T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
Dec., 191j
risen higher t h a n t h a t in t h e metal. On c u t t i n g t h e loaves t h e bread in t h e glass p a n s was found t o be more thoroughly baked t h a n t h a t i n t h e metal p a n s (see Figs. 11, I11 a n d I V ) . T h i s experiment was repeated with t h e same result. A more accurate comparison of t h e r a t e s of heating in various materials was obtained b y placing t h e dishes containing one liter of water on t h e shelf in t h e oven for a definite t i m e a n d noting t h e temperatures reached b y t h e water. T h e determinations are given in Table I. TABLEI-RATES Test NO.
OF
HEATING n‘ATER
TIIIXGLASS
1. . . . . . . . . . . 90°
2........... 3... . . . . . . . . 4. . . . . . . . . . . 5........... 6. . . . . . . . . . . Average. . . . . . .
88
91° 86
89
89.5 89
89 94 88
89.4
.. ..
Aluminum 78‘
I N VARIOUS XIATERIALS
Enameled earthenware
Tin
..
THICK GLASS
.. ..
86’
..
..
73 73
83
78
74
Si
86
7,a8
53.3
82
86
.. ..
..
.. ..
..
..
T h e resistance of t h e glassware t o fracture b y blow was compared with t h a t of crockery ( “ c h i n a ” ) a n d ename!ed earthenware by dropping a 3 j o - g r a m weight on t h e b o t t o m s of inverted dishes of approximately t h e s a m e shape. T h e height of drop was increased TABLE 11-RESISTAWE T O F R A C T C R E B Y BLOW Thickness of bottom RESULT DISIIRSTESTED GLASS: 2-quart, . . . . . . . . . . . . . . . . . .4 . 8 mm. Cracked a t 2 2 in. 2-quart, . . . . . . . . . . . . . . . . . . 5 . 8 mm. A-ot broken at 34 i n ] - q u a r t . , . . . . . . . . . . . . . . . . . 4.9 mm. Cracked a t 26 in. I-quart. . . . . . . 4 . 3 mm. Cracked a t 22 in.
EA-AMELED EART
German, 2 - q u a r t . . . . . . . . . . . 5 . 4 mm. Domestic, 2-quart . . . . . . . . . . 8 . 1 mm. German. I-quart. 5 . 6 mm. CROCKERY: h-ew J e r s - g . , . . . . . . . . . . . . . . 6 . 2 0 mm. Doniehtic . . . . . . . . . . . . . . . 4.27 mm. Ziipiish. . . . . . . . . . . . . . . . . . 5 .fi2 mm. Gcrinan . . . . . . . . . . . . . . . . . . 2 . / 6 mm.
cracked
a t 6 in. Crazed a t 8 in. Shattered a t 18 in. Cracked a t 14 in, Broke Broke Broke Broke
at at at at
16 in. 10 in. 12 in. 4 in.
b y 2-in. intervals until t h e dishes failed. T h e results a p p e a r i n Table I1 a n d show t h a t t h e glassware withstancis a blow very m u c h better t h a n t h e other ware. C O R N I N GGLASS~ V O R U S .CORNING, AT. Y.
THE REVERSED HEAT ENGINE A S A DRYING MACHINE BY P ~ U J.L Fox’ Received June 21, 1915
As far back as 18j2, Lord Kelvin2 called a t t e n t i o n t o t h e inefficiency of heating (e. g., houses) b y t h e direct combustion of coal. I t would appear t o common sense t h a t , if all t h e heat escaping in a n y m a n n e r whatever f r o m t h e combustion of t h e fuel were used for heating, perfect economy of t h e fuel would result. “ I t is not my present object to determine how exactly in . practice this degree of economy may be approximated to; but t o point out how the limit which has hitherto appeared absolute may be surpassed, and a current of warm air at such a temperature as is convenient for heating and ventilating a building may be obtained mechanically, either by water power without any consumption of coals, or, by means of a steam engine, driven by a fire burning actually less coals than are capable of generating by their combustion the required heat;” . . . . . (even admitting that) “ a very good steam engine converts about one-tenth of the heat generated in its furnace into mechanical effect.”3
’ Scientist in Soil Investigations,
Bureau of Soils. Sir William Thomson, “Mathematical and Physical (1852), 5 1 5 . Lord Kelvin, LOC.c d . , pp. 516-517.
