EVAPORATING THE SOLVENT – THE SOLVENT HEATING SECTION

Last month I explained the basic processes of the most common type of distillation system found in dry cleaning machinery. In this issue, I’ll be describing in more detail the components associated with the distillation process. There are many variations in the distillation systems used in dry cleaning; all of the items discussed, and those shown in the illustrations may not be present on your machinery. 

The steam heating section of the still contains the following components which correlate to the numbers shown in the circles on the accompanying Illustration. The numbers assigned to the paragraphs indicate the numbers attached to the components shown in the illustration:

1.    STEAM PRESSURE REGULATOR

Controls the pressure of the steam in the still heating system. Steam temperature directly correlates to steam pressure so what you are doing when you are adjusting the steam pressure is varying the temperature of the still heat. The correct steam pressure setting is usually between 40-60 lbs/sq. in.  This will vary depending on the machine manufacturer and the solvent being used so be sure to follow the suggested manufacturer’s setting.
Common problems encountered with this device are:

a.    REGULATOR SET TOO HIGH OR NOT REGULATING- If the pressure in the still is excessively high it’s possible to overheat the solvent and cause it to boil too violently. This forces the raw still content up the still riser pipe into the still condenser contaminating the condensed solvent (condensate) in the condenser, the condensate flow path throughout the machine, and in the water separator.  This contamination will usually contain dry cleaning detergent which when mixed with the water present in the condensate prevents the proper separation of water and solvent in the water separator. A milky white mixture of solvent or water can usually be observed when this condition is present.

b.    REGULATOR SET TOO LOW OR STUCK SHUT- Should the regulator provide no, or insufficient steam pressure solvent will not evaporate at the expected rate. This results in very slow or no distillation at all. Overfilling of the still will result If during the dry cleaning machine cycle solvent is automatically pumped to the still.

2.    STEAM CONTROL VALVE

This valve starts and stops the flow of steam to the still. Its operation is usually selectable from a button on the control panel and is also controlled by safety devices which I will be explaining later in the article.

Common problems encountered with this device are: 

a.    VALVE NOT OPENING- Most of the time this is a pneumatic (air system) problem. Either the air pressure to open the valve is too low which is likely an incoming air pressure regulator problem, or the air is blowing by the valve’s piston which used to open the valve. A symptom of this valve not opening would be no or very slow distillation.

b.    VALVE NOT SHUTTING- Usually a steam control valve sticking open is caused by a build-up of scale on the shaft connecting the valve disk to the piston causing friction which exceeds the force exerted by the spring which shuts the valve. Symptoms of this problem during normal distillation would be a continuously heated still which may result in higher utility costs. During a failure of some other component in the distillation system, this valve sticking open may result in a still boil-over. 

3.    STEAM PRESSURE RELIEF VALVE

This valve is used to relieve steam pressure in the still heating system if it exceeds safe design limits. It’s important to understand that this is the pressure in the steam jacket which surrounds the lower portion of the still not to be confused with the area of the still which contains solvent.
Common problems encountered with this device are: 

a.    VALVE NOT OPENING- If the steam pressure should exceed the limits of the still steam jacket there could be steam leaks or a rupture of some component. I have found no way of realistically testing this valve and have never seen one fail, so this paragraph may be an academic exercise.

b.    VALVE LEAKING- If this valve leaks the escaping steam would be obvious and likely require the replacement of the valve.

That’s it for now, I’ll be carrying on from here in the next issue.