Page:Journal of the American Society of Mechanical Engineers, Volume 33.pdf/665

1332 :$$C_{pa}$$ = mean speciﬁc heat of air at constant pressure between temperature $$t$$ and $$t'$$
 * $$C_{ps}$$ = speciﬁc heat of steam at constant pressure between $$t$$ and $$t'$$
 * $$r'$$= latent heat of evaporation at wet-bulb temperature $$t'$$

Knowing any two of the three important values of $$t$$, $$t'$$ or $$W$$, we may solve for the third or for any other required relation.

50Determination of Weight of Moisture in 1 Lb. of Pure Air, having a Dry-Bulb Temperature $$t$$ and Wet-Bulb Temperature $$t'$$. To determine the equation of the adiabatic line corresponding to a given saturation, or true wet-bulb temperature $$t'$$, and dry-bulb temperature $$t$$, we have from equation [6]

51The diagonal adiabatic lines in the charts, Figs. 1 and 2, representing saturation or wet-bulb temperatures, are calculated from, this formula. It should be observed that they would be perfectly straight if it were not for the element $$C_{ps} (t - t')$$, which produces a slight curvature, becoming more pronounced at higher saturation temperatures. The dew point $$t_1$$ corresponds to $$W$$ on the saturation curve. The slope of these lines, neglecting $$C_{ps} (t - t')$$ is $$ \frac {dW}{dt} = - \frac {C_{pa}}{r'}$$. This will always give the intercept $$t$$ for $$W = 0$$.

52Wet-Bulb Depression and Cooling Effect. The wet-bulb depression or cooling effect obtained by having $$t$$ and $$W$$ known is

and

53Having $$t$$ and $$W$$ known, $$t$$ cannot be calculated except by relating $$W$$ to $$t$$ by an empirical equation. By referring to the psychrometric charts, Figs. 1 and 2, constructed chieﬂy for that purpose, $$t$$ is conveniently determined. The cooling effect $$(t - t')$$, to be obtained by saturating air of known temperature and moisture content, is likewise obtained from the chart.