Papers,”
1
1065
T h e suggestion seems never t o h a v e been adopted a n d t h a t for reasons obvious enough. Anybody can b u r n coal in a stove or furnace. It requires neither expensive installation nor skilled a t t e n d a n c e , a n d there are thermodynamic complications t o be mentioned later. Besides, in t h e d a y s when t h e extract given above was written, people did not go in for efficiency a n d conservation. It seems t o t h e a u t h o r , however, t h a t t h e ideas of Kelvin are well worth reviving a t t h e present time, especially in view of conditions likely soon t o come a b o u t i n t h e electrochemical industry. Kelvin’s proposal seems t o h a v e been confined t o t h e application t o house warming. This application might be valuable i n t h e case of large buildings such as hotels, a p a r t m e n t s , houses, a n d factories, where more or less elaborate installations of machinery a r e t h e rule. T o t h e writer, however, a n industrial use seems more interesting; t h a t is, t h e use of large volumes of warm air for drying a n d evaporating operations on a n industrial scale in cases where electrical or water power is cheap a n d coal expensive. I n this way i t would, in some instances, s u p p l a n t heat for drying developed b y electrical resistance methods, a n d in some instances render drying operations possible where resistance methods mould be too expensive; a n d in some cases it might even be advantageous t o generate t h e heat b y this method, with t h e use of a s t e a m engine. This is especially t h e case, since t h e drying engine includes a pump. T h e writer will s t a t e t h e t h e r m o d y n a m i c principles of t h e engine proposed, making use for t h i s purpose, of various papers b y Lord Kelvin,’ b u t with presentd a y notation, a n d s u b m i t some observations a n d calculations a s t o how t h e machine would have t o be modified t o serve for drying. As is well known, a heat engine, i. e . , a n engine for transforming heat into mechanical work, absorbs heat from a reservoir a t a high t e m p e r a t u r e , transforms a p a r t (relatively small) of i t i n t o mechanical work, a n d rejects a p a r t (relatively large) into a heat reservoir (often t h e atmosphere) a t a lower temperature. A large p a r t of t h e heat is t h u s necessarily lost, even though t h e heat engine were a perfect transformer. T h e essential thing f o r operation is t w o heat reservoirs a t two different temperatures. Conversely, when a heat engine is reversed, i t can absorb a large a m o u n t of heat f r o m t h e lower reservoir, have a relatively small a m o u n t of mechanical work performed on i t , a n d deliver a large a m o u n t of heat a t t h e higher temperature. These relations are most simply exhibited in a q u a n t i t a t i v e manner in t h e well-known C a r n o t cycle. This consists of two adiabatic a n d two isothermal operations. T h e working substance, e. g., air, expands a t a high t e m p e r a t u r e , tl, doing external work, a n d absorbing q u a n t i t y of h e a t , Q1. I t t h e n exp a n d s adiabatically, cooling itself t o t z a n d doing external work. I t is now compressed isothermally at t ~ b, u t t h e compression results in t h e production of heat i n t h e working substance, which must be a b See especially, besides the paper cited, Sir William Thomson, “Mathematical and Physical Papers,” 1 (1853). 326.
