In this series on the tools of the trade, I've been looking at some of the methods of obtaining the required field data to enable an effective dryer audit. I left off with the discussing simple mercury bulb thermometers, which are suitable for performing both dry and wet bulb temperature measurements, and the infrared (IR) thermometer, which measures surface temperature and can be targeted and focused. I'll pick it up there.
Each thermometer is suited to obtain different data readings. Infrared is great for measuring surfaces to calculate losses as well as product temperatures before and after the drying process for the heat and mass balance. Digital thermometers can accurately measure the process gas temperature inside ducts, the dryer and, of course, the inlet and exhaust condition. Be sure to use a thermometer that is calibrated to the process range. Measuring the flame temperature of your burner with a thermometer that is only rated to 500oF (260oC) will, obviously, give erroneous results and may damage the instrument.
Learn how to use the thermometer and especially limitations of the technology. This is especially true of infrared thermometers because their accuracy is affected by the color of the product or surface (emissivity) as well as the angle and distance at which it is measured. Basic thermometers are pretty foolproof and give accurate information.
A "tach" measures rotational velocity or revolutions per minute (rpm). Again, there are a variety available, broadly grouped into contact and noncontact instruments. Noncontact devices require tape or other reflective media to enable them to work. A strobe can be used on spoked devices to obtain rpm.
Contact devices have inserts that are used on the shafts or surface of the rotating device. For dryers, these are used to measure actual fan rpm, valve rpm and belt rpm. The rpm can be converted to a linear speed based on the radius of the driven device.
Hygrometers are inexpensive instruments that provide the local temperature, atmospheric pressure and humidity. This is valuable as a datum and necessary to enable an accurate heat and mass balance. However, they do not function at elevated temperatures with respect to humidity, so use traditional methods to obtain the web bulb temperature.
In my humble and subjective opinion, combination instruments are the way to go when purchasing an instrument. These instruments offer measurement of more than one of the required parameters in an integrated package. Some manufacturers have modules that can be added to their instruments to turn them from multimeters to infrared thermometers, probe thermometers or manometers. In my view, you lose value with too much flexibility because it frequently is advantageous to use two instruments simultaneously.
Most ammeters double as multimeters, and there are a host of good ones out there. Some manufacturers make a micro-manometer that has a built-in thermometer to enable more accurate measurement of gas velocity at the local process point.
One manufacturer has released an instrument that offers the potential of three simultaneous pressure readings while monitoring process temperature or atmospheric temperature, relative humidity, barometric pressure and calculating the true gas density, giving a real-time true velocity. This includes the effects of water vapor on the gas. In addition, it has a damping system that assists in obtaining accurate, meaningful data simply. This instrument illustrates that the developers have invested in designing an instrument that answers many of industry's calls.
With the proliferation of computerized data collection, many of these instruments provide called for datalogging functionality. Some store the data internally for downloading later while others link directly into the PC.
Measuring the performance of a dryer is critical to ensure that the unit operates to its maximum potential. Measurements identify weaknesses and areas that require improvement. Obtaining process data will permit a heat and mass balance across the system to verify performance. The above tools are needed to obtain this data. Use them well.
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