| Some components that can be added to standard | | | | can be easier to read from a distance than an LED |
| timer boards to achieve demand cleaning are either a | | | | numeric) for the ability to read from a distance. Some |
| Photohelic type device that combines a pressure | | | | devices do not require jumpers, and only have one set |
| indicator (like our standard Magnehelic gauge) with a | | | | of connection terminals to the timer. Most of these |
| set of pressure switches, or (1) to (2) pressure | | | | devices are available in a range of voltages, and with |
| switches with a separate differential pressure gauge. | | | | options for 4-20mA Analog output. |
| The indicating switches like the Photohelic are very | | | | Using pressure switches in conjunction with a |
| convenient to use. They have some drawbacks and | | | | differential pressure gauge |
| limitations when it comes to mounting and wiring. | | | | Care must be taken when selecting one single |
| The current version of the Dwyer brand Photohelic | | | | pressure switch to ensure the desired result. Dwyer |
| must be split and mounted with the face on the | | | | models 1626 for example, use a dead band with a |
| outside, and the relays on the inside of an approved | | | | very small adjustment range to achieve the start and |
| enclosure that has to be drilled for the gauge. A lot of | | | | stop function of the timer board. The dead band |
| labor is required just to achieve a NEMA 4 electrical | | | | settings and the max differential between set points |
| rating. This is less than convenient, and still relies on the | | | | should be considered when making a selection. Other |
| installer to provide a bead of silicone rubber to make | | | | single switches can work, but some will require an |
| the seal. | | | | external relay to perform the latch function we desire. |
| Wiring for our use requires the installation of two reset | | | | Two separate pressure switches and a relay are |
| jumpers on the terminal board of the Photohelic unit, | | | | another (very good) method to use when a differential |
| and requires prior knowledge or reading of the manual | | | | pressure indicator is already in place. The Dwyer |
| (must be on hand) because the jumper locations are | | | | Series 1800 are a good low cost alternative. The -5 |
| not clearly marked on the back of the Photohelic. | | | | and -10 models suit most of our needs. There are |
| Photohelic theory of operation is that the Photohelic | | | | miniature pressure switches that cost even less. |
| gauge has two set-point needles. One is for the high | | | | Pressure switches have the advantage of being very |
| pressure set point, and one is for the low pressure set | | | | user repairable, and removes the more costly |
| point. The wiring from our standard timer board to the | | | | Photohelic from the stores room shelves. |
| Photohelic, requires the use of dry contacts on the | | | | Pressure switches are easily mounted and enclosed, |
| Photohelic. The reason for this is that the timer board | | | | but beware of pressure connections into the enclosure |
| puts out a control voltage on one of the two terminals | | | | that can drop the electrical rating. |
| labeled "pressure switch". If the jumper is installed on | | | | One pressure switch disadvantage is the difficulty of |
| our timer board (no pressure switch used), then the | | | | set-up and calibration. One has to use a hand held |
| board will always run and clean the bags any time it is | | | | vacuum pump (available at most auto parts stores for |
| powered on. If the jumper is not installed (in the case | | | | under $30.00) to simulate the differential pressure, a |
| of a pressure switch or Photohelic), the timer will only | | | | Magnehelic gauge to view the current vacuum, a |
| run when the contacts on the pressure switch or | | | | screwdriver to adjust the switch, and a test light or |
| Photohelic are closed. | | | | continuity meter to hear or see when the adjusted |
| The term "dry contact" refers to using the device as a | | | | contact trips, all at the same time. |
| switch only that does not provide power (since the | | | | Is "Demand Cleaning" right for your application? |
| timer board supplies the power to the pressure switch | | | | This question can NOT be answered based on gut |
| terminals). The way we use the Photohelic is to lock | | | | feelings or opinions. This question can not be |
| (with the jumpers on the Photohelic) in the "high" relay | | | | answered on past experience with an identical |
| when the pressure exceeds that of the high needle | | | | application, unless that identical application is in the |
| set point. This is effectively like pressing the start | | | | same plant with the exact same material and |
| button. The jumpers use a set of contacts on the high | | | | conditions, and this in itself is very rare. |
| relay to lock in the relay (like a motor starter does with | | | | Only experience and testing can determine if any |
| it's AUX contact to "latch" the start button) until the low | | | | given application can benefit from "On-Demand |
| relay is activated which is effectively like pressing the | | | | Cleaning". This can usually be determined within a few |
| stop button. | | | | hours of run time. |
| Photohelic operations simplified: | | | | Applications that must clean continuously to keep the |
| The filter cleaning timer will run when the differential | | | | differential pressure in check, will rarely benefit from |
| pressure needle (black) meets or exceeds that of the | | | | this technology. |
| high pressure needle (red needle on the right). | | | | Component Selection Considerations |
| The filter actively cleaning will cause the differential | | | | 1) Ensure components are selected with the proper |
| pressure to drop. | | | | range, and voltage. |
| Once the differential needle (black) drops to or below | | | | 2) What electrical enclosure is required for the |
| the low pressure needle (red needle on the left), the | | | | location? |
| Photohelic will unlatch and cause the filter cleaning to | | | | 3) Will the location be inside or outside? (affects |
| stop. | | | | enclosure type, and should be a consideration on any |
| Photohelic operations simplified: | | | | electronic displays. LED for in and outside. Non- backlit |
| The filter cleaning timer will run when the differential | | | | LCD for well lit indoor locations. Backlit LCD or LED for |
| pressure needle (black) meets or exceeds that of the | | | | poorly illuminated locations) |
| high pressure needle (red needle on the right). | | | | 4) Should the system contain a transmitter? (useful |
| The filter actively cleaning will cause the differential | | | | when the gauge is not easy to access, when |
| pressure to drop. | | | | continuous monitoring is required and, when |
| Once the differential needle (black) drops to or below | | | | recordkeeping requirements dictate it) |
| the low pressure needle (red needle on the left), the | | | | 5) Is a data logging system required? |
| Photohelic will unlatch and cause the filter cleaning to | | | | 6) If data logging is a requirement, should broken bag |
| stop. | | | | detector outputs be logged also? |
| There are other devices out there to do the job the | | | | 7) If data logging is a requirement, what other items |
| Photohelic does, and some do a better job than others. | | | | need to be logged? |
| Consider devices with LED displays (LED bar-graph | | | | |