Blower control system

Abstract

A system for mitigating the flow of vapors from contaminated soil into a building has a blower pulling an exhaust from underneath the sub slab of the building and blowing the vapors away from the building. The blower is controlled by pressure sensors that measure the pressure drop across the sub slab. The sensors are monitored to make sure they all have a minimum pressure drop. If any of the sensors has an inadequate pressure drop, the blower speed is increased by a small amount. If all of the sensors have adequate pressure drop, then the blower speed is decreased by a small amount. The system then rechecks the pressure drops on a periodic basis and makes appropriate blower speed adjustments after each measurement.

Claims

1. A sub slab exhaust system comprising: a) an exhaust blower; b) a duct connected to said exhaust blower, said duct having an inlet located below a sub slab of a building; c) one or more pressure sensors, each of said pressure sensors measuring a pressure drop at a location below said sub slab relative to a pressure above said sub slab; d) a controller comprising an input for said pressure sensors, an output to control said blower, a microprocessor programmed to perform calculations based on said input to control said output, and a permanent memory comprising computer readable instructions to cause said microprocessor to carry out at least the steps of: i) at the beginning of a first control cycle period, measure said one or more pressure drops indicated by said one or more pressure sensors; ii) determine the ratio of said measured pressure drops to desired pressure drops for each of said sensors; iii) determine the minimum of said pressure drop ratios and either: 1) increase said blower speed by a first fractional increase if said minimum of said pressure drop ratios is between a first amount and 1 wherein said first amount is less than 1; 2) increase said blower speed by a second fractional increase if said minimum of said pressure drop ratios is less than said first amount; or 3) decrease said blower speed by a first fractional decrease if said minimum of said pressure drop ratios is greater than or equal to 1; and iv) execute steps i), ii) and iii) at the beginning of a second control cycle period.

2. The system of claim 1 wherein said first fractional increase is about 1% and said first control cycle period is about 1 second.

3. The system of claim 1 wherein said second fractional increase is about 5% and said first amount is about 0.95.

4. The system of claim 1 wherein said first fractional increase is zero and said second fractional increase is greater than zero.

5. The system of claim 1 wherein said steps comprise: a) increase said blower speed by a third fractional increase if said minimum of said pressure drop ratios is less than a second amount wherein said second amount is less than said first amount.

6. The system of claim 5 wherein said third fractional increase is about 10% and said second amount is about 0.85.

7. The system of claim 1 wherein said first fractional increase is more than said first fractional decrease.

8. The system of claim 1 which further comprises an internet communication system for receiving remote instructions to control said microprocessor and transmitting compliance data from said microprocessor.

9. The system of claim 1 which further comprises three or more pressure sensors.

10. The system of claim 1 wherein said first control cycle period is less than about 10% of the time it takes the pressure in said building to respond to a sudden change in building configuration.

11. The system of claim 10 wherein said sudden change in building configuration is an exterior door opening.

12. A sub slab exhaust system comprising: a) an exhaust blower; b) a duct connected to said exhaust blower, said duct having an inlet located below a sub slab of a building; c) one or more sensors, each of said sensors measuring an indication of adequate air flow from said building through said sub slab; d) a controller comprising an input for said sensors, an output to control said blower, a microprocessor for performing calculations based on said input to control said output, and a permanent memory comprising instructions to cause said microprocessor to carry out at least the steps of: i) at the beginning of a first control period, read the outputs of said one or more sensors; ii) determine the ratio each of said measured sensor outputs to desired outputs for each of said sensors; iii) determine the minimum of said measured ratios and either: 1) increase said blower speed by a first fractional increase if said minimum of said measured ratios is between a first amount and 1 wherein said first amount is less than 1; 2) increase said blower speed by a second fractional increase if said minimum of said measured ratios is less than said first amount; or 3) decrease said blower speed by a first fractional decrease if said minimum of said measured ratios is greater than or equal to 1; and iv) execute steps i), ii) and iii) during a second control period.

