With summer upon us your dry cleaning machines’ cooling system will be under greater stress. It doesn’t matter if you are using a water tower, chiller, or city water for cooling, conditions that weren’t an issue during the cooler months may become problems as the surrounding (ambient) temperature rises. Most of these hot weather problems will appear during the drying segment of the dry cleaning cycle and are caused by faulty cooling.
Drying, when used in the dry cleaning machine sense, is all about moving heat energy from one location to another. The media moving this heat is called a “refrigerant”, a fluid that absorbs heat when changing state from a liquid to a gas (evaporating) and releases heat when changing its state from a gas to a liquid (condensing). This cycle of evaporation and condensation is referred to as the “refrigeration cycle” and takes place within coils found in different locations within the dry cleaning machine. In dry cleaning machinery, the refrigerant used is almost always a Freon of some sort. Liquid refrigerant, upon entering a coil referred to as an “evaporator” rapidly changes state into a gas thereby absorbing heat and cooling the air passing through the coil. The hot, compressed refrigerant gas is forced under pressure by the “refrigeration compressor” into another type of coil called a refrigeration “condenser” where it is cooled by a liquid called a “coolant”, usually water. This cooling as well as the high pressure placed upon the system by the refrigeration compressor, causes the refrigerant to turn back into a liquid for reuse; the cycle repeats itself throughout the drying process.
Problems with the cooling system are most likely to appear towards the end of the dry cycle when the dry cleaning machine goes into cool down mode. During the drying mode, the heat exchanger coil is transferring much of the heat from the refrigerant back into the air stream and only a small portion of the heat load is being dumped into the refrigeration condenser. Near the end of the dry cycle, during the cool-down mode, solenoids and/or dampers dump almost the entire heat load into the refrigeration condenser. The extra heat load overloads the capacity of the refrigeration condenser to properly cool and condense the refrigerant. This type of problem is indicated by the refrigeration compressor entering a high-pressure condition which trips the refrigeration high-pressure safety switch. On most, but not all, machines, this would cause an error message on the programmer screen and require a manual reset of the refrigeration high-pressure safety switch.
This condition is almost always caused by either a clogged inlet Y strainer or excessive scaling inside the refrigeration condenser. On most dry cleaning refrigeration systems, there is a pressure-sensing coolant flow valve called a “modulating” valve controlling the refrigeration compressor’s outlet pressure by varying the flow of coolant through the refrigeration condenser. This valve is adjusted to give proper refrigeration head pressure for efficient refrigeration system operation.
When scale builds up in the refrigeration system past the point where the normal adjustment of the head pressure adjusting valve can no longer compensate for, the flow of coolant through the refrigeration condenser, many dry cleaners simply adjust the valve wider in the very mistaken belief they have fixed something. What they have actually done is “kicked the can down the road”, until the refrigeration high-pressure safety switch trips continuously and in the process will pay lots of extra money in solvent and energy costs as well as possibly damaging the refrigeration compressor for the privilege. Maintaining the proper heat transfer between the hot, high-pressure refrigerant gas and the flow of coolant through the condenser is usually where the problems occur during hot weather operations.
THREE METHODS OF SUPPLYING COOLANT TO THE REFRIGERATED CONDENSER
- CITY WATER– water from the city main is forced through the dry cleaning machines refrigeration condenser propelled by existing city water pressure. The exiting water is then sent down the drain.
- WATER TOWER– water is circulated by a pump through the dry cleaning machines refrigeration condenser; over an evaporative cooling media (looks like corrugated plastic cubes) which has air forced through it to aid in evaporation; into a sump where the suction inlet of the pump is connected. The evaporation of the water from the surface of the media cools the remaining water which drops into the sump for recirculation through the dry cleaning machines refrigeration condenser.
- CHILLER– water or a mixture of water and glycol (this mixture is called a brine) is circulated by a pump through the dry cleaning machines refrigeration condenser; cooled by a separate refrigeration system; re-circulated through the dry cleaning machines refrigeration condenser.
No matter what method is used for cooling and circulating the coolant through the refrigeration condenser, the efficiency of the process is dependent on two factors:
A. COOLANT FLOW– volume of coolant moving through the condenser.
B. HEAT-EXCHANGE EFFICIENCY– the ability of the condenser coil to exchange heat from the hot refrigerant to the coolant.
Each of these factors can be easily monitored using pressure and temperature as indicators of what is going on in the coolant system. A pressure gauge and thermometer at the inlet and outlet of the coolant system located near the dry cleaning machine are vital for monitoring the cooling system performance. Let’s take a look at what the pressure gauge can tell us about the condition of the coolant system. The pump must supply enough pressure as well as flow in sufficient quantity to push coolant through the components that require cooling on the dry cleaning machine (on most machines this would include not only the refrigeration condenser but also the still condenser and solvent cooler) and in the case of water towers and chillers sufficient pressure to lift the water back to the tower or chiller sump which is often located on the roof. Unfortunately, there is no hard-fast rule for what the pressure readings should be. The best method for obtaining the “numbers” is to note for future reference the inlet and outlet pressure and temperature readings when the dry cleaning machine is working properly. The following are some possible scenarios for a different combination of qualitative data:
1. Inlet pressure high/outlet pressure low
A. Check for blockage inlet flow path- a clogged Y strainer between the pump and the dry cleaning machine.
B. Heavy scale accumulation- a build-up of scale inside the piping and or condenser of the dry cleaning machine.
2. Inlet and outlet pressure rapidly changing:
A. Low coolant level in the pumping system.
3. Low inlet/high outlet pressure
A. Open bypass valve on pumping system
4. Low inlet/low outlet pressure
A. Pump off
B. Clogged inlet strainer
5. High inlet temperature/high outlet temperature
A. Water tower fan not working/chiller refrigeration system faulty
B. Pump off
6. Slightly higher than normal inlet temperature/high outlet temperature
A. Heavy scale accumulation- a build-up of scale inside the piping and or condenser of the dry cleaning machine.
B. Excessive heat source- still boil over or steam sweep valve open; steam supply valve to steam boost coil or carbon adsorber stuck open.
The accompanying illustration shows a schematic for a typical cooling water installation for use with a water tower or chiller WITH THE ADDITION OF VALVES 1,4,5 which are used when city water hook-up is included in the installation. This city water hook-up provides an emergency back-up for continuing operations when there is a failure in the normal cooling system (it will not help if excessive internal scaling is the cause of the problem).