Remember, refined does not equate to more accurate -- the simple U-tube or inclined-tube manometers still provide reliable and accurate measurements of pressure. Several manufacturers offer low cost instruments for measuring pressure in both hand-held analog and digital manometers. These types of instrument range in pressure from a few inches of water to several thousand psi. For most drying applications, process gas pressures in excess of 40 inches of water are rare.
Pressure in drying systems is fundamental. Note that vacuum is negative pressure, so when I refer to pressure, I am talking about an absolute value. Because gas is being moved around the dryer, there is a requirement for the primary and secondary movers to be able to produce sufficient pressure to overcome all of the losses as the gas moves across the system. A manometer is used to establish the static pressure across the system, and a static pressure profile can be developed to illustrate the losses across the system.
There are other pressures in systems that require more accurate measurement than a standard manometer can accurately read. For these pressures, a special type of manometer is required.
There are various methods to obtain the velocities of a gas in a dryer. One of the most common and accurate is to use a pitot tube, which measures the difference between the static and the dynamic pressure of the gas as you traverse the duct or cross-section. This differential, coupled to the true gas density at the ensuing pressure and temperature conditions, will yield the velocity of the gas in the duct.
This pressure difference can be small at lower velocities, and an inclined manometer or digital micro-manometer with a much lower range may be required to take the readings. In addition, without obtaining the true density of the gas, the velocity and resulting volume and mass are only estimates.
To measure an accurate velocity in a processing system is really a challenge with a digital instrument because there is almost always some turbulence in the system. This causes the readings to bounce erratically and fluctuate dramatically. Accuracy here is paramount, and the instrument must have damping abilities if it is to be of any real use.
Various companies manufacture digital micro-manometers. The cost of these instruments is higher -- sometimes significantly so -- than regular manometers but is needed to obtain meaningful data.
Most velometers are basically micro-manometers calibrated to a specific pitot tube. Their packaging and displays vary and often are integrated into a convenient box. They have been used almost exclusively in the HVAC industry for balancing air-conditioning ducts.
Another valuable instrument for measuring gas velocity is an anemometer. There are two primary types available. The first of these is the vane anemometer, which is essentially a propeller that is driven by the gas flow. A sensor counts the revolutions per minute (rpm) of the propeller, and algorithms extrapolate this to give a velocity.
A hot wire anemometer provides a head with an exposed wire concealed in a protective loop. Most hot wire anemometers are termed constant temperature anemometers. The sensor (wire) forms one leg of a wheatstone bridge. The anemometer's feedback circuitry keeps this wire at a fixed temperature, which is above atmospheric. Changing velocity and, consequently, heat transfer rates from the wire cause the anemometer electronics to vary the voltage supplied to the wire to maintain its temperature. Calibration of the instrument then yields a relationship between this voltage and gas velocity.
The neat thing about anemometers is that the velocity obtained is independent of the gas density. However, to perform a heat and mass balance across the dryer, you need the true density, so this benefit is only short lived. Anemometers have other limitations, specifically as they relate to dryers. Gases flowing in dryer ducts often have solid particles at the location of the traverse. Solids damage anemometers and affect their performance. Additionally, vane anemometers have large heads, and it is difficult to get them into ducts.
They are of most value if they are used outside of the system such as at filter inlets and exhausts -- but be careful of the high temperatures.
Simple mercury bulb thermometers are versatile when auditing a dryer. They are suitable for performing both dry and wet bulb temperature measurements and are reliable.
Dial thermometers also are useful. They can be placed in a suitable process-measuring port and provide information from some distance. Newer digital thermometers provide great flexibility, and the temperature-sensing elements can be designed for gas, liquid, surface or penetration measurement. They also can be designed for high-speed response or reading. There are numerous manufacturers of digital thermometers, and they all function within an acceptable range.
Another valuable type of thermometer that is becoming more available is the infrared (IR) thermometer. This device measures the surface temperature, and units with laser pointers allow you to target and focus.
Next month in Tools of the Trade, Part 3, I'll explain which thermometer to use where and look at a few other tools of the trade.