Evaporation rates are slow, we learned, because the rate of heat transfer from air to parts is quite low because gas-solid heat transfer coefficients are so poor. In last month’s column, a figure showed how these coefficients have little dependence on temperature, but de-pend greatly on air velocity.
Is heat transfer coefficient the whole story? No, indeed. It is the rate at which heat can be transferred from the moving air stream to the water which controls drying rate. What’s the difference be-tween coefficient and rate? The defining equation is:
heat transfer rate = (heat transfer coefficient) x (part surface area) x (temperature difference)
The units are:
BTU/hr = (BTU/hr - ft2 - °F) x (ft2) x (°F)
Temperature difference is that between the free stream air and the water on the parts.
A key point is that heat transfer coefficient has little dependence upon air temperature, but heat transfer rate is highly dependent upon air temperature, and that rate limits drying rate. Let’s examine some practical situations. We’ll use 10 ft2 for surface area and ambient temperature (75°F) for water temperature on parts. Remember, that the heat of evaporation of water is about 1000 BTU/lb. Figure 1 of this column will control.
The same format is shown above for heat transfer rate. Suppose we need to evaporate ~10 lb/hr of water. The heat transfer rate is ~10,000 BTU/hr.
Which should we prefer?
From the standpoint of utility costs, the one with the highest free air velocity will be significantly cheaper. It costs much more to heat air than to make it flow. How about part integrity? Some parts can’t stand temperatures of up to 350°F. Plastic components may warp. Metal components may be damaged by unwanted chemical reactions. From the standpoint of particle contamination, the least air flow is the better choice.
There are two aspects to this point. First, suppose this operation is done in a cleanroom. Then particle loading is probably light, and it is of sub-micron particles. The velocities required for drying of parts are at least 10X higher than would be observed at the face of a filter. A higher air velocity, or volumetric flow, will contact the presumably cleaned part surface with more sub-micron particles. Thus the good cleaning work may be negated.
Second, suppose the operation is not done in a cleanroom. These air velocities, 25 to 50 ft/sec, are large and will convey large, small, and sub-micron particles onto the cleaned part surface. The velocity in the center column of Table 1 is equal to the calculated terminal velocity for a particle whose size is given in the right column of that table. Here, the cleaned part surfaces become infected with particles of all sizes.
There are valid reasons to chose both high and low air velocities, depending upon what will happen to your parts after processing.
Vacuum drying, another method of evaporating water, can resolve the dilemma. Vapor pressure and vaporization temperature are correspondingly reduced. Hence evaporation rate is enhanced without having to use high air velocities.
Drying can be done by converting the liquid water to a solid, liquid, or a gas. One saves time and money by using impact-based methods to remove liquid water before using evaporation. The latter is expensive and can make clean parts dirty.