US2350401A - Draft producing apparatus - Google Patents

Description

s. KNEASS, JR 2,350,401 DRAFT Pnonucme APPARATUS June 6, 1944.-

Filed Sept. 25, 1941 lo I 5 JNVENTOR v I SJRICKLAND KNEASS,JR. BY Qaw ATTOR NEY Patented June 6, 1944 DRAFT PRODUCING APPARATUS Strickland Kneass, Jr., Boylston, Mass, assignor to Morgan Construction Company, Worcester, Mass, a corporation of Massachusetts Application September 25, 1941, Serial No. 412,330

7 Claims.

This invention relates to draft producing apparatus, and more particularly to the construction of an improved apparatus in the form of an ejector stack arranged to utilize air under pressure for the entrainment and discharge of hot gases, such as the waste gaseous products of combustion from a furnace or the like.

The air is ordinarily supplied to such stacks by means of a power driven fan, and the power required by the fan is the principal item of expense in the operation of the apparatus.

It is accordingly one object of the invention to provide a draft producing apparatus particularly adapted for handling hot gases and of such a nature as to require comparatively little power.

It is a further object of the invention to provide an ejector stack arranged to be supplied with ejector air from a fan and so constructed that the power required to drive the fan will be greatly reduced as compared with prior apparatus.

With these and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claims appended hereto.

Referring to the drawing illustrating one embodiment of the invention and in which like reference numerals indicate like parts,

Fig. 1 is a vertical section through an ejector stack, the section being taken on the line l--l of Fig. 2;

Fig. 2 is a section on the line 2-2 of Fig. 1;

Fig. 3 is a section on the line 33 of Fig. 1;

Fig. 4 is a fragmentary view similar to Fig. 1 but showing a modification; and

Fig. 5 is a section on the line 55 of Fig. 4.

The embodiment illustrated comprises a vertical passage l0 for the upward flow of the hot gaseous products of combustion from a furnace (not shown). In the type of installation for which the invention is particularly adapted, these gases will ordinarily be at a temperature of over 1000 degrees Fahrenheit, and in some cases they may be as high as 2000 degrees. Above the passage I0 and connected thereto is an upright metal stack II with its upper end open to the atmosphere. This stack is constructed in the form of a Venturi tube, with a throat l2 spaced from the lower end of the stack a distance somewhat less than half the height thereof. Directly below-the throat there is an upwardly converging portion I4, whereas the portion above the throat diverges gradually in the upward direction to form a so-called diffuser. An upwardly directed jet of air is discharged into the stack adjacent the throat l2, and for this purpose a nozzle I6 is provided. Air under pressure is supplied to this nozzle by means of a rotary fan or blower l8.

It will now be understood that with the construction as so far described the air discharged by the nozzle IE will entrain the hot gases in the stack and eject them upwardly to the atmosphere, producing a desired draft or suction in the passage 10. However, a considerable quantity of air will be required at an appreciable pressure, so that the power necessary to drive the fan will be a substantial item of expense. I have discovered that this power can be very materially reduced by transferring heat from the hotgases to the air at a sufiicient rate to raise the temperature of the air leaving the nozzle to a comparatively high value. This discovery is substantiated by the following theoretical discussion relating to the behaviour of a jet of air discharged through a nozzle under a pressure head.

Let

P=power available from the jet in foot-pounds per second.

11) =weight of air in pounds per second.

H=head of the air in inches of water.

h =head of the air in feet of air.

T=absolute temperature for standard conditions,

in degrees Fahrenheit (460+).

t =absolute temperature of air entering nozzle, in degrees Fahrenheit.

y =pounds of air per cubic foot at standard conditions (30" Hg and 60 degrees F).

g =acceleration due to gravity in feet per second v =velocity of air leaving nozzle in feet per second.

a =area of nozzle in square feet.

Then We have as basic formulae From the known weight of water, and the effect of temperature on the density of gases, We have From 1) and (4),

From (2) and (5), and the effect of temperature on the density of gases,

From (3), (4) and (6),

Assuming that the head or pressure of the air is constant, there are no variables in Equation 8 except it. Hence 9) I Pam Y where K is aconstant.

Thus the power available from the jet is proportional to the square root of the absolute temperature of the air entering the nozzle. Hence if the air is heated to 600 F. after compression by the fan, we would have V (10) P= K 600+460=K /1060=32.6K I

If the air is not heated and enters the nozzle at 60 F wewould have From the above it will be seen that the air when heatedto 600 F. will deliver 1.43 times as much power through the same nozzle area as the cold air.

Furthermore, theweight of the air discharged will be appreciably less when the air i heated, as will now be shown. In Equation 6 the factors a, g, H, y and T are all constants. Therefore we canwrite AL: l M /c0+460 /'2'0*2Ts From (13) and (14) we have the ratio 22.8 7 70 Thus by heating the air to 600 after it has been compressed by the fan there is obtained 1.43 times the amount of power from the same nozzle, and the weight of air handled by the fan is only .7 of that when discharging at 60. Accordingly from a given weight of air at 600 I will obtain the hot gases which are being ejected. For this purpose the stack H is enclosed by an external casing or jacket to provide an annular passage 2| through which the air may flow downwardly in counterflow heat-transfer relationship to the hot gases. This casing preferably extends for a substantial distance both above and below the throat I2, to provide a long path of travel for the air.

