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Is Parts Drying Hot Air or Something Else?


John Durkee, PhD, PE

This column discusses why parts drying is a problem with aqueous cleaning agents used as replacements for some solvents. I wrote about this topic in 1989. I have frequently updated the material, to protect the guilty. But basically, drying of parts is similar to how it was handled years earlier, and before that.

For most, drying is synonymous with evaporation. Drying of water by evaporation can be the most costly and time-consuming operation in metal cleaning. It frequently causes more problems than soil removal. Drying can be nonevaporative.

There are other methods for getting water off parts. If you are considering replacing your current cleaning/rinsing/ drying system, you should consider some of these alternative methods. They are described in this column. There is guidance about when they should and should not be used.

An Introduction

The phaseout of CFCs (chloroflurocarbons) as cleaning agents caused a global revolution in the way products are manufactured and repaired. After January 1, 1996, it became illegal to manufacture and sell CFCs identified as capable of depleting ozone from the earth’s stratosphere. This change was accomplished while meeting stringent regulations about emissions to water and air.

CFCs provided both cleaning and drying functions. The drying function was attained by evaporation of the cleaning agent. Dry parts could be obtained in just a few minutes without surface defect (residue) or effort on the part of the user. The cleaning agent was the drying agent.

That capability essentially is gone—forever.1 There are other quick-drying cleaning solvents,2 but their use adds problems not faced by users of the banned materials.

Firms doing product or maintenance cleaning with CFC-113 (“Freon” 113) or TCA (1,1,1-trichloro-ethane) had to find another way of getting clean and dry parts.

Drying Process Problems

Drying usually is a synonym for evaporation. There are three types of problems with drying of cleaning agents:

  • Aqueous and semi-aqueous ones dry (evaporate) and leave surface residues (often called “watermarks”).
  • Nonaqueous ones dry (evaporate) and leave a environmental problem (VOCs), a safety problem (flammability) or a human problem (odor) with the emissions.
  • Aqueous, semi-aqueous, and nonaqueous ones don’t dry (evaporate) well from internal part sections.

These problems occur when a cleaning agent is replaced, as the chemical structure isn’t the same as that of the replaced materials.

Why Is Drying of Water Difficult?

As stated above, drying generally means evaporation of water. It takes a lot of energy.3 The heat of vaporization of water is about five times higher than that of solvents, and it takes a long time to evaporate a little water.4 For example, 100 SI of surface wetted with 10 mils of water film (a typical number for a wet part), contained about 15 grams of water. To evaporate (dry) this small amount of water in five minutes from those 100 SI might require 7,500 ft3 of air heated to 212°F.

The rate of drying parts is limited by the rate at which heat can be transferred to the water, causing it to evaporate. Slow heat transfer from heated gas to wet parts is normally the rate-limiting process step. Even worse, air doesn’t have a high capacity to carry heat or water. Consequently, huge volumes of hot air can be required.

Here is an example of why drying water from parts can require a lot of energy and time. Suppose you have a 1-qt. stainless steel saucepan half-full of water. The pan is on an electric cooktop, and you want to evaporate all the water in 5 minutes. This is an easy job, no doubt.

The energy demand to do this evaporation of the water is equivalent to one ton of refrigeration (12,000 BTU/hr). But since we have to heat the stainless steel saucepan as well, to evaporate the water in it, the energy requirement is equivalent to the refrigeration requirement for cooling of a large home.

Now, suppose we want to do this job without heating the saucepan! That’s what we do when we dry parts. We don’t normally heat all the parts to the temperature necessary to cause evaporation at a sufficient rate. That would be likely to damage most parts.

The conventional approach would be to get a hair dryer and blow hot air across the top of the saucepan. That’s how drying of parts is usually done. Can you imagine how long that would take you? Hours.

Suppose the job is just to evaporate the top surface skin or film of water. That’s what’s done in parts drying. You’re still going to need a lot of air. My estimates are around 1,500 CFM heated to the boiling point of water.

In other words, drying with hot air combines two inefficient processes in series: Heating of air and humidification of that hot air.

Drying Without Evaporation

Methods of drying other than open evaporation should be considered when replacement cleaning agents are used. These methods are called nonevaporative. There are at least six different methods: Centrifugal force; displacement by insoluble material.5 The solvents used for displacement drying can be PFs (perfluorinateds), HFEs (hydrofluoro ethers of which there are two flavors), HCFCs (hydrochlorofluorocarbons), OSs (methyl siloxanes), or HFCs (hydrofluorocarbons); drainage (gravity force) enhanced by vibration; entrainment into moving stream of air (vacuum); dislodgement by high velocity air; and evaporation under vacuum where liquid is recovered.

