Let's share some insider information. Performing a "dryer audit" is a mainstay of any drying specialist, and every reputable company that sells equipment has people specifically trained to perform these audits. Even with vast experience and all the necessary equipment, it takes someone skilled in the art significant time to capture all of the pertinent field data. Thereafter, another considerable investment is made into analyzing the data to manipulate it into a meaningful form.
A few years ago, I questioned my accountant on what I considered to be high fees for his services. His response was classic and something that made a meaningful impact on me. He nonchalantly responded, "My fees for my services are extremely low," he said. "I am only charging you one penny for all of my services. The rest of the money you are paying is for me knowing how to perform these services!"
The same is true for any professional service -- especially if the true value of what you receive will save you significantly more than your investment. If you, however, can perform these services in-house, you will save the investment for the specialist and reap the rewards of the audits conclusions.
So, I am going inside to share with you some of the methods of obtaining the required field data to enable an effective dryer audit. So much more than simply obtaining the data from the dryer itself is involved in a system analysis. Each system is different, so I will be focusing exclusively on obtaining the data from the dryer.
To do this, you need specific tools. These tools include a screw driver, flashlight, drill (preferably cordless) and other hand tools. However, the most noteworthy tools are more sophisticated, will likely cost more and are referred to as instruments. This is the topic of this Drying File -- tools of the trade. There are more instruments available than you can begin to imagine, and they are designed to measure anything you can think of. They all have their target applications. So, without going overboard in sophistication, cost or need, I will detail the basic types of instruments used in performing a dryer audit and discuss how they are used to gather meaningful information. Please note that I am offering no endorsement of any instrument, and any manufacturer or model is referenced purely to illustrate the technology.
Numerous manufacturers offer multimeters that vary in sophistication from simple to "awesome." Awesome multimeters are of real value for instrument technicians and the electronics industry. As a plant requirement, a simple instrument is sufficient for almost everything that is required. The two most functional uses of the multimeter for this application are voltage and resistance.
Most industrial high voltage applications are moving away from direct current (DC), so you typically will use the alternating current (AC) setting on the meter. Voltage or potential difference will tell you if the component has "juice," or power. This is important for confirming that motors and other field devices are powered.
As a direct measure, resistance is meaningful in determining such things as electric element condition. More commonly, the resistance feature of the instrument is used to determine continuity -- that is that there is no break in a wire or that a switch is closed when it needs to be. By applying the probes across the terminations, the instrument typically will emit a sound confirming the continuity. This is used to ensure that all switches and output signals are operating as they are required to do.
A feature that is nice to have on a multimeter is analog field reading capability. Analog transmitters commonly are 4 to 20 mA or 0 to 5 mV. You can test discrete or digital field signals with a DC (low voltage) or AC (120 VAC) volt setting, depending on the system configuration. This will validate that the correct inputs and outputs (I/O) are being sent and received from the control system.
A really cool instrument also will provide an analog output (although this typically is done with a separate instrument). This feature is another nice to have. With it, you can verify positions and conditions of field devices by changing the signal incrementally to illustrate whether each device is working properly.
Remember that working with multimeters requires that you work on "hot" panels and devices. Electricity is extremely dangerous, and only qualified individuals should be performing these tests.
An ammeter, or "amp clamp" as it is endearingly referred to, measures the current that a device draws. Most commonly, the device is a motor that is powering a rotating mechanical component such as a fan, valve or conveyor.
To obtain the current, a ring (coil) is placed around the conductor that creates a magnetic field, and by flux (that's magic in English) measures the current. On three-phase systems, each leg should be measured to check for balance and to obtain the average amp draw. The amount of current a device draws can be used to estimate the performance of the device.
A manometer is used to read the pressure of a fluid within a system. Pressure can be static or dynamic and, for certain applications, you need both. Most the pressure readings of interest to us are differential pressure readings. What this means is that the pressure obtained from the instrument will indicate the pressure at the process point relative to another pressure. If the other pressure is atmospheric pressure, the differential pressure obtained is called the gauge pressure.
Manometers come in a variety of instruments. The most simple is the U-tube manometer, where a fluid of known density is used to measure the pressure. One leg of the tube is connected to the process point, and the other is left open to atmosphere. For a pressure drop or loss measurement (differential pressure), one leg would be connected before the equipment and the other after the equipment. The difference in the two heights is measured and the resultant pressure is the difference in height (a distance) for that fluid.
If the fluid is water, the reading would be millimeters or inches of water. If the fluid is mercury, the reading would be millimeters (i.e., torr) or inches of mercury. These are the two most common fluids, and conversions for these fluids to kPa (kiloPascals) or psi (pounds per square inch) are available.
There are more refined instruments to read pressure. Note that refined does not equate to more accurate -- the simple U-tube or inclined-tube manometers still provide reliable and accurate measurement of pressure. I'll look at those as well as other devices you need to perform a dryer audit in my next column, "The Tools of the Trade, Part 2."