DE3731318C2 - - Google Patents

Description

The invention relates to a method according to the preamble of claim 1.

When operating a gas-fired furnace, the combustion efficiency can be reduced be optimized in that the correct ratio between the amount of gas supplied and the amount of combustion air is maintained. In general is the ideal ratio for security reasons slightly shifted by adding a little more combustion air (i.e. Excess air) than for optimal combustion efficiency is required is used. About heat loss To keep the oven small, it is important that the Excess air volume is controlled.

A method of the type specified at the outset is in DE 29 25 031 A1 when used on a chimney draft regulator with a speed-adjustable motor for driving a suction fan described. The suction fan creates a vacuum in an existing boiler. When commissioning the Boiler is first turned on and the fan accelerates until a desired, manually adjustable Negative pressure is reached. During operation, the actual vacuum continuously with the set vacuum compared and accordingly the speed control takes place of the drive motor for the suction fan. For this type the speed control therefore requires a pressure transducer, its control function with considerable effort connected is. 3 of DE 29 25 031 is the Chimney draft regulator with the power control of the boiler connected. To increase the performance of the boiler a servomotor is provided, which is the gas valve  operated. If the gas valve is adjusted at the same time also the speed of the suction fan motor changed, so that there is no mismatch between the amount of exhaust gas and the flow rate of the exhaust gases in the exhaust pipe. Accordingly, in the known method before Ignite the gas oven by accelerating the drive motor blown through until the pressure drop is a predetermined amount reached, and then the ignition process is initiated.

It is common for gas-fired ovens to have two different ones To have firing levels, each level having its own supplied Amount of gas. The two-stage operation can be done with one motor with two fixed speeds for driving the suction blower motor and the blower motor can be achieved. The electricity consumption such engines, limited to two speeds are, however, during operation with the lower Speed significantly greater than z. B. that of an electronic Reversed polarity motor with variable speed (ECM). Since further the suction fan motor only works at two fixed speeds, the system can not be connected to different operating and System conditions, such as B. with a ventilation system different lengths, be adjusted so that the excess air volume not set to the desired size unless the system is special Established coordinated.

The object of the invention is that of the generic type Train the method so that the speed control of the Drive motor for the suction fan without using a Pressure transducer can be performed.

This object is solved by the features of claim 1. Advantageous developments of the invention are in the Subclaims described.

In contrast to the known method, the The inventive method requires no pressure transducer.  Rather, when the predetermined pressure drop is reached the associated engine speed using a pressure switch detected when the predetermined pressure drop is reached is operated. The engine speed determined in this way is recorded and after ignition, the engine speed depending on the recorded engine speed calculated, lower speed reduced. Hereby an optimal amount of excess air is set, which leads to leads to an optimal combustion efficiency.

The method according to the invention is suitable for gas ovens which both according to the one-tier system and the multi-tier system operate. In both cases, the Improve the economy of the gas oven. With the multi-stage system each stage has its own supplied amount of gas assigned. The two-stage operation can be done with a motor two fixed speeds for driving the suction fan and the drive of the blower for the room air to be heated be achieved.

The method according to the invention enables the gas furnace Use of air ducts of different lengths. This is advantageous because the method according to the invention without significant change even on gas stoves different Use types, especially different sizes leaves. In addition, the method according to the invention enables Maintenance of desired excess air quantities if the Operation of the gas furnace in a one-stage or multi-stage system he follows.

In the method according to the invention, during the blow-by cycle the suction fan motor accelerates to the pressure increase in the heat exchanger so that the pressure switch closes when its closing point is reached. If the Pressure switch closes, the suction fan motor speed captured and recorded by means of a microprocessor. To the gas oven is ignited by blowing, and the is left  Stabilize suction fan motor. After a short Period of time, the suction fan motor speed will rise to a level which decreases the recorded engine speed and is based on a formula derived from data that were obtained experimentally from a sample operating system, the desired amount of excess air in strong burning conditions generated. In this way the pressure switch used during pre-ignition to determine an engine speed to be recorded, which are then used for this can, the desired operating speed of the suction fan motor to get under burning conditions. According to another Aspect of the invention are in the heat exchanger of the gas furnace Pair of pressure switches are provided, including a low pressure switch is provided that responds to a pressure level that with the desired theoretical excess air quantity in the low combustion mode is in line and a high pressure switch is present that is responsive to pressure that with a desired theoretical excess air volume in Heavy combustion operating state is in line. During the Blow-by cycle, the aspirator motor is accelerated to increase the pressure in the heat exchanger so that the Low pressure switch and the high pressure switch one after the other shut down. When the switches close, the suction fan motor speeds captured by a microprocessor and recorded. Then the speed at which the low pressure switch closes and the speed, at which the high pressure switch closes, the Quotient calculated. This quotient is then for one recorded later use. After blowing through the gas stove is ignited in the strong combustion mode, and the is left Stabilize suction fan motor. After a short Period of time, the suction fan motor speed becomes one size which decreases the recorded engine speed and is based on a calculation formula derived from data is experimental from a sample operating system were obtained that the desired amount of excess air below contains various heavy combustion operating conditions. To  After a short period of time, the gas furnace goes into the weak combustion mode above, with the suction fan motor speed up a value is reduced by multiplying the previous engine speed with the previously calculated Quotient is calculated. In this way, the Pressure switch used during pre-ignition operation to determine a quotient that will be used later can be the desired operating speed for the To get weak burning.