There is efficient commercial equipment to implement three of these methods while others can be implemented by your fabrication staff.6 It is quite common for a site to construct its own drying equipment. The most commonly used are centrifugal force, displacement by insoluble material, and evaporation under vacuum. These six methods for drying without evaporation are described in Table 1.

Table 1: Nonevaporative Drying Methods
Nonevaporative Drying Method Application Positives & Negatives
Centrifugal force Very low cost. Practical only for parts that will fit into dryer ( i.e., “smaller than a
breadbox”). Liquid recovered for reuse. Can dry some complicationed internal
sections. Commercially available.
Displacement by insoluble materials Uses PFs, HFEs, HCFCs, OSs, or HFCs.
Useful for “spot-free” drying of complicated sections. Liquid recovered for reuse. Commercially available.
Drainage (gravity force) enhanced by vibration Cheap and easy, but only for removes and
recovers approximately two-thirds of liquid.
May be useful for goods about to be stored. Custom applications.
Entrainment into moving stream of air (vacuum) Practical only for flat sections, like sheet. Dry to “the touch.” Liquid recovered for reuse. Custom applications.
Blowoff by high-velocity air Useful for small parts without internal sections. Dry to “the touch.” Liquid recovered for reuse. Custom applications.
Evaporation under vacuum where liquid is recovered Very expensive. Used only for drying last approximately 100 ppm of “moisture.” Commercially available.

 

Selection of the Proper Method

What is the right drying method for your situation? The answer depends on two factors. Both are covered below. They are:

  • The nature of your parts, and;
  • the degree of dryness you require.

Specific recommendations for a variety of situations are listed in Table 2. Other recommendations are possible based on additional information.

Table 2: Recommendations for Nonevaporative and Evaporative Drying Processes
  Parts Can Have Some Remaining "Moisture" Dryness to "the Touch" Is Satisfactory Very High Level of Dryness Is Needed
Nature of Parts Aqueous or Semi-aqueous Cleaning Agents Solvent Cleaning Agents Aqueous or Semi-aqueous Cleaning Agents Solvent Cleaning Agents Aqueous or Semi-aqueous Cleaning Agents Solvent Cleaning Agents
Parts "smaller than a breadbox," have simple or no internal sections Drainage or centrifugal Drainage or centrifugal Centrifugal, hot air, or high-velocity air blowoff Centrifugal , or high-velocity air blowoff Hot air, followed by vacuum evaporation Centrifugal, possibly followed by vacuum evaporation
Parts "larger than a breadbox," have simple or no internal sections Drainage Drainage Hot air, or high-velocity air blowoff High-velocity air blowoff, or hot air Extended time with hot air High-velocity air blowoff, followed by extended hot air
Parts "smaller than a desk," have very complicated internal sections Displacement by insoluble materials Displacement by insoluble materials Displacement by insoluble materials Displacement by insoluble materials Displacement by insoluble materials Displacement by insoluble materials

 

The belief is that the cleaning agent should be chosen based on the nature of the soil, and the rest of the process be chosen based on the nature of the parts. Most parts drying problems are described in the middle column of Table 2. Dryness “to the touch” is satisfactory and the liquid being removed is either a solvent or water.

Thus the two most common drying techniques can be hot air (for water removal), and centrifugal dryers (for either removal of water or solvent). High-velocity air blowoff is a useful second choice for either removal of water or solvent, but air consumption may be costly.

References

  1. CFC-11, CFC-12, CFC-113, methyl chloroform (1,1,1-trichloroethane, TCE, 111TRI, or MCF), halons, and carbon tetrachloride haven’t been manufactured in the U.S .and other countries after 1995.
  2. N-propyl bromide (n-PB) dries as does TCA, but its use replaces concerns about depletion of the earth’s ozone layer with concerns about human toxicity. Manufacture of TCA is banned. Use of n-PB is limited in the U.S. to where parts are dried in some piece of equipment, and not in the open air.
  3. HCFC-225 ca/cb, HFC-43-10 meet both types of HFEs (which are ethers), and the OS silicon-based solvents all force evaluation of a tradeoff between operating cost and investment because of their selling price.
  4. The use of acetone and methyl acetate frees users in the U.S. from concern about VOC regulation, but forces learning of the electrical safety codes because they are flammable.
  5. These choices, and many others, are described in more detail in the forthcoming book by this author: On Solvent Cleaning, published in 2005 by Elsevier, ISBN 185617 4328.
  6. The heat of vaporization of water is about five times higher than that of solvents.
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