c is a check valve to prevent mercury from passing into the system in case of a sudden decrease in the pressure in the system. a and b are stainless-steel needle valves wherein the movement of the needle is effected by a bellows. e is a trap to collect mercury which may distill through the disk. In operation the desired pressure for the system is set by connecting mercury reservoir i and manometer rn by means of threeway stopcock f; mercury is added to or withdrawn from m through the use of stopcock g. Stopcock f is then turned to connect m with the system, and valves a and b we adjusted until the sintered-glass disk d, is partly covered with mercury. The pressure in the system is then measured from h, which is the difference in the levels of the mercury in m and at d. I t is necessary to throttle valve a only at very low flows through the system. Valve b is used at large flows, since the gas will flow through d a t small rates only because of its high resistance to gas flow. When the pressure in the system is higher than the desired value, the sintered-glass disk is uncovered and gas flows through the disk to the vacuum side, whereas, when the pressure is low, the disk becomes covered with mercury and gas does not pass through, thus allowing the pressure to build up. Figure 2 shows the maximum rates of flow of air through dthat is, with b closed. These values depend to a certain extent on the rapacity of the pump in the vacuum system. Although the medium-prade filter has the advantage of low flow resistance, its use is limited to pressure differentials up to 1.5 atmospheres, a t which point mercury passes through the disk. The fine-grade disk was tested up to 2 atmospheres' pressure differential, and no passage of mercury through it was observed. The principal advantage of this type of control over the float method is that the problem of the relationship between the weight of the florLt and the pressure differential is eliminated One sintered-glass disk functions at all pressures until the pressure differential is reached where mercury passes through the disk. Certain limitations in the use of a sinteredglass disk in a manostat are imposed by passage of mercury through the disk and restriction of gas flow. The relation governing the passage of mercury can be shown to be
2 ycos 8 = where pl
- (PI - p*)T
(1)
surface tension of the mercury pressure differential across the disk T = radius of pores (average) 8 = contactangle y = pl =
That for flow resistance can be closely approximated by either the Poiseuille equation
G = - dm -
-
p2b4
8vd
Lv RT
(2)
or the Knudsen equation when the mean free path is comparable with T 4 G = 6 ( P I - PZ) (3)
dfi
FIGURE 1. APPARATUS FOR CONTROL OB PRESSURE AND FLOW OF GASES 637
where G
=
7
= = =
mass of gas flowing through per second viscosity of gas p average pressure in the disk M molecular weight of gas R = gasconstant T = absolute temperature d = thicknessofdisk and the other terms are as defined for Equation 1.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
638
Vol. 15, No. 10
therefore, that a disk with low resistance t o flow will also have a law resistance to passage of mercury, which destroys the regulating function of the disk. For example, the medium disk passed mercury at ahout 1.5 atmospheres pressure differential. Using the following values in Equation 1
4
= 485 dynes per cm. = 1.5 X 10' dynes per sq. C O S 8 = -1 y
p,
-p,
cm.
r is calculated to he about 6.6 microns. The fine disk waa tested up to 2 stmospheres without passing mercury. From this one would estimate I to he of the order of 3 microns.
3
e
2
b
I e
While this disk can be used at greater preasure differentials, the gas flow, as shown by Equations 2 and 3 and by Figure 2, is considerably reduced. To obviate this difficulty valve b was inserted in parallel with the disk. This valve can he adjusted t o the point where the flow of gas to the disk is of a magnitude capable of being handled by the disk and the regulating property of the disk functions. Because of its ready availability a fritted-glass disk WES used. A much better scheme would involve the use of a long, cylindrical cup of fritted glass, whereby the mercury would find ita level by covering or uncovering the cylinder for passage of pas. This was done by Wanshrough-Jones (S), who used a clay tube closed a t one end for regulating flow of gases at very low pressure differences. In this case, however, the presetting of a desired pressure would he more difficult since d , Figure 1, would no longer he a flxed point and height h would he a varinhle. Pyrex brand sintered-glass disks manufactured by the Corning Glass Co. were used.
Y a
'0
Y
k
_I
2
1
0
20
40 PRESSURE. CM. Hg IN SYSTEM
SO
FIGURE2. MAXIMUMRATESOF GAS FLOW THROUGR S m E R E n - G w s DISKS
(1)
Literature Cited Galdwell. M. J.. and Barham, H. N., IND.ENB.Ca~aa..ANAL. ED.,14.485 (1942).
As can he seen from these relations, the limiting pressure differential for nonpassage of mercury varies inversely as I, while the flow of gas varies as the cube or 4th power of r. It follows,
(2) Lewis, F.M., Ibid.. 13,418 (1941). (3) WansbroughJones, 0.H., Pwc. Roy. Soc., A , 127.530 (1930).
Pne~raamby ~ermisaionof the director, Bureau of Minea. United States Department of Interior.
Modified Hershberg Melting-Point Apparatus MORRIS M. GRAFF, Southern Regional Resear-L
T
-L---+--r
?inn Rnh.r+ P 1
-
RlvA
Nsu n.lPPns1.-
A.
CCURATELY controlled temperature and heating of n mg-point baths have long been a problem in the determ tion of melting points up to ahout 150' C. Where the temp ture is controlled by heating resistance units, one is apt t o counter a lag or overshoot in the temperature when contar made or broken. The Herahberg precision melting-point apparatus' has t modified, so that the melting-point temperatures of low-me1 solids may he maintained to within 0.1' C. over 8. longer perio time (Figure 1). Heat is applied through radiation from an frared drying bulb of 105 to 120 volts and 250 watts. When lamp is turned on or off, heat is applied or discontinued ins taneously, thus avoiding any heat Isg or overshoot. The bu placed within a metal container sufficientlyventilated to allom heat dissipation and is focused on the heating chamber of melting-point bath, The chamber consists of a flittened por of the Thiele tube made by sealing two 6cm. Pyrex petri di into the tube. By using the infrared bulb specified the limi temperature is ahout 150' C. A higher temperature coulc attained by using two heating units, one on either side of heating chamber, or by using lamps of higher wattage. heating is eontrolled by means of a variable-voltage transfon 1
Hershberg, E. B.. IN". E N..
C a m . , An*=. Eo., 8 , 312 (19361.
FIGURE 1