An embodiment of the invention is in the drawings shown and is described in more detail below. It shows

Fig. 1 is a perspective view of a gas furnace, which is designed according to the invention,

Fig. 2 is a schematic representation of two built-in pressure switches, as used in the heat exchanger system, and

Fig. 3 to 5 are graphs showing changes in the Sauggebläsemotordrehzahlen during typical operation cycles.

Reference is first made to FIG. 1, which shows a gas furnace designed according to the invention. A burner 11 is connected to a burner box 12 of a main heat exchanger 13 . At the other end of the main heat exchanger 13 , a condensation heat exchanger 14 is connected in terms of flow, the outlet end of which is connected in terms of flow with a collecting box 16 and an outflow opening or an outflow channel 17 . In operation, a gas valve 18 measures the gas flow to the burner 11 in which combustion air is mixed in from the air inlet 19 and the mixture is ignited by the igniter 21 . The hot gas is then passed through the main heat exchanger 13 and the condensation heat exchanger 14 , as shown by the arrows. The relatively cold exhaust gases then pass through the header 16 and outlet 17 to escape to the atmosphere while the condensate flows from the header 16 through a condensate drain line 22 , from where it is appropriately discharged to a sewage port or the like. The flow of the combustion air into the air inlet 19 through the heat exchangers 13 and 14 and the outflow opening 17 is promoted by a suction fan 23 which is driven by a motor 24 in response to control signals from a microprocessor.

The domestic air is drawn into a blower 26 which is driven by a drive motor 27 , again in response to signals received from the microprocessor. The air discharged from the blower 26 flows through the condensation heat exchanger 14 and the main heat exchanger 13 in countercurrent to the hot combustion gases, thereby heating the domestic air, which then flows from the outlet opening 28 into the duct system inside the house.

The microprocessor mentioned here is contained in a microprocessor control device 29 . In response to electrical signals from the thermostat and other signals to be described below, the microprocessor controller 29 operates to control the suction fan motor 24 and the fan motor 27 so that an efficient combustion process is obtained at two different power levels.

In order to help control the excess air, a pair of pressure switches 31 and 32 are arranged across the burner box 12 and the collecting box 16 , respectively, in order to be able to measure the pressure drop across the heat exchanger system. Switches 31 and 32 are mechanically connected to the system to sense the heat exchanger pressure drop, as shown in FIG. 2.

As can be seen, a fuel box pipe 33 leads away from the pressure tap 36 and a collecting box pipe 34 leads away from the pressure tap 37 . In between, the low pressure switch 31 and the high pressure switch 32 are arranged in terms of flow in a parallel circuit. These switches are calibrated so that they close at certain pressure differences as determined in a manner described in more detail below. Switches that have been found to be satisfactory for use in this manner are commercially available from Tridelta as part numbers FS 6003-250 (high pressure) and FS 6002-249 (low pressure).

Since the system normally operates under negative pressure conditions, the outlet opening of the gas valve 18 with the pipe 38 is connected to the pipes 33 and 39 via a T-piece 40 in order to connect the low pressure switch 31 , the high pressure switch 32 and the gas valve 18 to those in the burner box 12 Obtain negative pressure while the suction fan motor 24 is in operation.

