UNIT-7
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Refrigeration System Operating Characteristics
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General
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Refrigeration systems must operate at all hours of the year, even when the
building is unoccupied. Warmer weather tends to push refrigeration equipment
to its capacity limit, thus creating a maximum operating kW and kWh.
Evaporators
- must be selected to provide the
required cooling at all expected ambient conditions even with the maximum
frost on the coils (i.e., just prior to defrosting). Evaporator coils used
include two types of refrigeration systems: flooded evaporator and direct
expansion. For direct expansion systems, two of the most commonly used
refrigerant liquid metering devices are the capillary tube and the
thermostatic expansion valve.
In addition, proper provisions must be made for
periodic defrosting of evaporator air-side surfaces. Defrosting may be
accomplished using refrigerant compressor discharge hot-gas, water spray, or
manually as selected to meet the user's objectives. Suitable drain
connections should be provided to carry off the water resulting from defrost
operations.
Condensers
- must be selected to operate at all
outdoor weather conditions in the area. Air-cooled condensers must be
supplied with the proper controls to permit operation at low outdoor ambient
conditions. Water-cooled condensers may require water regulating valves to
keep condensing pressure high enough to enable the thermal expansion valves
to function. The type of condenser selected depends largely on the size of
the cooling load, refrigerant used, quality and temperature of available
cooling water (if any), and noise considerations.
Water-cooled condensers require cooling
water from an external cooling tower, or from a lake, well, river or other
similar source. Once-through use of city water for condensing purposes is
prohibited in most locations. Air-cooled condensers are the most popular
since they avoid other problems of water acquisition, treatment and disposal.
The trade-off may be higher electrical consumption. As seen here, the
evaporative condenser is a combination of a water cooled condenser and an
air-cooled condenser that rejects heat through the evaporation of water into
an airstream traveling across a condenser coil.
Compressors
- must be sized to meet the varying needs
of each application. Provision must be made to protect the compressor from
liquid carry over from the evaporator, in addition to the normal safety
controls (high and low pressure cutout. oil pressure, etc.). The most common
type of compressor used for commercial refrigeration systems is the
reciprocating compressor. Reciprocating compressor types include single-stage
(booster or high state), internally compounded, and open, hermetic or
semi-hermetic.
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HEAT TRANSFER
The second important law of
thermodynamics is that heat always travels from a warm object to a colder one.
The rate of heat travel is in direct proportion to the temperature difference
between the two bodies.
Assume that two steel halls
are side by side in a perfectly insulated box. One ball weighs one pound and
has a temperature of 400° F., while the second ball weighs 1,000 pounds and has
a temperature of 390° F. The heat content of the larger ball is tremendously
greater than the small one, but because of the temperature difference, heat
will travel from the small ball to the large one until the temperatures
equalize.
Heat can travel in any of three
ways: radiation, conduction, or convection.
Radiation is the transfer
of heat by waves similar to light waves or radio waves. For example, the sun's
energy is transferred to the Earth by radiation. One need only step from the
shade into direct sunlight to feel the impact of the heat waves, even though
the temperature of the surrounding air is identical in both places. There is
little radiation at low temperatures, and at small temperature differences, so
radiation is of little importance in the actual refrigeration process. However,
radiation to the refrigerated space or product from the outside environment,
particularly the sun, may be a major factor in the refrigeration load.
Conduction is the flow of
heat through a substance. Actual physical' contact is required for heat
transfer to take place between two bodies by this means. Conduction is a highly
efficient means of heat transfer as any service-man who has touched a piece of
hot metal can testify.
Convection is the flow of
heat by means of a fluid medium, either gas or liquid, normally air or water.
Air may be heated by a furnace, and then discharged into a room to heat objects
in the room by convection.
In a typical refrigeration
application, heat normally will travel by a combination of processes, and the
ability of a piece of equipment to transfer heat is referred to as the overall
rate of heat transfer. While heat transfer cannot take place without a
temperature difference, different materials vary in their ability to con-duct
heat. Metal is a very good heat conductor, while asbestos has so much
resistance to heat flow it can be used as insulation.
What
is refrigerant:-
At any hardware store or electronics
store you can buy a can of electronics duster. This is just a spray can that
blasts a stream of gas when you pull the trigger. You use it to blow the dust
out of tiny crevices in electronic circuits. Your teacher is going to bring
some cans of electronics duster to class and you can feel for yourself what
happens when you spray this stuff. If you spray it long enough, the can will
get very cold. It can even get cold enough to give you frostbite.
If you're inquisitive, you may be
asking "what gas does the electronics duster shoot out?" It so
happens that the gas is a hydrofluorocarbon, or an HFC. Remember that HFCs are the
family of compounds used to replace chlorofluorocarbons as refrigerants. It's obvious from feeling
the can that HFCs can make things cold, but just how do they do that?
Heat and Changes of
State
This is a change of state, of
course. As you may remember, for every change of state there is a heat of transition.
When a solid becomes a liquid, it absorbs heat in the process of melting. This
is called the heat of melting. When a liquid becomes a gas, it absorbs
heat in the process. This is called the heat of vaporization. This works
backward, too. When gases condense to become liquids they give off heat, and
when liquids freeze to become solids, they give off heat as well.
