1. Field of the Invention
The present invention relates to vapor generators and, more particularly, to a vapor generator fluid flow control system, permitting controlled fuel, air and water flow and pressure regulation.
2. History of the Prior Art
Vapor generators of the kind in which a fuel-air mixture is combusted in the direct presence of feed water to produce a useful mixture of steam and non-condensibles are known. Such vapor generators are shown in U.S. Pat. No. 4,211,071 issued to the assignee of the present invention. In accordance with that invention, a vapor generator is provided in which several interrelated means are employed to improve the quality of combustion in the generator so that a product steam substantially free of carbon monoxide results. In effecting this result, means are provided for dividing the air feed into two parts which are both delivered to the generator for combustion. Such methods and apparatus are effective in producing a well-mixed stoichiometric mixture of fuel and air to provide completeness of combustion and reducing production of carbon monoxide to extremely low levels. Such an accomplishment is a marked advance over the prior art and illustrates the emphasis placed on controlled flow rates and mixture conditions.
The prior art is also replete with apparatus manifesting high temperature product vapor having high carbon monoxide content. Such conditions are objectionable for many applications and dangerous for some of them. High carbon monoxide production is traceable to incomplete combustion. This is, in turn, traceable in part to difficulties in maintaining a stable lean flame and, in part, to excessive quenching of the flame through direct radiation and convective contact between the flame and the feed water. Once a stoichiometric mixture is obtained, it is, therefore, necessary to maintain the combustion level without varying any of the three major constituents of fuel, air, or water. Variations would affect the combustion and the carbon monoxide production therefrom.
Other problems occur in certain ones of the prior art vapor generators when they are operated at low pressures. Conventionally, as shown in U.S. Pat. No. 4,288,978, a vapor generator may be especially adapted for low pressure operation by the provision of certain features such as a pilot burner for striking a stable flame. It may be seen that low pressure operation, likewise, requires a specific reduction in the air, fuel and water input for a low volume output from the vapor generator. Disproportionate variances in any of the three constituents will seriously affect the combustion level and the resulting heat and vapor content of the downstream product.
Few prior art systems effectively address dual output vapor generator operation. The ability to fine tune the fuel, air and water flow for a single vapor generator output is critical enough. Once this critical equilibrium is reached, it is tantamount to starting over to vary the generator operation to another flow. Varying the generator volume necessitates adjustment in the air, fuel and water flow rates which will directly affect both the heat and the water vapor level of the combustion products. When such generators are used in industrial applications necessitating precise control of heat and vapor, these variations are not tolerable. One such application is the curing of concrete where the temperature and moisture level of the vapor products have a direct effect on the curing process and resulting structural condition of the cast product.
The use of steam for concrete curing is not novel in and of itself. Steam has been produced from conventional boilers for such methods and processes, but the cost of a boiler operation is much higher than that of a vapor generator type device. Moreover, the temperature and vapor level of the steam product can be more closely controlled than with a conventional boiler. Moreover, super heated vapor can be provided for specific types of aggregate to be cured. The flexibility of the vapor generator also permits substantially instantaneous operation as compared to the threshold period for a boiler method. The obvious disadvantage is the substantially single flow rate afforded by most conventional vapor generators. Since any number of kilns may be run at one time for particular production schedules, excess vapor from a single flow rate vapor generator system would, thereby, be wasted.
Conventional techniques for varying the output of standard vapor generators includes reducing the rate of combustion and water flow while maintaining a constant air flow rate. The advantage of such a system is simplicity and cost in that reliable variations in air flow rates have been, to date, difficult to attain. The paramount disadvantage to the continuous air flow volume is the excess air ingressing into the kiln and the adverse effects to the curing process which affects the ultimate humidity level within the kiln and imparts non-uniform cooling characteristics. Variations in the air flow volume have, to date, been addressed by multiple speed blowers and throttle systems which either decrease the air intake to the blower or exhaust therefrom. Unfortunately, the relatively large motors and blower units necessary for vapor generator volumes generate large amounts of heat which the air volume dissipates. When the air volume is throttled for constant motor speed, heat dissipation is inhibited and variations in flow rate result as well as having degenerative effects on the motor and the blower unit. The obvious alternative to such flow problems is a multiple speed motor, but the cost and availability for such systems are generally disproportionate to the vapor generator construction.
Dual output vapor generator applications also require dual flow feedwater systems which may be accurately set in preselected flow configurations as well as precisely maintained. Precise fluid flow is an integral element of the aforesaid combustion efficiency particularly in stoichiometric mixtures. The primary area of fluid flow consideration in such vapor generator systems is in the water flow controlling network. Prior art flow control devices though capable of preselect flow volumes are generally not sensitive to variations in flow pressure which affects the flow volume.
It would be an advantage therefore to overcome the problems of the prior art by providing a variable output vapor generator having a constant volume fluid flow system which affords automatic operation between high and low fluid flow rates and which is sensitive to fluctuations in fluid flow pressure. One approach to dual output vapor generators and a discussion of prior art problems associated therewith is discussed in co-pending U.S. patent application Ser. No. 554,780 assigned to the assignee of the present invention. The method and apparatus of the present invention comprises an advanced system overcoming the problems set forth above by providing a combination of a pressure responsive valve and fine adjustment valve in parallel flow communication. This parallel flow system is itself aligned in parallel flow communication with a second matching system adapted for selective actuation for high and low volume operation. In this manner both the high volume and low volume flow rates within the water flow system of a vapor generator may be pre-adjusted for automatic actuation as needed by demand conditions. This has been done in a configuration facilitating automatic temperature control and which is pressure sensitive for accurate volumetric control without adversely affecting the efficiency of the generator system.