13. The system of claim 12 wherein said sensors measure a quantity that is inversely related to the flow of air through said sub slab and wherein said sensor ratios are computed by dividing said desired sensor outputs by said measured sensor outputs.

14. The system of claim 13 wherein said sensor outputs indicates a contaminant level in said building, said contaminant being a vapor originating below said sub slab.

15. The system of claim 12 wherein said sensor outputs indicate a flow rate in said duct.

16. The system of claim 12 wherein said sensor outputs indicate a pressure in said duct relative to a pressure in said building.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is an illustration of an exemplary blower control system.

(2) FIG. 2 is a flow chart of a suitable algorithm for controlling an exhaust blower.

(3) FIG. 3 is a graph comparing a bimodal control algorithm with a progressive algorithm.

(4) FIG. 4 is a graph of 30 minute averages of blower speed for a field installation of a blower control system.

(5) FIG. 5 is a graph of monthly averages of blower speed for the field installation of FIG. 4.

DETAILED DESCRIPTION

(6) The detailed description is exemplary and not limiting. Any features described herein may be combined with any other features to provide at least the benefits described herein.

(7) FIG. 2 is a flow chart of an exemplary control algorithm for a sub slab exhaust blower. A control period is first selected. Suitable control periods are in the range of once per second to once per 5 seconds. Longer or shorter control periods can be selected depending upon the response times needed. A suitable minimum control period is 10% of the time it takes the pressure in a building to respond to a sudden change in building configuration, such as a window opening or closing.

(8) At the beginning of each control period 202, the system checks the pressure drop at each sensor (Sensor #1, Sensor #2, Sensor #3). Any number of sensors may be provided. The ratio of sensor value to desired value is calculated for each sensor. Each sensor may have its own desired value or the same desired value may be used for multiple sensors. If the minimum sensor ratio is less than 1 (i.e. not enough pressure drop at the sensor location), then the control system increases the blower speed 204. If the minimum sensor ratio is greater than or equal to 0.5, for example, then the blower speed is increased by 25%. If the minimum sensor ratio is in the range of 0.75 to 0.5, then the blower speed is increased 15%. Reducing the fractional change in blower speed with increasing minimum sensor ratios is termed progressive control. In this example, the minimum increase in blower speed is 1% if the minimum sensor ratio is in the range of 0.95 to 1.0. The minimum increase in blower speed may be set to zero. This would correspond to there being a dead band where no change would be made to blower speed if the minimum sensor ratio was below a minimum value.

(9) If all of the sensors are at or above their desired values, the system then checks to see how much the blower power should be decreased 206. If the minimum sensor ratio, for example, is greater than 1.5, then the blower speed is decreased by 10%. If the minimum sensor ratio is in the range of 1.25 to 1.5, then the blower speed is decreased by 5%. The minimum decrease in blower speed for any given control period is a decrease of 1% when the minimum sensor ratio is in the range of 1.0 to 1.1.

(10) After the adjustment to blower speed is made, the system repeats the process at the next control period.

(11) An advantage of progressive control is that the same system can accommodate changes in pressure drop on widely varying time scales. If there is sudden change in pressure drop during a control period, such as a large door to the building opening or closing, the system will take large actions to correct for it and hence respond quickly. On the other hand, if small changes happened over the course of a large number of control periods, such as a change of seasons, then the system will constantly make small adjustments to respond over the corresponding time scale.

(12) The relationship between fractional change in blower power or speed and minimum sensor ratio may be linear. It may also be non-linear. Any number of different control levels may be provided including continuous analog control. A very simple embodiment is that the system increases the blower speed by a fractional increase amount if the minimum sensor ratio is less than 1 or decreases the blower by a fractional decrease amount if the minimum sensor ratio is more than 1. This is termed bimodal control.

