第 69 节
作者:
朝令夕改 更新:2021-02-21 11:24 字数:9322
it is evident that 1111 loaves; instead of 1102 loaves; ought to have been produced; for 100 lbs。 of meal are to 64 loaves as 1736 lbs; to 1111 loaves。 Hence it appears that 9 loaves less were produced in these experiments than ought to have been produced。
There were reasons to suspect that this was so contrived by the baker; with a design to get the number of loaves he was obliged to deliver for each 100 lbs。 of meal lessened;but in this attempt he did not succeed。
Quantity of Fuel consumed in these Experiments。
Dry pine…wood。 lbs。 loths。 In heating the oven first time; 366 16 second time; 134 16 third time; 100 0 fourth time; 90 0 fifth time; 80 0 sixth time; 74 16 Total; 845 16 Employed in keeping up a small fire near the mouth of the oven while the bread was putting into it; 34 16
Total consumption of wood in the six experiments; 880 lbs。
The results of these experiments show; in a striking manner; how important it is to the saving of fuel in baking bread; to keep the oven continually going; without ever letting it cool: for in the first experiment when the oven was cold; when it was begun to be heated; the quantity of wood required to heat it was 366 1/2 lbs。; but in the sixth experiment; after the oven had been well warmed in the preceding experiments; the quantity of fuel required was only 74 1/2 lbs。
As in these experiments 2393 lbs。 13 loths of bread were baked with the heat generated in the combustion of 880 lbs。 of wood; this gives to each pound of bread 11 1/3 loths; or 34/96 of a pound; of wood。
In the fifth experiment; or batch; 186 loaves weighing (at 2 lbs。 5 1/2 loths each) 304 lbs。 were baked; and only 80 lbs。 of wood consumed; which gives but a trifle more than 1/4 of a pound of wood to each pound of bread; or 1 pound of wood to 4 pounds of bread。
As each loaf weighed 2 lbs。 16 loths when it was put into the oven; and only 2 lbs。 5 1/2 loths when it came out of it; the loss of weight each loaf sustained in being baked was 10 1/2 loths; as has already been observed。 Now this loss of weight could only arise from the evaporation of the superabundant water existing in the dough; and as it is known how much heat; and consequently HOW MUCH FUEL is required to reduce any given quantity of water; at any given temperature; to steam; it is possible; from these data; to determine how much fuel would be required to bake any given quantity of bread; upon the supposition that NO PART OF THE HEAT GENERATED IN THE COMBUSTION OF THE FUEL WAS LOST; either in heating the apparatus; or in any other way; but that the whole of it was employed in baking the bread; and in that process alone。 And though these computations will not show how the heat which is lost might be saved; yet; as they ascertain what the amount of this loss really is in any given case; they enable us to determine; with a considerable degree of precision; not only the relative merit of different arrangements for economizing fuel in the process of baking; but they show also; at the sane time; the precise distance of each from that point of perfection; where any farther improvements would be impossible: And on that account; these computations are certainly interesting。
In computing how much heat is NECESSARY to bake any given quantity of bread; it will tend much to simplify the investigation; if we consider the loaf as being first heated to the temperature of boiling water; and then baked in consequence of its redundant water being sent off from it in steam。
But as the dough is composed of two different substances; viz。 rye meal and water; and as these substances have been found by experiment to contain different quantities of absolute heat; or; in other words; to require different quantities of heat; to heat equal quantities or weights of them to any given temperature; or any given number of degrees; it will be necessary to determine how much of each of the ingredients is employed in forming any given quantity of dough。
Now; in the foregoing experiments; as 1102 loaves of bread were formed of 1736 lbs。 of rye meal; it appears; that there must have been 1。47 lb。 of the meal in each loaf; and as these loaves weighed 2 1/2 lbs。 each when they were put into the oven; each of them must; in a state of dough; have been composed of 1。47 lb。 of rye meal; and 1。03 lb。 of water。