Furthermore, the width of the passage 2| (measured radially) is comparatively small so that the air velocity will be maintained high enough to ensure a rapid transfer of heat through the metal times as much energy as would be obtained from a the same weight of air at 60.

wall of the stack ll. At the upper end of the casing 20 the air passage is enlarged to provide an annular inlet chamber 23 which is connected to the fan l8 by a duct 24. Similarly, the air passage is enlarged at the lower end of the casing to provide an annular outlet chamber 26 which is connected to the nozzle l6 by a duct 21. In order to accommodate differential expansion between the stack l and the casing 20, an annular water seal 28 is provided at the top of the casing. In-

the event the stack is to be employedin connection with a reversible furnace as a part of the wellknown Isley system, I may mount a slide'damper 30 at the throat l2, as shown in Figs. 4 and 5. In this construction the passage 2| is divided at the throat into upper and lower portions which are connected by three short externalv ducts 3| arranged'to avoid interference with the damper 30.

It will now be apparent that in the operation of the invention the fan l8 will deliver air at atmospheric temperature through th duct 24 to the chamber 23, and this air will flow downwardly through the passage 2| to the chamber 26, and thence through the duct 21 to the nozzle IS. The nozzle will discharge the air in an upwardly directed jet, which will entrain the hot gases from the passage l0 and cause them to flow upwardly through the stack II to the atmosphere. In the course of its downward flow through the passage 2| the air will be heated to acomparatively high temperature of say 600 F. by the transfer of heat from the hot gases. Because of the increased efiiciency of the hot air jet in ejecting the gases, as explainedabove, it is possible to reduce the weight of air discharged as compared with prior ejectors utilizing cold air. Thus the area of the nozzle I6 will be less, the fan |8 will be smaller, and the motor or other device which drives the fan will be less powerful, thereby greatly decreasing the original cost as well as the expenditure for operating power. The slide damper 30 of Figs. 4 and 5 will of course be open when the hot gases are being ejected, and closed when the furnace is reversed and the air is to be used for combustion purposes.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent is: I

1. Apparatus for the drafting of hot gases comprising a conduit for the flow of the gases to the atmosphere, a nozzle arranged to discharge air into the conduit to entrain the gases and cause flow thereof in the direction desired, means providing a passage through which air may flow and be subjected to heating by the gases flowing in a region anterior to the nozzle, means to sup ply air to the passage at a pres-sure above atmosphere, and means'to conduct the heated air from the passage to the nozzle. fl

I 2. Apparatus for the drafting of hot gases comprising a conduit for the flow of the gases to the atmosphere, a nozzle arranged to disharge air into the conduit to entrain the gases and cause flow thereof in the direction desired, means providing a passage through which air may travel in counterflow heat-transfer relationship with the gases flowing in a region anterior to the nozzle, means to supply air to the passage at a pressure above atmosphere and in such a manner as to cause the air to flow therethrough in a direction generally opposite to that of the gases in the conduit, and means to conduct the heated air from the passage to the nozzle.

3. Apparatus for the drafting of hot gases comprising a conduit for the flow of the gases to the atmosphere, a nozzle arranged to discharge air into the conduit to entrain the gases and cause flow thereof in the direction desired, means providing a passage through which air may travel in counterflow heat-transfer relationship first with the gases flowing in a region posterior to the nozzle and then with the gases flowing in a region anterior to the nozzle, means to supply air to the passage at a pressure above atmospheric and in such a manner as to cause flow therethrough in the said counterflow relationship, and means to conduct the heated air from the passage to the nozzle.

4. Apparatus for the drafting of hot gases comprising an upright stack open at its upper end to the atmosphere, means to deliver hot gases to the lower end of the stack, a nozzle arranged to discharge air upwardly into the stack and thereby entrain the gases and cause upward flow thereof, a casing surrounding the stack to progases to the lower end of the stack, a nozzle mounted within the stack in a position to discharge air upwardly and thereby entrain the gases and cause upward flow thereof, a casing surrounding the stack to provide an annular passage which extends for a substantial distance below the nozzle, means to supply air at a pressure above atmospheric to the upper end of the passage for downward flow therethrough in counterflow heat-transfer relationship with the gases flowing upwardly beneath the nozzle, and

'means to conduct the heated air from the lower end of the passage to the nozzle.

6. Apparatus for the drafting of hot gases comprising an upright Venturi-shaped stack having a restricted throat and open at its upper end to the atmosphere, means to deliver hot gases to the lower end of the stack, a nozzle mounted within the stack in a position to discharge air upwardly into the throat and thereby entrain the gases and cause upward flow thereof, a casing surrounding the stack to provide an annular passage, the passage extending from a region located a substantial distance above the nozzle to a region located a substantial distance below the nozzle, means to supply air at a pressure above atmospheric to the upper end of the passage for downward flow therethrough in counterflow heat-transfer relationship with the gases flowing upwardly both above and below the nozzle, and means to conduct the heated air from the lower end of the passage to the nozzle.

7. Apparatus for the drafting of hot gases comprising an upright Venturi-shaped stack having a restricted throat and open at its upper end to the atmosphere, means to..;deliver hot gases to the lower end of the stack, a nozzle mounted within the stack in a position to discharge air upwardly into the throat and thereby entrain the gases and cause upwardflow thereof, a casing surrounding the stack to provide an annular passage havin a relatively small width radially, the passage extending from a region located a substantial distance above the nozzle to a region located a substantial distance below the nozzle, an annular inlet chamber communicating directly with the upper end of the passage and having an outside diameter appreciably exceeding that of the adjacent portion of the passage, means to supply air at a pressure above atmospheric to the inlet chamberv for downward flow therefrom through the passage in counter-flow heat-transfer relationship with the gases flowing upwardly both above and below the nozzle, an annular outlet passage communicating with the lower end of the passage and having an outside diameter appreciably exceeding that of the adjacent portion of the passage, and means to conduct the heated air from the outlet chamber to the nozzle.

STRICKLAND KNEASS, JR.

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