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
1066
sorbed b y t h e refrigerator (or heat reservoir at t h e lower temperature). L e t QZ equal t h e a m o u n t absorbed. Finally, t h e working substance is compressed adiabatically t o t h e s a m e condition a s i t was i n t h e beginning of t h e cycle. T h e work yielded b y t h e first adiabatic operation is j u s t sufficient t o cancel t h e work necessary t o apply t o effect t h e second adiabatic operation. T h e work actually t u r n e d o u t b y t h e machine is t h e difference between t h e work yielded b y t h e isothermal expansion a t t h e higher t e m p e r a t u r e , a n d t h e work required t o effect t h e isot h e r m a l compression at t h e lower t e m p e r a t u r e . T h e working substance or gas i n t h e ,machine remains i n t h e machine a n d is considered p a r t of i t . If Q1 is t h e h e a t given t o t h e machine b y t h e upper heat reservoir at t l a n d Qa is t h e h e a t absorbed f r o m t h e machine b y t h e refrigerator or lower h e a t reservoir a t t z , t h e n 2
tz -Q1. -tl
-
T h e heat disappearing, Q1 - Qz, is t h e
mechanical work done. I t is unnecessary t o develop these expressions here a s t h e y m a y be f o u n d i n a n y text-book of engineering thermodynamics. T h e t e m peratures tl a n d t 2 a r e expressed on t h e absolute scale. If we apply .this t o a n engine working between s a y 338" F. (100lbs. gauge pressure) a n d So", we see t h a t 6 7 . 7 per cent of t h e heat supplied is absorbed b y t h e refrigerator. It is scarcely necessary t o r e m a r k t h a t no a c t u a l engine even approaches so small a loss. T h e facts mentioned above h a v e relevancy only as elucidating t h e operation of t h e proposed heat engine. T h e machine discussed above is reversible, i. e., i t m a y absorb h e a t f r o m t h e refrigerator or lower h e a t reservoir, h a v e a small a m o u n t of mechanical work done u p o n i t , a n d deliver a relatively large a m o u n t of h e a t t o t h e upper heat reservoir, which i n this case is presumed t o be t h e drying chamber. I n t h i s case, of course, precisely t h e s a m e mathematical relations hold. We would scarcely use t h e drying engine, however, between t h e temperatures 338 "--Soo, i. e . , 338" would be higher drying t e m p e r a t u r e t h a n ordinarily necessary. Consider t h e atmosphere at 60" a n d t h e drying chamber 125". I n t h i s case, Q1 or t h e a m o u n t of h e a t delivered t o t h e drying chamber amounts
to
t Q1 = QZ1
tz
= I . 125
Q2. T h e
difference
Q1 - Q2 = 0 . 1 2j Q2 is t h e mechanical work necess a r y t o b e applied. I n other words, Q1 consists of t w o p a r t s , t h e greater one derived from t h e lower h e a t reservoir, a s t h e air, a n d t h e smaller f r o m t h e mechanical work applied t o t h e machine. It is easily seen i n t h e present ca'se t h a t 8 8 . 9 per cent comes from t h e atmosphere a n d 1 1 . I per cent f r o m t h e mechanical work, i. e., Q1 = Q2 0.125 Q2. Stated i n a n o t h e r way, t h e h e a t obtained is 9 . 0 times t h e mechanical work done, or applied t o t h e machine. T h e Carnot cycle has been used a b o v e on account of i t s familiarity, a n d of t h e clearness with which i t exhibits t h e essential thermodynamic principles of t h e engine. In practice, a different procedure would be followed. T h e gas would be cooled b y expansion below t h e atmospheric temperature; allowed t o come u p t o t h e atmospheric t e m p e r a t u r e at t h e reduced
+
Vol. 7 , No.
