Last month I focused on the operation of typical cooling systems used in the
process. This month I’ll write about heat and what’s going on during the drycleaning process
that necessitates a cooling system.
What are we really talking about when we use the term “cooling” in reference to the
A more accurate term would be
“heat moving,” for what we are
actually doing is moving heat from
one place to another. “Heat moving”
is accomplished by exchanging the
heat from one “fluid” (for our
applications the definition of a fluid
is a gas or liquid) to another fluid.
This “heat exchange” takes place
in a device strangely enough called a “heat exchanger.” The generic term most frequently used
for heat exchangers in the drycleaning/laundry industry is a “coil.” The type of coil used to
exchange heat with air is the “finned coil;” the type of coil used to exchange heat with still vapor
and liquids is the “serpentine coil.”
There are three operations performed in the typical drycleaning machine in which
exchanges take place; drying, distillation and solvent cooling.
I’m going cover the fourth- and fifth-generation machine only and because of space
limitations I won’t get into a detailed study of refrigeration or the carbon adsorption system.
I’m going to use the dry cycle as an example for the heat exchange study because
incorporates all of the pertinent heating and cooling components. Future
articles will contain
detailed descriptions of other machine cycles.
Drying is that portion of the drycleaning cycle which removes liquid solvent
from the garments
after the cleaning portion of the drycleaning cycle. Drying is all about air
flow and temperature
changes in that flowing air.
First heat exchange: Freon to air
The air flow is first heated in the “heat pump coil.” Hot, high pressure Freon gas leaving the
refrigeration compressor flows through a solenoid into the tubes of this coil,
heating the attached
plates which in turn heats the air flowing through the coil.
Second heat exchange: Steam to air
More heat is added to the air stream as it flows through the “steam boost coil.” This coil is
surprisingly heated by steam. Steam flow through the coil is regulated by a
controlled steam shut off valve (used to control drying temperature).
Inside the coils tubes the steam changes state, condensing into water. This
condensation releases heat (puts heat into) the metal of attached plates,
further heating the air
flowing through the coil.
Third heat exchange: Air to Freon
This air, rich in solvent vapor, then flows through the “cooling or evaporator coil”. Liquid Freon
droplets are sprayed into the coil tubes using a metering device called an “expansion valve.”
Inside the tubes the droplets change state (evaporate) into Freon gas. This
absorbs heat (takes heat away, cools) from attached plates; in effect cooling
the air stream
passing through. Cooling the air condenses the solvent vapor present in the air
stream back into
liquid solvent (liquid solvent flows into the water separator and the separated
solvent then flows
back into a storage tank for reuse). The air with most of the solvent removed is
in the heat exchange coil in a continuous loop during the dry cycle.
Fourth heat exchange: Freon to water
The air stream has at this point flowed through the entire drying pathway. Heat
removed from the air stream in the evaporator coil and transferred to the Freon
drying process needs to be shed.
The device that accomplishes this is call the “refrigeration condenser.” Hot Freon vapor exiting
the evaporator coil is drawn into the “refrigeration compressor” where it is compressed into a
very hot dense gas and forced into the refrigeration condenser. Inside the
condenser there is a
length of serpentine coil through which cool water (coolant) is circulated. The
heat from the Freon, cooling and condensing it back into a liquid for
re-circulation to the
Fifth heat exchange: Water to air, Freon or water
Last in the chain of heat exchanges is the removal of heat from the coolant.
accomplished in any of three ways:
1. City water. Water from the city main is forced through the drycleaning machines
refrigeration condenser propelled by existing city water pressure. The exiting
water is then sent
down the drain. In this case there is no further heat exchange.
2. Water tower. Water is circulated by a pump through the drycleaning machine’s
refrigeration condenser; over an evaporative cooling media (looks like
corrugated plastic cubes)
which has air forced though it to aid in evaporation; and 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
drops into the sump for recirculation through the drycleaning machines
In this case the heat exchange is from water to air.
3. Chiller. Water or a mixture of water and glycol (this mixture is called a brine) is
by a pump through the drycleaning machine’s refrigeration condenser; cooled by a separate
refrigeration system; and re-circulated through the drycleaning machines
In this case, the heat exchange is from water to Freon to either water or air
what media is used to cool the refrigeration condenser in the chiller.