Induatrial UIartes Heat used in drying paper is an example of waste often concealed in accounting, but it can be evaluated and often recovered
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accountant in B certain prosperous manufacturing plant is required to keep a factual record of those intangible items of expense which do not appear specifically in his cost statements. The president of this company believes that his accounting department should be aware of such hidden expenses and constantly bring emphasis within the organization on the benefits that could be realized from improving or eliminating these hazy expenses. Every 3 months the president sits down with his accountant, engineer, and chemist to review these 'intangible expenses. Efforts are niade to determine how these items can he evaluated tangibly and how to prevent their existence or to utilize the waste products in the manufacturing process or sales. When B possible means is conceived, tlie cost accountant, with the assistance of the engineering and chemical staff, assumes the responsibility in preparing an econoniic engineering analysis of the scheme to determine the practical potentialities of the idea before any money is spent on laboratory or plant studies. Only when the group approves the analysis are funds appropriated to develop the reality of the plan. l'his approach has been outstandingly successful in several chemical process industries. What are these intangible itenis? The yield of salable product from the raw material used is an example. Improvement of yield is an outstanding means of iniproving cost of manufacture. Process residuals, wliicti do not compose the finished product, rarely reach tlie cost statement as such. These are hidden expense items; wastes which have to be burned or purged into the air or streanis are certainly in this same category. Management is too willing to accept such wastes as inevitable because they have always existed. Improved technology is occurring almost daily and justifies alertness by any chemical process plant in studying new processes and equipments. Possibly the most elusive and intangible of these hidden expenses is found in the use of air and water in procewes. Because these two basic raw materials are cheap and abundant, we are often careless in their use. I n this discussion we do not wish to emphasize the use of excessive amounts of power in transporting materials through the process. Our concern is the loss of heat in the process where air and water are used as heat carriers. ,To illustrate this point we will consider the use of air in drying paper on the paper machine. The basic idea is applicable to many other processes. Paper is formed as a wet continuous web by depositing a water suspension of prepared wood pulp on a horizontally moving endless woyen wire belt revolving a t a rapid rate. The water drains through the belt and leaves a uhiform wet paper web on the wire. At the point where the endless wire belt wraps about a suction roll and returns to the initial forniing location, the wet web is transferred mechanically to a traveling woven felt carrier which then conveys the web through a series of heavy rubber- and stone-surfaced rolls to press out the entrained water so that the paper web enters a series of steam heated rolls carrying 2 pounds of water for every pound of dry paper weight. COST
A paper machine (producing 100 tons of paper per day), which is equipped with a hood over the dryer section, will require 112,000 cubic feet per minute of make-up air (basis 70" F.) or 6000 tons of air' per day. I n other words, 60 tons of air are required forkvery ton of paper produced. Assuming that this air is heated from 32" F. to an exhaust temperatuer of 112" F., 240,000, pounds of steam will be required to produce 100 tons of paper or 1.2 pounds of steam per pound of paper to heat the air to carry off the water vapor from drying the paper. This is about one third of all steam required in the paper machine room operation. If the steam cost is 75 cents per 1000 pounds then the air for this operation costs $180 per day. If the outside air is at -20" instead of 32" F., as assumed above, then the steam cost inmeases to $297. On the other hand if the inlet air is 70" F. the $180 per day value will be reduced to $100. This is only the cost of heating the air to carry away the water vapors from the paper drying in contact with the dryer drums. The heat input for these two steps passes out with the exhaust vapors leaving the paper machine dryers. This heat is as good as coal, if recovered, and exhaustive efforts are being taken to realize fully on this loss. I n order to conserve this heat, the first and most obvious thing to do is decrease the amount of air used. I n other words, concentrate the water vapor a t its source by using a hood over the dryers with a controlled exhaust exit. This step alone will reduce the air needed by 20 to 25%, and will give better working conditions in the room. It is natural to think that with a given hood and given amount of vapor to remove, reducing the exhaust volume would cause the relative humidity of the exhaust to rise proportionately. Such is not the case. Actually, as the exhaust volume is cut down the relative humidity or percentage saturation may either rise or fall, or perhaps first do one and then the other. Invariably, however, a 'decrease in exhaust volume means an increase in temperature and, of course, an increase in absolute humidity. It is therefore a mistake, with any given hood condition, to assume that the relative humidity can be used alone as a criterion of the hood performance or that by varying the exhaust volume the percentage humidity can be varied to some presumably correct amount-for example, 80%. Let's see why this is true. Suppose we mix equal volumes of air 50% saturated a t 75" F. and 50y0 saturated a t 125" F. Will the resultant mixture be 50% saturated a t 100" F.? No, it will actually be 67% saturated at 98" F. Or, suppose that air 50% saturated a t 70" F. is mixed with an equal volume of air 50% saturated a t 150" F. Will it be 50% saturated at 110" F.? No, this mixture will be 96% saturated at 106" F. Incidentally, fog (which is nothing but an air-water vapor mixture over 100% saturated), if formed in a machine room or between the dryers, is (Continued on page 89 A ) commonly due to a well saturated 81 A
mixture a t high temperature being mixed with another airwater vapor mixture a t lower temperature. But to return to the hood, reducing the exhaust volume is tantamount to reducing the amount of the cooler room air to be mixed with the high temperature and high humidity air from the machine dryers, with the consequence that the temperature of the mixture is higher, and the percentage saturation may possibly even be lower. This points to the importance, in hood design for maximum economy, of having ample gathering space under the hood into which the very hot, humid air may rise and stratify before removal by fans or exhaust stacks. Perhaps the worst offender against economy is the natural draft stack or monitor. In winter, paper machines require the least exhaust, and it is then that the heating of excess air from the outdoor temperature to the exhaust temperature is the most expensive. But it is precisely then that the natural draft stack pulls the hardest. I n the summer when the largest volume is required and an excess for comfort is desirable, the natural draft stack lays down. Fans afford a constant and controllable exhaust volume. Fortunately the first cost of fan exhaust is not more than the cost of natural draft stacks, because the stack area may be so much smaller and still handle the same volume. Methods have been suggested and used to some extent, though not so commonly as it seems they should be, for controlling the exhaust and supply volumes by the temperature and humidity condition of the machine exhaust. Thus, whenever the paper goes off the machine the volume of air is drastically reduced automatically. Also it is adjusted automatically as necessary to changes in production rate and climatic conditions. Certainly the cost of such automatic control, if successfully applied, can be returned many times over in the course of 1 year. Having controlled the amount of air used in the drying operation, further reduction in the over-all plant economy is possible by recovering the heat in the exhaust air. A. E. Montgomery of J. 0. Ross Engineering Corporation, an expert in process air engineering, has contributed greatly to this problem. We have used some of his data in the above discussion. I n paper mills, particularly in northern climates, Montgomery advises, that the Briner economizer has “become almost standard equipment.” This equipment is a simple, efficient heat exchanger which heats incoming cold air with the hot exhaust air leaving the hood over the paper machine, Montgomery reports the following savings under different outdoor air temperatures : Temu. after Briner
Outdoor Temp.,
Economizer,.
Rise,
O F .
O F .
O F .
72 78 84 90
92 78 64 50 36
- 20
0 20 40 60
96
Equivalent Steam ___ Lb./hr.
Boiler hp:
11,700 9,900 8,100 6,350
338 287 235 184 132
4,100
Savings/ M o n t h of
25 D~~~ (754 Steam) $5260 4450 3640 2860 1840
These savings are real and tangible. There are many applications in the chemical process industries for such methods in controlling the cost of drying and evaporation. Textile and paper production are not the only industries that should study hidden expense items. a2 A
