Page 174 - Boiler_Operators_Handbook,_Second_Edition

Refrigeration & AC 159

centrifugal compressors is water chillers and you should Free Cooling
read about those later to learn more about the operation When it’s possible to obtain a source of fluid that’s
of centrifugal compressors. colder than what you want to cool, the refrigerant cycle
can function without the compression and expansion
(throttling) stages. The refrigerant gas will migrate (a
CONDENSERS term used to describe the flow of the gas because it’s
a natural thing) to the coldest spot in a system because
Once the compressor has increased the pressure that’s where it will condense if the pressure in the sys-
of the refrigerant high enough to produce a saturation tem is below the saturation pressure that matches that
temperature above the temperature of the substance that temperature. By assembling the equipment so the con-
receives the heat (heat removed in the evaporator and denser is above the evaporator the gas will condense in
added by the compressor) the condenser serves to trans- the condenser, giving up heat to a substance and the liq-
fer the heat from the refrigerant vapor to that substance. uid will drain to the evaporator where it can cool anoth-
Operating pressures and temperatures in the condens- er substance by boiling. When equipment is arranged to
er are dependent upon the substance used to cool the do this it’s called free cooling because no energy is input
condenser and its temperature. The condenser has to re- to compress the gas.
move the superheat, simply cooling the refrigerant gas Free cooling can also be accomplished with parallel
until it can start to condense, then condense it. systems that simply use the colder substance to remove
Because heat transfer from a gas to metal is a frac- heat from whatever it is you want to cool. One example
tion of the rate of heat transfer from a condensing liq- would be using cold river water to create chilled water.
uid to metal a fair portion of the condenser does noth- To be truly effective these systems have to allow warmer
ing but desuperheat the gas. Once the gas temperature chilled water temperatures. That’s perfectly acceptable
drops to saturation temperature the gas condenses to a because when the river water is cold it’s cold outside
liquid which drizzles over the heat exchange surfaces and the warmer chilled water will still do the job. Free
and drops to the bottom of the condenser. Many con- cooling can also be utilized in more complex systems
densers are designed so the hot superheated gas flows to pre-cool chilled water or cooling tower water before
around and through the condensed droplets because mechanical equipment is used to achieve the necessary
a small portion of them will be evaporated in cooling water temperature.
the superheated gas and the rest will be heated a bit
because the liquid in contact with the heat exchange Heat Pipes
surfaces is cooled below the saturation temperature of Devices that exchange heat between incoming ven-
the refrigerant. The evaporative cooling of the gas by tilation air and indoor air exhausted to make room for
the droplets is more efficient than the cooling of the the ventilation air can perform free cooling. Devices call
metal surfaces. heat pipes consist of a number of finned tubes sealed at
Finally, the condensed liquid must be subcooled in the ends, installed in a split casing, and sloped appropri-
most systems so it will remain a liquid until it reaches ately, can automatically transfer heat from warm indoor
the throttling device. The amount of liquid in a system exhaust air to hotter outside air and, with airflow re-
serves to balance the gas to liquid ratio in order to com- versed, heat cold outside air with warm indoor exhaust
pensate for variations in load and operating conditions. air. Each heat pipe is a refrigeration system unto itself,
Your refrigerator, window unit, automobile, and house the half at the lower end of the slope absorbs the heat
air conditioning systems hold varying amounts of liquid by evaporating the liquid refrigerant that drains down
refrigerant in the bottom of the condenser depending on into it. The evaporated gas then travels up the pipe to
those factors. The amount of subcooling is an indication the high end where it is condensed, thereby giving up
of how much liquid is retained in the condenser. A small its heat.
amount of subcooling can indicate insufficient refriger-
ant and a large amount would indicate excessive refrig- Air Cooled Condensers
erant. A measure of subcooling is a good measure of the You are familiar with many air cooled condensers.
refrigerant in a system when the liquid is stored in the You have one in your car, your refrigerator, your freez-
condenser but the subcooling has to be measured at a er if you have one, and your home air conditioning to
consistent condenser pressure. Systems with receivers, name the principal ones. These have to be designed for
discussed later, have other measures. operating temperatures of the cooling air at maximum