Last month I needed to segue into distillation to provide the foundation for this month’s article on condensation. When we speak of condensation the condensed liquids involved will be water and some type of dry cleaning solvent. Condensing a vapor to a liquid can be accomplished by two methods. 1. Cooling a vapor, 2. Pressurizing a vapor. For this article we will be dealing with cooling.
Since condensing water and solvent vapors are somewhat similar, let’s think about condensation as it applies to water. We’ve all listened to weather reports which include the term %RH (percent relative humidity), seen water droplets on a cold glass and wiped a bathroom mirror after a shower (well at least some of us have wiped the mirror). All three of these involve the phenomenon of condensation. Here are the definitions of the words we’ll be dealing with:
GAS – for our purposes a gas will mean air.
VAPOR – molecules of solvent andor water dissolved in the air.
HUMIDITY – the amount of solvent andor water held in the air.
CONDENSE- change from a vapor to a liquid.
The hotter the gas the more vapor a gas can hold, the lower the pressure the more vapor a gas can hold. With these physical laws in mind % relative humidity is the amount of vapor the gas is actually holding divided the maximum amount of vapor that a gas can hold at a given temperature and pressure. What happens when the % relative humidity approaches 100% (the maximum amount of vapor a gas can hold)? At or near this point the vapor will change back into a liquid. This change is known as condensation and it is the flip side of evaporation which we covered in last month’s article on distillation.
Are you all enthused now? Good, because we are going to reveal one of the great mysteries of dry cleaning machinery by coupling the Condenser (the cylindrical device at the end of the large pipe coming out of the top of your Still) to the still. When you look at the Still Condenser you see a large cylindrical device with the Still Riser (the large pipe coming out of the top of the still going into the side of the condenser), two smaller Coolant Lines (two pipes usually coming out the top of the condenser) and a Condensate Outlet line (a large pipe exiting from the bottom of the condenser. Inside the still condenser, connected to the coolant lines is a spiral coil (also called a serpentine coil) which is a heat exchanger. The coolant flows through this spiral coil. The still riser conducts hot solvent vapor laden gas (which is air) from the still into the still condenser. Keep in mind that the gas entering the condenser is at 100% relative humidity; the hotter the gas the more vapor it can hold, conversely, the colder the gas the less vapor can be held. The hot solvent laden air flows around this spiral coil which adsorbs heat from air, cooling it. As soon as the solvent laden air is cooled the vapor changes into a liquid (condenses) and falls to the bottom of the still condenser housing. Inside most still condensers there are also baffles which direct the vapor and solvent flow. Along with solvent vapor, water vapor is also condensed into a liquid. There needs to be some method to separate these two liquids before the condensed solvent enters the Rinse Tank (the tank on the dry cleaning machine that typically stores distilled solvent). Fortunately, solvent and water are immiscible (they do not easily mix together or dissolve into each other; they act like oil and water) and they both have different specific gravities. Perc is heavier; DF-2000, Green Earth and K4 are lighter than water. These qualities allow the use of “specific gravity separation” in a container amazingly called a Water Separator. The water separator directs the solvent into the rinse tank and the water to a small storage tank for later disposal outside the dry cleaning system. This “separator” or “contact water” is contaminated. It must be handled as a hazardous waste and hauled away or treated in some manner before legally being disposed of on location at the dry cleaning plant. (In a blatant plug for one of my product lines, EZtimers Manufacturing, manufactures and sells inexpensive devices that will allow you to dispose of this water safely and legally saving hazardous waste hauling cost. Visit our web site at eztimers.com for more information).
Seems simple enough, what could go wrong? Well, not much except the flow of coolant through the spiral coil. Like any heat exchanger, there will be an accumulation of scale over time which will decrease the flow of coolant. This will gradually decrease the efficiency of the condenser to a point where the condensate will heat up, causing a decrease of the efficiency of solvent/water separation which can result in water contamination of the rinse tank. A rule of thumb for evaluating condenser performance is: At the upper side of the condenser it is too hot to touch for more than a moment and at the bottom it should not be much more than warm to the touch. Of course, this is just a qualitative test but it works well. Be cautious, the upper inch will burn you if you leave your finger on it for more than a moment. If the bottom is too hot look for a water flow problem or a still boil over. Most modern machines have a thermostat sensor monitoring the outlet of the condenser; if there is a problem a machine error message and/or shut down is executed.
That’s it for this month. Don’t exactly know what to do next month. Any suggestions (JUST SHUT UP! will not be treated as a valid suggestion) send them to me I’m reachable at: bruce@eztimers.com