Page 166 - Boiler_Operators_Handbook,_Second_Edition

Refrigeration & AC 151

EVAPORATORS moisture from the air but freeze it on the coil surface.
As that ice builds up it serves as an insulator restricting
Evaporators absorb heat by evaporating the re- the flow of heat. Timers, or devices monitoring either
frigerant. Ignoring, for the moment, the pressure drop the superheat of the refrigerant or temperature of the
associated with the friction of flow through the evap- fins on the coil, are used to initiate a defrosting cycle.
orator the heat is absorbed at a constant temperature, Defrosting of freezer coils can be accomplished in two
the saturation temperature of the refrigerant at the pres- ways. One way is using an external heating source, nor-
sure in the evaporator. With the exception of chillers all mally found in frost-free refrigerators, where an electric
evaporators are intentionally operated to evaporate all heating element is used to melt the ice on the coils while
the liquid before it leaves the evaporator. Once all the the refrigeration compressor is temporarily shut down.
liquid is evaporated the refrigerant, as a gas, absorbs ad- You may have opened the freezer side of the refrigerator
ditional heat to become superheated. The absorption to and heard the hissing sound of melted water dripping
superheat is intentional in order to prevent liquid enter- on the heating element. In larger commercial freezers
ing the compressor and damaging it. The superheat is defrosting is achieved by first shutting off the supply of
maintained automatically in most systems by the throt- liquid refrigerant to the evaporator then admitting hot
tling device. refrigerant gas coming from the discharge of the com-
The evaporator most people can relate to is the one pressor. Drain pans under those coils are designed to
that’s visible in the window air-conditioning unit when separate the ice that falls off the coils from the melted
you remove the cover and filter so you can clean the fil- water so the water is drained. Sometimes maintenance
ter. You should recall that what you see are rows of tub- is required to remove the ice. Fins on the cooling coils of
ing wrapped with aluminum that looks shredded or the a walk-in freezer would not permit effective defrosting
tubing runs through sheets of aluminum. We commonly because they would hold the ice in place. However, fins
refer to those as fins. We also refer to it as extended sur- on the cooling coils of your refrigerator are effectively
face. Heat transfer from a solid to a boiling liquid and defrosted by the heater. In both the walk-in freezer and
through highly conductive metal is much faster than it is your refrigerator flow of air over the coils while defrost-
between a metal surface and air. Therefore an evaporator ing is reduced by shutting off air circulating fans.
to cool air is designed with a lot of surface area where It pays to check the condition of the drains that re-
the metal contacts the air. An evaporator in a window move the water melted off the coils after a defrost cycle
air conditioner cools the air and removes moisture by to ensure they are flowing clear. The drains in a typical
condensing it. The velocity of the airflow over the cool- household freezer drop the water into an external pan
ing surface has to be restricted to prevent blowing the positioned at the condenser cooling air outlet where the
droplets of moisture off the coil and into the circulating heated air can evaporate the water. The tubing between
fan or the room. The additional surface compensates, to the pan installed in the freezer to collect the melted ice
a degree, for the loss of turbulence that would be pro- water and the external pan or drain can get plugged
vided by higher airflow rates. The fins are also designed with chunks of ice to block flow. That results in the pan
to accelerate the drainage of the condensed water off the inside the freezer overflowing to make the bottom of
heat transfer surfaces. the freezer into an ice rink, a dangerous condition for a
That water cooler you stop to drink at has an evap- walk-in freezer.
orator that’s a coil of two tubes, one inside the other, Heat transfer between metal and flowing water
with the refrigerant inside the inner tube and the wa- occurs at a rate much faster than between metal and
ter that you’re drinking in the annular space between flowing air. Therefore you will seldom see fins on tubing
the two tubes. Heat transfer rates from metal to flowing used for cooling water or freezing water. Chillers fre-
water is higher than it is to a gas like air so fins aren’t quently have a form of fin that will be covered later.
economically justified on something so small that only When making ice the standing water and the ice
requires a little extra length of tubing to make up the dif- itself restrict heat flow. That requires additional surface
ference between heat transfer to boiling liquid and heat which has to be smooth to permit removal of the ice. Ice
transfer to flowing liquid. makers can use one of the two methods described for
If you entered a walk-in freezer you would prob- defrosting freezers to melt a thin layer of the ice at the
ably note few if any fins on the cooling coils. That’s surface of the freezer to allow removal of the ice.
because fins would hinder defrosting and maintaining You might see what looks like a P trap used in
cleanliness. A freezer’s cooling coils not only condense plumbing in the piping of an evaporator or the gas pip-