12
pressure b y retention i n a suitable vessel, a n d finally compressed again t o atmospheric pressure, t h e compression resulting in a n increase of t e m p e r a t u r e . Accordingly, t h e engine consists of t h r e e essential p a r t s : ( I ) a large metal expansion cylinder with a piston; ( 2 ) a vessel i n which t h e air in t h e machine is allowed t o come t o atmospheric t e m p e r a t u r e , a n d which we m a y call t h e "atmospheric h e a t ; " a n d (3) a large metal compression cylinder, also with a piston. T h e t w o pistons a r e connected t o one piston rod. A single half stroke, or single complete movement of t h e piston i n one direction completes a set of operations, a n d delivers t o t h e drying chamber t h e weight of air t a k e n i n , which, i n t h e following, is assumed t o be one pound i n all cases. T h e expansion cylinder t a k e s in air at pressure p , specific volume V a n d t e m perature t on t h e left (for example) of t h e piston d u r ing a p a r t of i t s stroke, a n d t h e ports being closed, expands i t t o 01 a n d p1 a n d cools i t t o t e m p e r a t u r e tl. T h e a m o u n t of expansion will depend, of course, o n t h e a m o u n t of heat t o be a d d e d t o t h e drying air. T h e expansion is assumed t o be adiabatic. On t h e right of t h e piston, a t t h e same t i m e , t h e air which h a s been expanded in t h e previous stroke is discharged i n t o t h e atmospheric heater. T h e atmospheric heater is merely a vessel for allowing t h e cooled a n d expanded air t o come t o atmospheric t e m p e r a t u r e . It would naturally be a long t u b e or block or coil of t u b e s of t h i n metal t o conduct h e a t t o t h e enclosed air cis rapidly as possible, a n d t o avoid mixing w a r m a n d cool gas. T h e t u b e would be of larger bore where t h e t e m p e r a t u r e difference between t h e atmosphere a n d t h e atmospheric heater was greatest. T h e r e would be a progressive increase of t e m p e r a t u r e a n d specific volume a s t h e air passed t h r o u g h t h e heater, b u t only such slight pressure gradient as was necessary t o move t h e air along. Water might be used t o bring t h e t e m p e r a t u r e of t h e heater u p t o atmospheric t e m p e r a t u r e , b y circulating t h e water a s in a condenser. I n t h e house warming engine proposed b y Lord Kelvin, it is implied t h a t t h e expansion of air in the c y l i n d e r should be isothermal, which would, of course, require less power t o operate, b y t h e difference between adiabatic a n d isothermal expansion. However, considering t h e slowness with which heat is communicated t o a gas, t h e operation would be substantially adiabatic, at least if t h e engine is operated at t h e speed common now. It is assurncd adiabatic i n t h e following. T h e atmospheric heater, t h e n , is essential t o bring t h e cooled a n d expanded air u p t o t h e atmospheric t e m p e r a t u r e . As t h e reduction in pressure a m o u n t s t o only a few pounds, t h e walls of t h e heater would not h a v e t o be very heavy. I n t h e heater t h e air expands from specific volume v1 t o v 2 a n d passes f r o m t e m p e r a t u r e tl t o t . I n t h e compression cylinder, on t h e left, for example, of t h e piston, air is t a k e n from t h e atmospheric heater, throughout t h e whole stroke. On t h e right, t h e air t a k e n f r o m t h e heater during t h e previous stroke is compressed during p a r t of t h e stroke from $1 t o $ ( t h e atmospheric pressure) a n d heated b y t h e compression f r o m t t o t 3 , t h e drying t e m p e r a t u r e . T h e
T H E JOL'RNAL OF IiTTDCSTRIAL A N D ENGINEERING C H E M I S T R Y
Dec., 1915
r e t u r n half-stroke completes a n o t h e r cycle of operations a s is sufficiently obvious. I n order t o m a k e t h e t h e r m o d y n a m i c calculations for t h e drying engine, i t is necessary t o use t h e following c o n s t a n t s , where p , v, t mean t h e pressure i n lbs. per s q . i n . , specific volume i n cu. ft. per lb., a n d t t e m p e r a t u r e in degree Fahrenheit (or abs., absolute F a h r e n h e i t ) . Unit weight (one lb.) of air is considered t h r o u g h o u t . p , of outside atmosphere and of drying chamber. ZJ, t , of outside atmosphere. qt,, on entering atmospheric heater. w ~ t on , leaving atmospheric heater. p l of atmospheric heater throughout. v3f3 on leaving the compressor, and in drying chamber. y = ratio' of specific heat a t constant pressure to specific heat a t constant volume = I ,406. 1
For dry air.
Corrections would have t o be made f o r moist air.
R is t h e gas c o n s t a n t j 3 . 3 7 for t h e English-American units. Of these constants, p , v , f are fixed b y t h e outside conditions, a n d v3 can be found immediately b y t h e ordinary characteristic equation of a gas PI7 = RTiiz, when we have decided on the drying temperat u r e . T h e air is raised t o t h e desired t e m p e r a t u r e b y compression, a n d t o compress i t t o a given t e m p e r a t u r e a t a given (atmospheric) pressure, i t is necessary previously t o h a v e brought i t t o a condition of lower pressure a n d t e m p e r a t u r e . T h e lower t e m p e r a t u r e in t h i s case is fixed b y t h e a t m o s p h e r e ; i. e., t h e air before heating b y compression is brought t o atmospheric t e m p e r a t u r e in t h e atmospheric heater. Hence we must first find pl or t h e pressure in t h e atmospheric heater, b y t h e formula for t h e adiabatic expansion of a gas. P V Y = K , where K is a constant. Applying t h i s t o t h e present case, we have pl"d-f = pv3-f.
