I
S e p t e q k r , 1924
G
sectional area to the space inchded by four adjacent tubes. The results later obtained- seemed equation in this way. -In solving Equat;on 10 for q., the mass flow of air, m, is ' .pading determined by the air density, flying speed;.%& and number of tubes, while Dehas a value also determined by the tube spacing. The diameter of a cirde equivalent in area to the space included by four tubes was used as an approximation. in which D, Dh
a and b
= equivalent diameter in Equation 10 = diameter of tube = vertical and transverse spacing of
tubes,
respectively
Since the exhaust gas consists of approximately 10 per cent steam, for which C, = 0.45, and 90 per cent other products of combustion, for which C, = 0.24, the equivalent C h was taken as 0.26. For the range below 55" C. the effective specific heat is very high, owing to the latent heat of condensation. This was determined approximately by computing the total heat of 1 gram of the gas mixture a t frequent temperature intervals below 55" C. and finding the rate of change of total heat with temperature. This rate of change was taken as the effective specific heat with a mean value of 2.83 in the range 55" to 45" C. and 1.92 in the range 45" to 35" C., or 2.20 for the entire range 55" to 35" C. The figure 45" C. for the exhaust temperature a t the end of the second condenser bank was secured by successive approximations, and is used only to determine the effective specific heats over the ranges indicated. These values were checked by a more rigorous mathematical determination, which can well be omitted here. The viscosity of the gas changes considerably with temperature, but an average value of p over the range 600" to poises, the viscosity of air a t 55" C. was taken as 2.4 X 150" C. For the range below 55" C. it was assumed the effect of the condensing vapor was negligible and the value 2.0 x 10-4 poises for the viscosity of air was used. For the air flowing outside the tubes, p = 1.85 X Thus, we have constant values for the variables in Equamay be tion 10, so that Qh and y,, and consequently Ph
+ 40
taken as constant for each range above and below the condensation point. For a three-bank condenser of the general form of Model 11, it is impossible to compute the necessary L directly, on account of the changing functions of the variables during parallel and counterflow as well as the change of C, with temperature of the exhaust gas. The most convenient method is to assume an over-all length for the condenser and solve Equation 8 or 9 for each section given. It is, of course, necessary to measure or assume the temperature of the exhaust gas received by the condenser as well as the air temperature. It will be found that an equation in two variables will be left in each case, but that one of these can be eliminated by solving simultaneously with the equation expressing the fact that the heat lost by the exhaust is equal to the heat taken up by tho air. (ra
-
Tb)
Hh =
(tb
- ta)
Hc
The curves of Fig. 8 show air temperature plotted against necessary length of a three-bank condenser for three air speeds and four initial exhaust temperatures. These summarize the effect of changes in the most important variables on heat transfer, for a constant final exhaust temperature of 32" C . This figure represents the conditions necessary for the condensation of 1 pound of water per pound of gasoline burned.
A New :
r
NORTON Co.,WORCSSTER, MASS.
N AN apparatus for analyzing gaseous mixtures of oxides of nitrogen it was necessary to have a gas-absorption bottle that would contain a known volume of liquid through which the gases could be passed, give excellent contact between the gas and the liquid so that complete absorption would result, circulate the liquid thoroughly so that all the liquid would be brought into contact with the gas, not trap air or gas at any point, and be all glass because the gases were, corrosive. Requirements of moderate ruggedness and accessibility for washing out completely also had to be met. Any bottle that satisfies all these conditions is an almost perfect gas-washing bottle for general laboratory purposes. The spiral gas-washing bottle designed for this use by the writer while working at Cornel1 University is shown in Fig, 1. There are three essential parts: B is a gas inlet tube sealed through a ground-glass stopper. C is a cylindrical tube open at both ends, the walls of which have been pressed in the form of a spiral, This slides on and'off over the inlet tube, but is held in position, when the bottle is assembled as shown in the figure, by the turned-up end of the inlet tube. A is the cylindrical outside bottle; the spiral fits this fairly closely and the top of the bottle is closed by the groundglass stopper of the inlet tube. This bottle is easy to make and is moderately rugged. At first it was feared that the spiral would tend to break off the end of the inlet tube by falling against it or by being jerked against it when the bottle is taken apart; but this difficulty was not experienced. The action of the bottle is simple. The gas carries the liquid up between the outside of the spiral and the inside of the bottle, and then the liquid goes down between the inside of the spiral and the outside of the inlet tube. The circulation is good, as can be seen when the lines of liquid flow are studied by putting a drop of dye solution into one of these bottles filled with water and then passing air through. There are no "dead" spaces to trap gas, and the spiral gives excellent contact between the gas and the liquid for a considerabIe period of time. The bottle is compact and oonvenient in form, and is easy to take apart and wash out for quantitative work. The opening a t the top of the spiral is elliptical, so that when gas bubbles tend to lift the spiral it is brought to rest against the little bulb at the center of the inlet tube. When the bottle is taken apart slide the spiral off over the end of the inlet t ture makes it easy to pack the bottIe for shipment, or possible to replace a broken spiral without getting a new bottle.
I
1
Received Jvne 25, 1924.
Gas Hazards in Street Manholes A study of gas hazards in street manholes has been completed by engineers attached to the Pittsburgh Experiment Station of the Bureau of Mines. Examinations were made of gases found in street manholes of Pittsburgh and Philadelphia to determine the health and explosion hazards of such gases, particularly from the appearance of natural gas, manufactured gas, or other in: flammable substances getting into the
