Cleaning on-demand is one small way industries can contribute to the reduction of plant compressed air usage and waste. Although not every system can be cleaned on-demand, those that do can benefit from:
* Lower compressed air usage
* Longer solenoid valve & diaphragm valve life
* Longer filter bag life
Electrically the savings will be insignificant at the collector, but great at the compressor. Some components that can be added to standard timer boards to achieve demand cleaning are either a Photohelic type device that combines a pressure indicator with a set of pressure switches, or (1) to (2) pressure switches with a separate differential pressure gauge. The indicating switches, like the Photohelic, are very convenient to use, but have some drawbacks and limitations when it comes to mounting and wiring.
Photohelic theory of operation is that the Photohelic gauge has two set-point needles. One is for the high pressure set point and one is for the low pressure set point. The wiring from a standard timer board to the Photohelic requires the use of dry contacts on the Photohelic. The reason for this is that the timer board puts out a control voltage on one of the two terminals labeled "pressure switch". If the jumper is already installed on a timer board (no pressure switch used), then the board will always run and clean the bags any time it is powered on. If the jumper is not installed (in the case of a pressure switch or Photohelic), the timer will only run when the contacts on the pressure switch or Photohelic are closed.
The term "dry contact" refers to using the device as a switch only that does not provide power. The timer board supplies the power to the pressure switch terminals.
The common use of the Photohelic is to lock (with the jumpers on the Photohelic) in the ‘high’ relay when the pressure exceeds that of the high needle set point. This is effectively like pressing the cleaning start button. The jumpers use a set of contacts on the high relay (like a motor starter does with its AUX contact to "latch" the start button) until the low relay is activated, which is effectively like pressing the stop cleaning button.
Using pressure switches in conjunction with a differential pressure gauge
Care must be taken when selecting one single pressure switch to ensure the desired result. The dead band settings and the max differential between set points should be considered when making a selection. Other single switches can work, but some will require an external relay to perform the latch function desired. Two separate pressure switches and a relay are another (very good) method to use when a differential pressure indicator is already in place.
Pressure switches have the advantage of being very user repairable and removes the more costly Photohelic from the storeroom’s shelves. Pressure switches are easily mounted and enclosed, but beware of pressure connections into the enclosure that can drop the electrical rating.
One pressure switch disadvantage is the difficulty of set-up and calibration. One has to use a hand held vacuum pump (available at most auto parts stores for under $30.00) to simulate the differential pressure, a Magnehelic gauge to view the current vacuum, a screwdriver to adjust the switch, and a test light or continuity meter to hear or see when the adjusted contact trips - all at the same time.
Photohelic operations simplified:
* The filter cleaning timer will run when the differential pressure needle (black) meets or exceeds that of the high pressure needle (red needle on the right).
* The filter actively cleaning will cause the differential pressure to drop.
* Once the differential needle (black) drops to or below the low pressure needle (red needle on the left), the Photohelic will unlatch and cause the filter cleaning to stop.
There are other devices out there to do the job the Photohelic does, and some do a better job than others. Consider devices with LED displays for the ability to read from a distance (LED bar-graph can be easier to read from a distance than an LED numeric). Some devices do not require jumpers, and only have one set of connection terminals to the timer. Most of these devices are available in a range of voltages and with options for 4-20mA Analog output.
Component Selection Considerations
* Ensure components are selected with the proper range, and voltage.
* What electrical enclosure is required for the location?
* Will the location be inside or outside? (Affects enclosure type and should be a consideration on any digital, electronic displays. LED for in and outside. Non– backlit LCD for well lit indoor locations. Backlit LCD or LED for poorly illuminated locations)
* Should the system contain a transmitter? (Useful when the gauge is not easy to access, when continuous monitoring is required, and when record keeping requirements dictate it.)
* Is a data logging system required?
* If data logging is a requirement, should broken bag detector outputs be logged also?
* If data logging is a requirement, what other items need to be logged?
Is "On-Demand Cleaning" right for your application?
This question can NOT be answered based on gut feelings or opinions. This question can not be answered on past experience with an identical application, unless that identical application is in the same plant with the exact same material and conditions (this in itself is very rare). Only experience and testing can determine if any given application can benefit from "On-Demand Cleaning". This can usually be determined within a few hours of run time. Applications that must clean continuously to keep the differential pressure in check will rarely benefit from this technology.