B u t v 2 , or t h e specific volume on leaving t h e atmospheric heater, b y t h e characteristic equation a m o u n t s Rt . Hence we h a v e pl(=) = p v 3 y , in which only p1 is unknown. T h e n u m b e r 1 4 4 is inserted merely because R i n t h e characteristic e q u a t i o n is t h e constant when p1 is expressed i n lbs. per sq. f t . (not sq. in.). T h e e q u a t i o n yields for pl
Of these, t h e first t w o a r e merely constants depending o n t h e u n i t s used a n d t h e atmospheric pressure. T h i s l a t t e r we t a k e a s 1 4 . 7 lbs. per sq. in., which yields 1
0.001334 for
p'-Y
0.7113
v which in t u r n gives tl b y t h e P V = R T m formula. Thus we have all t h e constants i n t h e list. T o apply t h e above t o a concrete case, t a k e t h e atmospheric t e m p e r a t u r e t = 519 absolute or 60" F. nearly, a n d suppose we desire t o d r y a t 100' F. or j j 9 " absolute, t h e n p = 14.7,'L' = 13.085, t = 3- 1 9 , 3.463
t3 = j j 9 .
(:)e (i) so =
T h e formula
that
$1
(i)
B
=
($)
gives PI =
0.7113
11.40;
and
F~ =
D
gives vl =
I j . 68.
The
characteristic equation PI' = R T m gives v 3 = 1 4 . 0 9 3 , = 1 6 . 8 7 , a n d fl = 482. 2 ' absolute. We m a y also
v2
(6)
,406
find t1 b y t h e equation t1 =
t , which gives a neat
check on t h e whole calculation. K e can now discuss q u a n t i t a t i v e l y t h e t h e r m o d y namic operation of t h e engine. T h e operations m a y be summarized as follows, each pair being correlative processes. It is convenient t o give a plus or minus sign t o each process, t h e plus sign being given when t h e process t e n d s t o t u r n t h e machine i n t h e s a m e direction as t h e power applied, a n d t h e minus sign when it opposes t h e power. I is the taking of air from the atmosphere by the expansion cylinder. I - is the delivery of air by the compression cylinder to the drying chamber. 2 is the expansion of the air in the espansion cylinder. z - is the compression of the air in the compression cylinder. 3 - is the discharge from the expansion cylinder t o the atmospher:c heater. 3 is the discharge from the atmospheric heater to the compression cylinder. T h u s there are three pairs of correlative processes, each pair containing one positive a n d one negative member. T h e handling i n pairs also greatly simplifies t h e work of computation. OPERATION I-The work a m o u n t s s i m p l y t o t h e volume times t h e pressure, t h e l a t t e r being c o n s t a n t . F o r t a k i n g i n air IVl = p t ~ , For rejecting air t o t h e drying chamber W z = p a 3 . Since t h e specific volume after heating a t c o n s t a n t pressure is larger, t h e network for operation I is negative a n d is
+ +
+
W 3
=
p
(83
- v)141.
a-The work is given b y t h e usual formula for t h e expansion or compression of a gas OPERATIOS
w
Let this constant = B. 3.463
T h e expression
T o find v1 a n d t l , however, i t is necessary t o apply t h e = K formula again, a n d p v y = plvly where p , v, a n d pl are known. This yields
pvr
a n d 0.03222 for
o . 0 0 0 0 4 2 9 8 for t h e product.
1067
=
JYdV va
which integrates i n t o t h e following 1y = R A e . if for P V , its equivalent R T is sub~
7-1'
Knowledge of PI (with t ) gives us t h e d a t a t o calculate or t h e volume on leaving t h e atmosuheric heater.
