Economizer for steam boiler

Claim 2

An economizer unit for use with a steam boiler having a feedwater inlet, a feedwater source, and an exhaust duct through which heated flue gases are discharged, said economizer unit comprising: an economizer heat transfer coil positioned in the duct so that the flue gases pass therearound in a heat transfer relationship therewith; and, flow control means for selectively directing feedwater to the feedwater inlet of said boiler through said economizer coil to heat the feedwater and directly to the feedwater inlet bypassing said economizer coil, said flow control means response to the average temperature of the flue gases after passage around said economizer coil to maintain the temperature of the flue gases after passage around said economizer coil within a prescribed temperature range above the dew point of the flue gases.

Claim 3

The economizer unit of claim 1 wherein said flow control means includes: a flow control valve operatively connecting said economizer coil and the feedwater inlet to the feedwater source, and connecting said economizer coil to the feedwater inlet to selectively direct feedwater from the feedwater source through said economizer coil to the feedwater inlet on the steam boiler and from the feedwater source to the feedwater inlet on the steam boiler by-passing said economizer coil; temperature sensing means for sensing the temperature of the flue gases after passage around said economizer coil; and, control circuit means operatively connected to said flow control valve and said temperature sensing means to cause said flow control valve to direct the flow of feedwater through said coil and by-passing said coil so as to maintain the flue gases in said prescribed temperature range after passage around said economizer coil.

Claim 4

The economizer unit of claim 2 wherein said temperature sensing means is an averaging thermocouple.

Claim 5

The economizer unit of claim 3 wherein said flow control valve is a three-way proportioning valve.

Claim 6

The economizer unit of claim 4 wherein said proportioning valve includes a pneumatically operated control mechanism for operating said valve; and wherein said control circuit means includes transducer means for converting the signal generated by said thermocouple into a pneumatic output operatively connected to said control mechanism on said valve.

Claim 7

An economizer unit as set forth in claim 1 wherein said flow control means further includes a three-way valve having a first inlet, a second inlet and an outlet, said valve selectively connecting said first and second inlets to said outlet, said first inlet operatively connected to the feedwater source through said economizeR coil, said second inlet connected to the feedwater source by-passing said coil, and said outlet connected to the feedwater inlet of the boiler; and temperature responsive control means operatively connected to said valve to cause the feedwater to be selectively directed through said coil and by-passing said coil to maintain the temperature of the flue gases after passage around said coil within said prescribed temperature range.

Claim 10

The method of claim 7 wherein the step of dividing the flow of feedwater includes controlling the amount of feedwater flowing through the economizer coil directly proportionally to the amount the actual temperature of the flue gases after passage around the coil is above said prescribed temperature range, and controlling the amount of feedwater by-passing the economizer coil inversely proportionally to the amount the actual temperature of the flue gases after passage around the coil is above said prescribed temperature range.