Air conditioning and refrigeration systems

1. Air Conditioning And Refrigeration
Systems
Air conditioning is the process of treating and distributing air to control temperature,
humidity, and air quality in selected areas. For temperature and humidity control,
air is moved over chilled or heated coils and/or a spray of water at a controlled
temperature. Direct water sprays also remove dust and odors.
Refrigeration is the process of lowering the temperature of a substance below that
of its surroundings and includes production of chilled water for air conditioning or
process applications. Chilled water for use in processes such as injection molding
may be in the same temperature range as chilled water used for air conditioning.
Refrigeration systems are also used to provide chilled antifreeze solutions (brines)
at temperatures below the freezing point of water. Brines are used in icemaking
and cold storage, in addition to a variety of chemical process applications..
Many methods are used to produce and distribute chilled air. In central air
conditioning systems, air is passed over coils chilled by water, by brine, or by direct
expansion of a volatile refrigerant. The chilled air is then distributed through
ductwork.
The basic mechanical components of an air conditioning system are the air and
water distribution systems, a refrigeration machine, and a heat rejection system.
Refrigeration for air conditioning is usually provided by either absorption or
compression cycles.
Absorption refrigeration uses low-pressure steam or high-temperature hot water as
the energy source, water as the refrigerant, and lithium bromide or lithium chloride
as the absorbent.
Compression refrigeration systems generally utilize a halocarbon compound or
ammonia as the refrigerant. An internal combustion engine, turbine, or electric
motor supplies the power to drive a centrifugal or positive displacement
compressor.
Refrigeration, or cooling, occurs when the liquid refrigerant absorbs heat by
evaporation, generally at low temperature and pressure. When the refrigerant
condenses, it releases heat to any available cooling medium, usually water or air.
SINGLE-STAGE REFRIGERATION CYCLE

2. The basic refrigeration cycle used for single-stage vapor compression has four
components in the system. They are the compressor, condenser, metering device,
and evaporator. Low-pressure liquid refrigerant in an evaporator extracts heat from
the fluid being cooled and evaporates. The low-pressure vapor is then compressed
to a pressure at which the refrigerant vapor can be condensed by the cooling
media available. The vapor then flows to the condenser, where it is cooled and
condensed. The liquid refrigerant flows from the condenser to a metering device,
where its pressure is reduced to that of the evaporator. The cycle is thus
completed.
In industrial or commercial air conditioning systems, the heat is usually rejected to
water. Once-through cooling may be used, but municipal restrictions and water
costs generally dictate recirculation and evaporative cooling processes.
Evaporative condensers or cooling towers are normally used for evaporative
cooling. A spray pond may be used as an alternative. Recirculation of the water in
a cooling system reduces the makeup water requirement to less than 3% of the
water that would be needed for once-through cooling.
Cooling capacity is measured in tons of refrigeration. A ton of refrigeration is
defined as the capacity to remove heat at a rate of 12,000 Btu/hr at the evaporator
or chiller.
An absorption refrigeration system that removes 12,000 Btu/hr (does 1 ton of air
conditioning) requires heat energy input of approximately 18,000 Btu/hr to drive the
absorption process. This means that the heat rejection at the cooling tower
approximates 30,000 Btu/hr per ton of refrigeration. With a 15°F (8°C) temperature
drop across the tower, the heat rejection of an absorption system requires
circulation of approximately 4 gpm of water per ton of air conditioning. Evaporation
of the recirculating water occurs at a rate of approximately 3.7 gph per ton.
Other than the solution and refrigerant pumps, there are no moving parts in an
absorption system. Although this is an economical design advantage, the cost of
producing the necessary low-pressure steam or high-temperature hot water
(HTHW) must also be considered.
Compression systems also impose an additional heat load. This is due to the
energy required to compress low-pressure, low-temperature refrigerant gas from
the evaporator and deliver it to the condenser at a higher pressure. The
compressor energy input is approximately 3,000 Btu/hr per ton of refrigeration.
Accordingly, normal heat rejection in a compression system approximates 15,000
Btu/hr per ton of refrigeration, requiring evaporation of about 2 gal/hr of cooling
water.

3. Compression refrigeration systems require a cooling water circulation rate of
approximately 3 gpm per ton of refrigeration, with a 10°F temperature drop across
the cooling tower.
The major energy consumer in a compression refrigeration system is the
compressor, which is designed to operate at a certain head pressure for a given
load. This pressure equals the refrigerant pressure in the condenser. The term
"high head pressure" refers to condenser pressure that is higher than it should be
at a specific load condition.
High head pressure can be costly in two ways. First, it presents the danger of a
system shutdown; a safety control will stop the compressor motor when the safe
maximum head pressure is exceeded in the compressor. Second, an increase in
power consumption results when a compressor operates at greater than design
head pressure.
Fouled condenser tubes are a common cause of high head pressures. Fouling
increases the resistance to heat transfer from the refrigerant to the cooling water.
In order to maintain the same heat transfer rate, the temperature of the refrigerant
must be increased. The compressor fulfills this need by increasing the pressure at
which the refrigerant is condensed. With a centrifugal chiller, a 1°F increase in
condensing temperature increases compressor energy consumption by
approximately 1.7%.
Fouling and the formation of scale in absorption systems also reduce operating
efficiency. Because the highest water temperatures exist in the condenser,
deposition first occurs in this unit. Under extreme conditions, scale formation can
also occur in the absorber.
Deposition in the condenser imposes a higher back-pressure on the generator, so
that increased steam or HTHW is required to liberate the refrigerant from the
absorbent. The result is an increase in refrigerant vapor pressure and a greater
temperature differential between the condensing water vapor and the cooling
water. Although this compensates for the resistance to heat flow, more energy is
required to provide the increased heat input.
If water conditions are severe enough to cause deposition in the absorber, less
refrigerant is removed by the absorber, and cooling capacity is reduced. The
reduction in refrigerant circulation diminishes the ability of the equipment to satisfy
cooling requirements.
If the absorption rate in the absorber is reduced while the absorbent is heated
above the normal temperature in the generator, the danger of over-concentrating
the brine solution also exists. This over-concentration can cause crystallization of
the brine, leading to a system shutdown.

4. Fouling and scale formation waste energy and can ultimately cause unscheduled
system shutdown. Effective water treatment can minimize the possibility of high
head pressure and excessive steam consumption caused by condenser
deposition.
Corrosion can cause problems in either the open recirculating or chilled water
circuits. When corrosion is not properly controlled, the resulting corrosion products
inhibit heat transfer, increasing energy consumption in the same manner as fouling
and scale formation. Unchecked corrosion can cause heat exchanger leaks and
catastrophic system failures.
In any cooling application, attention to cooling tower operation is important. Proper
tower maintenance maximizes cooling efficiency, or ability to reject heat. This is
critical for continuously running refrigeration machinery at full load conditions.
For best performance, the cooling tower fill should be kept clean and protected
from deterioration. The water distribution system must provide uniform wetting of
the fill for optimal air-water contact.
Other components, such as drift eliminators, fill supports, regulating valves,
distribution decks, and tower fans, should be kept clean to maintain efficient heat
rejection. Inefficient cooling or heat rejection increases the temperature of the
water in the cooling tower sump and, consequently, that of the water sent to the
condenser. This makes it necessary to condense the refrigerant at a higher
temperature (absorption) or higher temperature and pressure (compression) to
reject heat at the same rate into the warmer water. This increases the amount of
energy (steam, hot water, electricity) required to operate the system.