02
s t i t u t e d , a n d A0 is t h e t e m p e r a t u r e difference before a n d after expansion. I n t h e present case we h a v e R ( t - ti) a n d for t h e comfor t h e expansion Wd =
1068
T H E JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
pression W5 = R(t3 - t ) ; W 4 is 7-1
+ a n d W 5is -,
b u t WE
is always larger t h a n Wq. This l a t t e r m a y be shown b y using t h e adiabatic expansion formula tu? = constant, calling t h e new temperature t At a n d t h e new volume o Av, a n d repeating t h e process with a new At a n d Au. I t is also clear from t h e consideration t h a t t h e t e m p e r a t u r e of t h e air is warmer a t ' t h e same pressure at t h e end of t h e operation t h a n at t h e beginning.
+
+
J{T6 - J,-fT4
=
Ty6 =
R(t3
- 2t
+
tl)
--___~.__
7-1 O P E R A T I O N 3-This is t h e delivery i n t o a n d taking from t h e atmospheric heater. As t h e atmosphere heats t h e air (previously cooled b y expansion), a t constant pressure. not only is t h e temperature of t h e air raised, b u t a certain a m o u n t of work is done on i t , i. e . , its volume is increased. T ~ L I aS ,certain a m o u n t of work is yielded b y t h e heater o n account of t h e greater specific volume of t h e air o n leaving t h e atmospheric heater t h a n on entering. T h e work involved o n entering is negative a n d is W7 = plvl, a n d on leaving is positive, Wctig = p1v2: W S = p i ( P z - Vi)I44, t h e 144 being added since p , is expressed in lbs. per sq. in. a n d t h e work is desired in foot pounds. Let us now apply t h e calculations t o t h e concrete case before supposed, namely, t h a t t h e atmosphere is a t a t e m p e r a t u r e of 60' F. a n d we wish t o d r y a t 100'. T h e volumes, pressures a n d temperatures have been s t a t e d above, a n d t h e heat a n d energy relations are now given. T h e work a n d heat are given in foot pounds or in British thermal units. O P E R A T I O N I : Wa = p ( v 3 - v ) = 2116.8 X 1.008 = -2133. 7 , p being expressed in lbs. per sq. f t .
OPERATION 3 : W Q = p l ( v 2 - vl) = +1960, p1 being expressed in Ibs. per sq. f t . F r o m t h e above we see t h a t there is a net negative work of 601 foot pounds or 0 . 7 7 2 j B. t. U . T h e specific heat of air at constant pressure is 0 . 2 3 7 5 . Hence we have added 0 . 2 3 7 5 X 4 0 = 9 . 5 0 B. t. u . t o t h e one pound of air a t a n expenditure of 0 . 7 7 2 j B. t . u . in t h e form of mechanical energy, which gives a coefficient of performance of 1 2 . 3 ; i . ' e . . we obtain I 2 . 3 times a s much heat a s t h e heat equivalent of t h e mechanical energy consumed. S t a t e d in another way, 8 . I per cent of t h e heat came from t h e power, a n d 9 1 . 9 per cent from t h e atmosphere i n this case I n t h e accompanying curve, t h e temperature differences are plotted on t h e axis of abscissas, 60' F. being considered t h e lower temperature. T h e vertical axis gives t h e British thermal units for t h e "heat added" curve-which is merely t h e specific heat a t c o n s t a n t pressure multiplied b y t h e temperature difference-and also for t h e work done on t h e engine. For t h e performance ratio curve, t h e units on t h e ordinate axis are mere numbers equal t o t h e ratio of heat added t o work done. One pound of gas in each case is considered the unit. I t is more economical, generally speaking, t o heat the air through as few degrees as possible; t h i s is in accordance with t h e thermodynamic
Vol. 7 , No.
12
economy in avoiding as f a r as possible a n irreversible drop of temperature. I t will be noted t h a t t h e process is not cyclic, hence t h e formula
-t3
t3 -~cannot
-t
be used t o find t h e performance.