(13) Other factors may be monitored by one or more sensors in addition to, or instead of, pressure drop sensors across the sub slab. The air above the sub slab, for example, could be monitored for the contaminant that is in the ground below. Thus the system could increase the blower if contaminant is sensed within the building even if all of the pressure sensors indicate that there is adequate pressure drop across the slab at each monitored point. If a sensor measures a quantity that is inversely related to the flow of air through the sub slab, then the sensor ratio is computed by dividing the desired value by the measured value. If a desired contaminant level, for example is 100 ppm and a measured contaminant level in the building is 200 ppm then more exhaust flow is needed through the sub slab. The sensor ratio, therefore, is computed to be 100/200 or 0.5.

(14) The values for each blower increase or decrease and the length of the control periods can be selected to match the response times of the building and exhaust system to different changes. If a system overshoots its desired values, for example, the fractional increases or decreases in blower control can be reduced in magnitude or the cycle times can be increased.

Example 1

(15) A laboratory system was set up to simulate a building control system. A blower was attached to a duct that pulled air through a bed of gravel. The pressure in the gravel was monitored relative to the pressure above the gravel. This simulated a sub slab pressure drop. The system was set up with a bimodal control and a one-second cycle period. The results are shown in FIG. 3.

(16) FIG. 3 shows a graph 300 of the pressure above the gravel bed versus time. The units of pressure are inch WC. The units of time are min:sec. At time=0:00, the system was turned on. The desired value for the pressure sensor was 0.01 inch WC. For the bimodal control run, the measured pressure drop increased until the desired value was reached. The pressure, however, showed significant oscillations 304 about its set point. The oscillations could be reduced either by increasing the cycle time or decreasing the adjustment to the blower speed at each cycle. The system, however, would then respond slower to a sudden change in pressure drop.

(17) The experiment was repeated with a progressive control system and a cycle period of 5 seconds. This data is also plotted in FIG. 3. The fractional change in blower speed was proportional to the ratio of measured pressure to desired pressure. The pressure rose at about the same rate as the bimodal system and approached its set point asymptotically. The variations about the desired value 302, however, were much smaller than the bimodal system.

Example 2

(18) A progressive control system was set up on a sub slab exhaust system for a large industrial building. Sub slab pressure was monitored at multiple locations. The fractional change in blower speed was proportional to the minimum pressure drop ratio for the sensor locations. The blower speed was tracked over several days. The results are shown in FIG. 4.

(19) FIG. 4 shows a graph 400 of blower speed versus time. Blower speed is an indicator of blower power. Each point is a 30 minute average. Blower speed is shown on a relative scale with 100 corresponding to full power. Blower speed decreased during the day 402 and increased during the night 404. This may be due to warming and cooling of the building. Blower speed would also adjust to sudden changes 406. These may be due to door openings and closings. During these changes, the sub slab pressure drops always stayed within acceptable limits. The control system was also equipped with internet communications so its performance could be monitored remotely and reports could be prepared to show compliance with environmental regulations.

Example 3

(20) The same system as Example 2 was tracked over several months. The results are shown in FIG. 5.

(21) FIG. 5 shows a graph 500 of blower speed versus time. Each point is a one month average. Blower speed is shown on a relative scale. 100 corresponds to full power. Average monthly blower speed increased from October to January in response to overall weather changes. Thus power use was optimized on a seasonal basis.

Computer Control

(22) The control systems described herein may be implemented on a programmable device, such as a programmable controller or digital computer. The programmable device may comprise a microprocessor, an input device for receiving signals from sensor, an output device for controlling a blower and a permanent memory. The permanent memory may comprise instructions that will cause the microprocessor to execute the steps of the processes described herein.

Analog Control

(23) The control systems described herein may be implemented on electrical analog controllers where the voltage/current characteristics of the control circuit (e.g. operational amplifies) are designed to provide the appropriate control feedback.

Pneumatic Control

(24) The control systems described herein may be implemented on a pneumatic control system where the pressure flow characteristics of the pneumatic elements (e.g. pressure regulators) are designed to provide the appropriate control feedback.