Since the pressure drop across the heat exchangers gives an indication of the excess air in the combustion system, the low pressure and high pressure switches 31 and 32 are used to determine when the excess air quantity falls below the minimum theoretical quantities desired for low or high combustion conditions. For example, the low pressure switch 31 is calibrated so that it closes when the amount of excess air is equal to the desired theoretical value for low combustion conditions. At this point, the closure of the switch triggers a signal that is transmitted to the microprocessor, which then triggers the sensing and recording of the suction fan motor speed, RPM1. When the speed of the suction fan motor is increased, the excess air quantity increases until it finally reaches the desired theoretical value for the high combustion conditions, at which time the high pressure switch 32 closes accordingly and a signal is sent to the microprocessor. The suction fan motor speed is then sensed again at RPM2 and recorded. These RPM1 and RPM2 speeds are then mathematically modified to obtain the desired engine speeds in accordance with the present invention.

A typical operating cycle will now be described with reference to FIG. 3. When there is a need for heat, the controller checks the state of the high pressure switch and the low pressure switch 32 and 31 . If both switches are open, as it should be, the suction fan motor is accelerated until the pressure drop is equal to P₁; at this time, the low pressure switch 31 is closed and the suction fan motor speed RPM1 is recorded. The suction fan motor speed is allowed to accelerate further until the pressure drop is equal to P₂; at this time, the high pressure switch 32 closes and the suction fan motor speed RPM2 is recorded. The microprocessor controller 29 then calculates the quotient of the suction fan motor speeds at the switch closing points for weak and strong combustion conditions as follows:

The QUOTIENT is then recorded for later use.

After the high pressure switch 32 has closed, the system is subjected to a blow-through or flushing process and the pilot is ignited by the furnace control. Shortly after the pilot ignites and the main burners ignite, the controller calculates RPM4 using RPM2, as described in more detail below, and then reduces the aspirator engine speed to RPM4.

It is understood that when the ignition occurs, the overall temperature of the heat exchanger system increases and the overall density decreases. Again, this causes a significant increase in pressure drop, as shown in FIG. 3. To reduce the pressure drop to the level at which RPM2 was sensed, the speed of the suction fan motor must be reduced accordingly. However, it is not obvious how much the speed can be reduced. The various contributing factors include: the difference in temperature and density between the exhaust gas and the air, the gas valve opening characteristics and the length of the air duct system.

To determine standard operating points for different systems, was a pressure drop P₅ that of the desired theoretical Corresponds to excess air volume used. A sample system was run experimentally under different operating conditions (i.e. warm and cold starts for air duct conditions minimum length and maximum length), the speeds RPM1-RPM4 and the heat exchanger pressure drop (HXDP) are recorded were. The resulting data was then analyzed and modified to the heat exchanger pressure drop of Cycle to cycle for minimum and maximum air duct lengths to make it repeatable. For this purpose, a standard heat exchanger pressure drop of 18.3 mm water column for the high focal strength used. Where the deviation from this norm value was above a certain threshold, the following became Equation applied to correct the RPM4 values:

Using the average of the experimental results thus obtained The corrected RPM4 values were obtained to the RPM2 values, for minimum and maximum air duct conditions, as shown in Table I.  

Table I

Assuming that between the minimum and maximum Air duct conditions there was a linear relationship using a best fitting linear equation the RPM2 and RPM4 values are determined as follows:

RPM4 = 497.11 + (0.766 × RPM2) (Equation 3)

The speed of the suction fan motor is therefore up to RPM4 held at the end of the strong burning period. If the heat exchanger has been warmed up and the blower motor has been calibrated, switches the Control to a weak burn condition. This is done by that first the RPM5 suction fan motor speed below RPM4 suction fan motor speed is calculated. The blower motor speed will then decrease to a low burning speed decreases, and the furnace controller reduces the suction fan motor speed on RPM5, where:

RPM5 = RPM4 × QUOTIENT (Equation 4)

where the QUOTIENT is determined as in equation 1 and is measured during the blowing process.

When the RPM4 suction fan motor speed is reduced, the high pressure switch 32 opens and the high fire pot magnet is de-energized. The aspirator motor speed is then reduced to RPM5 and remains at that level during the period of low burn operation. If the thermostat is not satisfied within a prescribed period of time, the control switches from low-fire to high-fire operation. This is done by the suction fan motor first accelerating until the pressure switch for the high-power operation closes and thereby excites the pot magnet for the high-power operation. Then the speed RPM6 of the suction fan motor is recorded. The blower will then go to high speed, and the controller will increase the suction blower motor speed to RPM7. The relationship between the RPM6 and RPM7 values is determined experimentally in the same manner as described for RPM2 and RPM4, with the average speeds for minimum and maximum air duct lengths shown in Table II.