I n f a c t , i t m a y be worth while, incidentally, t o mention a n a p p a r e n t violation of t h e second law of thermodynamics in this connection. It serves t o emphasize t h e necessity of being sure a process is cyclic before applying t h e maximum efficiency law. T a k e again our s t a n d a r d temperatures of 60' a n d 100'. Imagine air a t atmospheric pressure a n d 60' F. i n a cylinder, t h e whole enclosed in a vacuum. Let t h e air expand isothermally, absorbing heat from t h e atmosphere. until i t s volume is doubled. Since i n isothermal expansion 6t- = Rt log 1, where r is t h e ratio of expansion, t h e work yielded in such isothermal expansion a m o u n t s t o 1 9 , 2 2 0 foot pounds, for unit mass of gas. B u t 1 j , 0 1 2 foot pounds of work is sufficient t o compress t h e air adiabatically t o t h e 40
30
20
IO
TEMPERATURE DIFFERENCEJLOWER TEMPERATURE I S 60 DEG.FAHR.)
original atmospheric pressure of 1 4 . 7 pounds, at t h e same time heating it t o 1 7 4 ' F., t h u s leaving k208 foot pounds in excess. I n other words, t h e gas h a s heated itself t o t h e t e m p e r a t u r e of 1 7 4 ' ~ a t t h e same pressure, with t h e heat of t h e atmosphere only. T h e second law requires, however, t h a t t h e process be cyclic, a n d it is not in this case. A matter of t h e first importance i n connection with t h e drying engine, is t h a t , when t h e air is cooled in t h e operation 2 + , t h e temperature, in m a n y cases, will go below t h e saturation temperature of aqueous vapor, t h a t is, t h e dew point, a n d water will condense out. I n adiabatic expansion of air s a t u r a t e d with aqueous vapor, while t h e volume of course is increased, a n d hence there is more space for aqueous vapor, t h e temperature reduction much more t h a n compensates t h e increase in volume. For example, in cooling air by adiabatic expansion from 60' t o 23', t h e ratio of expansion, i. e., t h e ratio of t h e new volume t o t h e old is 1 . 2 0 , while t h e vapor pressure drops from 13. 2 t o 3 millimeters of mercury. Hence i n t h i s there would doubtless be much condensation of water, even if t h e air was far from s a t u r a t e d with aqueous vapor
Dec., 1 9 1 j
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY
initially. All this means t h a t t h e air is dried i n passing through t h e machine. T h e great importance of t h i s from t h e drying point of view is obvious. T h e fact is t h a t in a n y case, d r y air or relatively d r y air is t h e real drying agent. This production of d r y air must be regarded as one of t h e most substantial a d v a n tages of t h e machine a s a drying agent, a n d would b e a n argument for running t h e machine relatively f a s t , in order t o approximate adiabatic conditions. On t h e other h a n d , t h e production of ice, b y t h e freezing of t h e condensed water, might hinder t h e operation of t h e engine or make i t necessary t o accomplish t h e cooling in steps. Finally, a s a house heating engine, while t h e large volumes of w a r m a n d especially moving air would facilitate ventilation, if t h e expansion was adiabatic, t h e resulting air might be drier t h a n considered advisable in houses, t h o u g h again, it m i g h t be remoistened b y appropriate arrangements. I t is hardly necessary t o s t a t e t h a t t h e calculations are of a strictly t h e r m o d y n a m i c character, a n d d o n o t consider losses b y friction. b o t h mechanical a n d gaseous, nor t h e other losses-avoidable a n d unavoidable-in actual engines. These losses v a r y so greatly according t o t h e size a n d construction of t h e installation, t h a t a general discussion is not very profitable. We must remember, however. t h a t t h e other methods of drying fall far short of complete efficiency, a n d few processes in practice even approxim a t e t h e calculated possibility. I n particular, t h e production of heat b y t h e transformation of t h e electric current is far from a n efficient process, if we a r e t o judge b y t h e d a t a furnished i n t h e papers on electric furnaces a t t h e September, 1914,meeting of t h e American I n s t i t u t e of Metals (especially papers b y G. H. Clamer a n d Carl Hering, H. G. Dorsey, a n d H . W. Gillett a n d J . 14.L o h r ) . On t h e practical side, however, it m a y be said t h a t t h e common objections t o air as a working substance do not apply. Air has a low specific h e a t , which causes t h e cylinders t o be bulky. B u t if drying is t o be effected, t h e d r y air must be p u m p e d or introduced in one way or a n o t h e r into t h e drying chamber even if only b y inefficient diffusion, a n d t h e machine suggested acts also a s a p u m p . T h e s a m e consideration applies also t o friction, whereas i n a s t e a m engine expansion can be stopped when counterbalanced b y friction. T h e difficulty of communicating heat t o air, unless the direct combustion products a r e brought into contact with t h e substance t o be dried, is a strong a r g u m e n t i n favor of t h e engine, for in it t h e air is
