Abstract: A furnace for heating a process fluid stream which includes means for efficiently controlling a flow of the process fluid stream through an adjunct loop of the fluid stream. The adjunct loop comprises a heat exchange system in which a portion of the process fluid stream collects heat from flue gases being emitted from the furnace and donates heat to combustion air being used in the furnace. Valve means is positioned downstream of a pump and in connection with the fluid stream in order to constantly replenish a small portion of fluid in the adjunct loop with fresh process fluid. The positioning of the valve means downstream of the pump permits more efficient control of the heat exchange system. One form of valve means may be a valve which will permit a small amount of fluid to continue to flow between the fluid stream and the adjunct loop during a non-operating mode of the adjunct loop heat exchange component.

Abstract: A furnace for heating a process fluid stream which includes means for precisely controlling the temperature of pre-heated combustion air. The system includes a process fluid stream being heated within the furnace and an adjunct loop fluid stream line which acts as a heat exchange system by communication with the process fluid stream. The adjunct loop comprises a heat collecting leg positionable within the combustion area of the furnace and a heat donating leg positionable adjacent a source of combustion air. Manipulation of the flow of process fluid through the heat donating leg provides control of the temperature of the combustion air and consequently control of the stack exhaust temperature. A valve system, which may be manually controlled or thermostatically controlled responds to a present, or desired, temperature of the stack exhaust.

Abstract: A method and apparatus for controlling combustion in a process heater provides for efficient control of the combustion with a minimum of components. The apparatus includes means for generating a signal from a condition of a process such as temperature, or pressure in the case of a boiler. In one embodiment the milliampere signal is presented in series to a resistor to provide an instant proportional voltage signal. This instant voltage signal is then fed into an R-C network to provide a ramp voltage signal. These two voltages are fed into voltage comparator switches which direct the instant voltage signal and the ramp voltage signal to one of two output amplifiers. The output amplifiers control the output signal to the external control devices. The external control devices adjust the flow of fuel and combustion air. The outputs are controlled so as to always provide a fuel-lean mixture during a change in the operating conditions of the process.

Abstract: A method and apparatus for controlling combustion in a process heater provides for efficient control of the combustion with a minimum of components. The apparatus includes means for generating a signal from a condition of a process, such as temperature, or pressure in the case of a boiler. The signal, which may be a pressure signal, is presented in parallel to a needle valve, to a first fluid-responsive relay, and to a second fluid-responsive relay. A volume tank is connected in series to the needle valve and in parallel to the first relay and to the second relay. The first relay communicates with means for controlling the flow of combustion air. The second relay communicates with means for controlling the flow of fuel. When the signal of the process condition is below the point at which a controller is set, then the signal is an increasing pressure signal. When the signal of the process condition is above the point at which the controller is set, then the signal is a decreasing pressure signal.

Abstract: A method of preheating combustion air being supplied to a furnace, and a furnace including the means for preheating the combustion air. The method comprises circulating a portion of fluid from a fluid stream first in non-contact heat exchange relationship with the flue gas for heat collection, then circulating this portion of fluid in non-contact heat exchange relationship with the combustion air for heat donation thereto. The rate of circulation and temperature of the portion of fluid is controllable substantially independently of the rate of flow and temperature of the parent fluid stream. The furnace comprises means to heat a portion of a fluid stream and includes means to circulate, at a controllable rate and temperature, such portion of fluid from a fluid stream through an adjunct loop connected to the fluid stream for collection of heat from the flue gases and donation of heat to the combustion air.