Supposing these loaves to have been at the temperature of 55 degrees of Fahrenheit's Thermometer when they were put into the oven; the heat necessary to heat one of them to the temperature of 212 degrees; or the point of boiling water; may be thus computed。
By an experiment; of which I intend hereafter to give an account to the Public; I found; that 20 lbs。 of ice…cold water might be made to boil; with the heat generated in the combustion of 1 lb。 of dry pine…wood; such as was used in baking the bread in the six experiments before mentioned。 Now; if 20 lbs。 of water may be heated 180 degrees; (namely from 32 to 212 degrees;) by the heat generated in the combustion of 1 lb。 of wood; 1。03 lb。 of water may be heated 157 degrees; (from 55 degrees; or temperate; to 212 degrees;) with 0。4436 of a pound of the wood。
Suppose now that rye meal contained the same quantity of absolute heat as water;as the quantity of meal in each loaf; was 1。47 lb。; it appears; that this quantity would have required; (upon the above supposition;) to heat it from the temperature of 55 degrees; to that of 212 degrees; a quantity of heat equal to that which would be generated in the combustion of 0。06405 of a pound of the wood in question。
But it appears; by the result of experiments published by Dr。 Crawford; that the quantities of heat required to heat any number of degrees; the same given quantity (in weight) of water and of wheat; (and it is presumed; that the specific or absolute heat of rye cannot be very different from that of wheat;) are to each other; as 2。9 to 1;water requiring more heat to it; than the grain in that proportion: Consequently; the quantity of wood required to heat from 55 to 212 degrees; the 1。47 lb。 of rye meal which entered into the composition of each loaf; instead of being 。06405 of a pound; as above determined; upon the false supposition that the specific heat of water and that of rye were the same; would; in fact; amount to no more than 0。02899; for 2。9 (the specific heat of water) is to 1 (the specific heat of rye); as 0。06405 is to 0。02899。
Hence it appears; that the wood required as fuel to heat (from the temperature of 55 degrees to that of 212 degrees) a loaf of rye bread (in the state of dough); weighing 2 1/2 lbs。; would be as follows; namely:
Of pine…wood; To heat 1。03 lb。 of water; which enters into the composition of the dough; 。。 0。04436
To heat the rye meal; 1。47 lb in weight; 。。 0。02899 Total; 0。07335 lb。
To complete the computation of the quantity of fuel necessary in the process of baking bread; it remains to determine; how much heat is required; to send off in steam; from one of the loaves in question (after it has been heated to the temperature of 212 degrees); the 10 1/2 loths; equal to 21/64 of a pound of water; which each loaf is known to lose in being baked。
Now it appears; from the result of Mr。 Watt's ingenious experiments on the quantity of latent heat in steam; that the quantity of heat necessary to change any given quantity of water ALREADY BOILING HOT to steam; is about five times and a half greater than would be sufficient to heat the same quantity of water; from the temperature of freezing; to that of boiling water。
But we have just observed; that 20 lbs。 of ice…cold water may be heated to the boiling point; with the heat generated in the combustion of 1 lb。 of pine…wood; it appears therefore that 20 lbs。 of boiling water would require 5 1/2 times as much; or 5 1/2 lbs。 of wood to reduce it to steam。
And if 20 lbs。 of boiling water require 5 1/2 lbs。 of wood; 21/64 of a pound of water boiling hot will require 0。09023 of a pound of wood to reduce it to steam。
If now; to this quantity of fuel; 0。09023 lb。 we add that necessary for heating the loaf to the temperature of boiling water; as above determined; 0。07335 lb。 this gives the total quantity of fuel necessary for baking one of these loaves of bread; 0。16353 lb。
Now as these loaves; when baked into bread; weighed 2 lbs。 5 1/2 loths = 2 11/64 lbs。 each and required; in being baked; the consumption of 0。16353 of a pound of wood; this gives for the expence of fuel in baking bread 0。07532 of a pound of pine…wood to each pound of rye bread; which is about 13 1/4 lbs。 of bread to each pound of wood。
But we have seen; from the results of the before…mentioned experiments; that when the bread was baked under circumstances the most favourable to the econ