Table II

Again assuming a linear relationship between the minimum and maximum air duct conditions was an am best fitting linear equation using RPM6 and RPM7 determined as follows:

RPM7 = 262.18 + (0.926 × RPM6) (Equation 5)

The suction fan motor speed then becomes constant at RPM7 for the high fire operation is held until the thermostat conditions are fulfilled or the system returns to the weak combustion mode switches.

If e.g. B. an obstacle was temporarily placed on the system air duct, the pressure drop would be reduced to a point at which the high pressure switch 32 opens, whereby the pot magnet is de-energized for the high combustion state. This is necessary because of the reduced combustion air flow, as shown in FIG. 5. The control then causes the suction fan motor speed to be increased until the high-pressure switch 32 closes again and the pot magnet is de-energized for the high combustion state. In this state, the suction fan motor speed RPM8 is recorded and the furnace controller increases the suction fan motor speed to RPM9, where:

RPM9 = 262.18 + (0.926 × RPM8) (Equation 6)

is. As can be seen, the suction fan motor speed Using RPM9 as a function of RPM8 speed the same mathematical relationship between RPM6 and RPM7 was found, as expressed in Equation 5.

It is clear that throughout the operation described above, control limits are in effect in the various operating states and the relevant states are monitored so that an error is indicated when the limits are exceeded and the cycle is reset accordingly. If e.g. For example, during the period of initial acceleration to RPM1, as shown in FIG. 3, either the low pressure switch does not close within a prescribed period or the RPM1 value is outside the prescribed limits, an error is indicated and the device turns off and try again. If the high pressure switch closes in front of the low pressure switch, an error is displayed and the device locks out. Similar limits and modified operating conditions are provided during the other phases of operation to ensure that the system operates within the intended parameters.

Although the present invention is related to use of a two-stage system, it should be clear that certain aspects of her are just as good with you Single-stage or multi-stage system can be used. For example, although Equations 3, 4, 5 and 6 are used the desired aspirator motor speeds for to maintain the strong combustion plant in a two-stage system, they are equally at determining the suction fan motor speeds for operation under firing conditions in a single stage or other multi-stage system applicable.

Claims (6)

1. A method of maintaining a desired amount of excess air in a gas furnace having a heat exchanger, a suction fan and a variable speed drive motor for the fan, the gas furnace being blown by accelerating the drive motor prior to ignition until a predetermined pressure drop across the heat exchanger is reached is, and then the ignition is initiated, characterized in that
that the actual engine speed (RPM2) when the predetermined pressure drop is reached is detected and recorded by means of a pressure switch ( 32 ) and
that after the ignition, the engine speed (RPM2) to a depending on the recorded engine speed (RPM2) and taking into account a given from given functional conditions, such as. B. Differences in temperature and density of air and exhaust gas, resulting factor (F) calculated, lower speed (RPM4) is reduced according to the formula RPM4 = RPM2 × F. 2. The method according to claim 1, characterized in that that the predetermined pressure drop is selected so that it with a theoretical desired amount of excess air below Burning conditions is in line. 3. The method according to claim 1, wherein the furnace is designed for low-firing operation and high-power operation and the predetermined pressure drop is designed for high-power operation and wherein the gas furnace is ignited in high-power operation, characterized in
that the weak combustion mode is also assigned a predetermined pressure drop which is run through when the drive motor is accelerated,
that the engine speed (RPM1) which is present when the predetermined pressure drop for low combustion mode is passed is recorded and recorded,
that the quotient is formed from the recorded engine speed (RPM1) at the pressure drop for low combustion mode and the recorded engine speed (RPM2) at the pressure drop for heavy combustion mode, and
that after the ignition has taken place and the engine speed (RPM4) is reduced to a lower engine speed (RPM5), the engine speed (RPM4) is calculated by multiplying the engine speed (RPM4) by the previously determined quotient from the engine speeds (RPM1 and RPM2). 4. The method according to claim 3, characterized in
that the drive motor is accelerated for a restart to the heavy combustion mode until the pressure drop for heavy combustion mode is reached again,
that the existing engine speed (RPM6) is recorded and recorded and
that the engine speed (RPM6) is increased to a higher engine speed calculated as a function of the recorded engine speed (RPM6). 5. The method according to any one of the preceding claims, characterized in that the engine speed (RPM4) from the following equation is calculated: RPM4 = 497.11 + (0.766 × RPM2) 6. The method according to any one of claims 4 or 5, characterized characterized in that the engine speed (RPM7) is from the following Equation is calculated: RPM7 = 262.18 + (0.926 × RPM6)