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heated evenly a n d t o t h e temperature desired. Again, heat loss can be minimized. I n a direct acting heat engine, as a s t e a m engine, t h e h e a t proceeds from i n outwards, a n d all loss of h e a t b y radiation, etc., is a loss of efficiency. I n t h e drying engine, however, t h e heat passes f r o m o u t inwards, a n d all gain of heat is gain in efficiency. This applies t o t h e expansion cylinder a n d atmospheric heater. So far as t h e compression cylinder is concerned, there is no reason why it should not be placed inside the d r y i n g chamber, a n d t h u s utilize for drying t h e various heat losses b y friction, etc., occasioned b y heating t h e air. For drying chemical products, t h e avoidance of local excessive heating, aside from i t s wastefulness, is of t h e first importance for t h e production of a uniform product. N o t only is t h e t e m p e r a t u r e uniform, b u t t h e circulation of t h e air, i. e . , t h e drying medium: does n o t depend upon accidental t e m p e r a t u r e differences, b u t can be thoroughly controlled. Besides, t h e uneconomical irreversible drop of t e m p e r a t u r e is avoided. A c h a m ber of particular construction, nor one fitted with electric resistance arrangements, nor with closed pipes, is necessary. Again, b y drying a t t h e lowest temperat u r e possible, t h e loss of heat b y r-adiation from t h e drying chamber is minimized. T h e low t e m p e r a t u r e drying is especially valuable in t h e case of organic materials as fertilizers, where it is i m p o r t a n t t h a t chemical changes should not t a k e place. T h e most serious object.ion t o t h e plan is doubtless found in t h e variation of t e m p e r a t u r e in t h e external atmosphere, since t h e machine heats t h e air through a fixed t e m p e r a t u r e difference for a given valve setting. This condition might prevent its use in certain places of extreme t e m p e r a t u r e variation. It m a y be said, however, t h a t b y adjusting t h e valves a n d varying t h e speed of operation t h e engine could be a d a p t e d for different temperature ranges; or a n u m b e r of machines could be set in series-each machine lifting t h e air through a certain t e m p e r a t u r e difference-and a varying number of t h e m used a s t h e atmospheric Or again, t h e machine t e m p e r a t u r e changed. could be set in a closed, heated room a n d t h e t e m p e r a t u r e range of t h e outside air compensated b y other mcans, leaving t h e final t e m p e r a t u r e lift a n d t h e d r y ing a n d circulation of t h e air t o t h e reversed h e a t engine. used as a drying machine. Or, finally, t h e drying air could be heated higher t h a n necessary, a n d a d j u s t ment made from time t o t i m e b y adding varying q u a n tities of cool atmospheric air. BUREAU OF SOILS, T A R I I T N G T O N D. , C.
ADDRESSES THE UNIVERSITY AND INDUSTRY' BY NICHOLAS MURRAY BUTLER
At the outset I wish to disabuse your minds of any notion that you may have that I have been invited to speak a t this conference because I am a chemist. 1111 the chemistry I know is such recollection as remains to me, after 35 years, of Dr. Chandler's most admirable and stimulating lectures. But I have an 1 Address before the New York Section of t h e American Chemical Society, Chemists' Club, 1-ovember 12, 1915.
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impression, somewhat strengthened by the number of journals and periodicals which come to my table day by day, that there have been some changes in chemical knowledge during that 35 years, and I am afraid that if I were to attempt to reproduce any of his excellent instruction you might think me a student not of chemistry but of archeology. The reason why I have been requested to speak is, I think, because in our University we have some very definite and longconsidered ideas as to the general problem of which this question
