Excess air control with dual pressure switches

• US 4,729,207 A
• Filed: 09/17/1986
• Issued: 03/08/1988
• Est. Priority Date: 09/17/1986
• Status: Expired due to Term

• Current Assignee: Carrier Corporation (Carrier Global Corporation)
• Original Assignee: Carrier Corporation (Carrier Global Corporation)
• Inventors: Thompson, Kevin D., Dempsey, Daniel J., Peitz, Robert W. Jr.
• Primary Examiner(s): Jones, Larry
• Application Number:
  US06/908,474
• Time in Patent Office: 538 Days
• Field of Search: 126/112, 126/110 R, 126/116 A, 237/2 A, 237/11, 237/12, 237/53, 236/DIG. 8, 236/10, 431/12, 431/2
• US Class Current: 126/112
• CPC Class Codes: F23N 1/02 conjointly with air supply

Abstract

A method and apparatus for maintaining the desired level of excess air in a two stage gas furnace system. Low and high pressure switches are placed across the heat exchanger and are successively closed, as the inducer motor accelerates during purging, when the pressure drop reaches the respective theoretically desired low and high firing pressure drop levels. As the switches are closed, the inducer motor speeds are sensed and recorded, with a ratio of the two then being calculated. After firing, the ratio is then directly applied to a desired high firing motor speed to obtain the desired low firing motor speed.

Claims

1. In a furnace of the type having a two stage gas valve, a two stage firing rate, a heat exchanger, and a variable speed inducer motor, a method of controlling excess air comprising the steps of:

providing a low pressure switch that is responsive to a selected first pressure drop level in the heat exchanger, said first pressure drop level being selected so as to be commensurate with a theoretically desired excess air level when operating the furnace in a low fire condition;

providing a high pressure switch that is responsive to a selected second pressure drop level in the heat exchanger, said second pressure drop level being selected so as to be commensurate with a theoretically desired excess air level when operating the furnace in a high fire condition;

accelerating the variable speed inducer motor until the low pressure switch closes, and sensing and recording a first motor speed at that time;

further accelerating the variable speed induce motor until the high pressure switch closes, and sensing and recording a second motor speed at that time;

computing and recording the ratio of said first and second motor speeds;

operating the system in a high fire condition with the inducer motor operating at a desired third motor speed; and

transitioning to low fire operation, with the speed of the inducer motor being reduced to a fourth speed which is calculated by multiplying said third speed by said ratio.

2. A method as set forth in claim 1 and including the step of calculating said desired third motor speed as a function of said second motor speed.

3. In a furnace of the type having a heat exchanger, a two stage firing rate, and a variable speed inducer motor, an excess air control apparatus, comprising:

a low pressure switch disposed across the heat exchanger and having a design threshold to close at a first pressure drop commensurate with a theoretically desired excess air level for operation under low fire conditions;

a high pressure switch disposed in the heat exchanger and having a design threshold to close at a second pressure drop commensurate with a theoretically desired excess air level for operation under high fire conditions;

means for sensing the two motor speeds when the inducer motor is accelerated to cause said low pressure and high pressure switches to close in succession;

means for calculating and storing a ratio of said two motor speeds;

means for arriving at a desired motor speed for high fire operation and for obtaining and recording said high fire speed; and

means for applying said ratio to said high fire speed to obtain a desired low fire speed.

4. An excess air control apparatus as set forth in claim 3 wherein said means for calculating and storing said ratio is a microprocessor.

5. An excess air control apparatus as set forth in claim 3 and including means for maintaining the inducer motor speed at said desired low fire speed during periods of low fire operation.

6. A method of controlling the level of excess air in a furnace of the type having a heat exchanger, a two stage firing rate, and a variable speed inducer motor, comprising the steps of:

establishing low and high fire heat exchanger pressure drops that are desirable for respective low and high fire operation;

while accelerating the variable speed inducer motor, sensing the pressure drop across the heat exchanger and sensing and recording the respective low and high motor speeds when said low and high fire heat exchanger pressure drops are reached;

calculating the ratio of said low and high motor speeds;

calculating a desired high motor speed for operation under stable high fire conditions; and

calculating a desired low fire motor speed by multiplying said high fire speed by said ratio

7. A method as set forth in claim 6, wherein said recording and calculating steps are accomplished with the use of a microprocessor.