Heat of Vaporization
and Your Refrigerator
A refrigerator works in the same
way. In a refrigerator, an HFC is pumped through a tube called a coil, like you
see in the animation below. In the coil, there is a plug with a small hole in
it called a throttle valve. Because this opening is so small, pressure builds
up behind the throttle valve, enough for the HFC to become a liquid. Slowly,
the HFC passes through the throttle valve. On the other side of the throttle
valve, the pressure is not as high. So the boiling point of the HFC drops low
enough for the HFC to evaporate. As it evaporates, the HFC absorbs heat from
its surroundings, specifically the inside of the refrigerator. The inside of
the refrigerator then gets cold.
But there's more to this story. The
HFC keeps moving through the coil. The coil passes to the outside of the
refrigerator and to the compressor. The compressor puts pressure on the HFC,
which condenses back into a liquid, and the whole process can start all over
again.
What Makes a Good Refrigerant?
Why is it so hard to find a good
refrigerant? To be a good refrigerant, a compound has to live up to a few
requirements. Obviously, we want something that is nontoxic. We also want
something that is unreactive. The refrigerant has to be stable for the lifetime
of the refrigerator. In addition, we want a compound that is ozone-safe. But on
top of all these criteria, we need a compound that has a low boiling point. So
why not use nitrogen (N2)? It's nontoxic
(we breathe it all the time), the atmosphere is already full of it, and its
boiling point is way down at -196°C. That's a little too low, as we'll soon
see. We want a refrigerant to a have a low boiling point but not too low,
because when a refrigerator is running, the refrigerant is constantly being
boiled from a liquid to a gas, and then being condensed back into a liquid
again. If the boiling point is too low, it will be hard to condense back
into a liquid.
VAPOUR
COMPRESSOR SYSTEM
Vapor compression refrigeration is the primary method used to provide
mechanical cooling. All vapor compression systems consist of four basic
components (plus the interconnecting piping): evaporator, compressor,
condenser, and an expansion device. The evaporator and condenser are heat
exchangers that evaporate and condense the refrigerant while absorbing and
rejecting heat. The compressor takes the refrigerant vapors from the evaporator
and raises the pressure sufficiently for the vapor to condense in the
condenser. The expansion device controls the flow of condensed refrigerant at
this higher pressure back into the evaporator.
Historically, the common refrigerants were R-11, R-12, R-22, and
compounds in the R-500 series. With the CFC phaseout, new refrigerants have
been developed to replace R-11 and R-12 in new equipment. These new
refrigerants can also be used to retrofit existing equipment in many cases.
However, these retrofits are not "drop-ins" and should be done by
trained technicians.
Food processors often use ammonia (R-717). While potentially hazardous,
ammonia is inexpensive and environmentally benign. Experts anticipate wider use
of ammonia due to concerns over CFC phase-out. Interestingly, R-22 was
developed as a safe alternative for cooling systems that would perform best at
ammonia refrigerant characteristics.
The manufacturer selects the specific refrigerant used in any equipment to
best match the cooling system design and size. The availability and cost of
these refrigerants and the consequences of refrigerant leaks and disposal have
become very serious concerns for today's building owners and the design
community. Each of these issues is addressed in other areas of this interactive
knowledge program.
Vapor Compression Systems - The
Evaporator
The evaporator and condenser are both heat exchangers. Whether they move
heat to or from air or water or refrigerant is merely a matter of design. On
the design day the evaporator typically cools either:
1. Air
returning from the building space (or outside air) to ~ 55 - 60°F
2. Water from about ~ 54°F as it returns from
building air handlers to ~ 44°F.
In both cases the evaporator
boils the selected refrigerant to provide this cooling. The pressure at which
the refrigerant boils is exactly that which satisfies the energy balance of
heat-in equals heat-out.
The refrigerant is circulated through numerous parallel paths. As the
refrigerant flows and evaporates along these paths the pressure will drop as
well. This in turn drops the temperature of the refrigerant as it evaporates.
Consequently, properly designed direct expansion coils operate with the coldest
refrigerant temperatures closest to the coil exit. However, the refrigerant
temperature coming out of this coil is usually a little warmer than this to
provide some level of superheat to be sure liquid refrigerant isn't leaving the
coil and entering compressor (where it could cause mechanical failure in some
designs).
Shell and tube heat exchangers commonly have water circulated through the
tubes and refrigerant boiling around the tubes. There are also designs where
refrigerant flows within the tubes and water flows over the tubes. Baffles are
normally used in this case to direct water flow in a serpentine fashion to
optimize heat transfer. Almost all large chillers use shell and tube
evaporators with water flowing through the tubes.
Vapor Compression Systems -
Evaporator Control
In comfort cooling applications, actual cooling loads are seldom at
full load conditions. Capacity control is achieved in finned coil evaporators
that directly chill air by splitting the coil into independent sections. The
principal reason is to permit coil sections to be activated and deactivated to
better match coil cooling capacity with compressor loading. The combination of
smaller coil sections controlled by correspondingly sized expansion valves
improves valve performance and part load humidity control.
Capacity control in shell and tube evaporators is usually handled using the
return water temperature. For example, if the full-load temperature range for
chilled water is from 44°F to 54°F, water returning at 50°F indicates the
cooling load is about 60%. Liquid refrigerant is metered to the evaporator to
match the load using an orifice plate system or an expansion valve. On large
chillers, the expansion valve is pilot operated.
Vapor Compression Systems -
The Condenser
The refrigerant is recovered by condensing it in a heat exchanger using air
or water to reject the heat. Air cooled condensers are most common in smaller
sizes, up to about 200 ton capacity. Technically, there is no upper limit on
the size of an air cooled condenser, but operating cost issues usually dictate
water cooled units for applications over about 100 tons.
There are two water cooled designs: cooling towers and evaporative
condensers. Both work on the principal of cooling by evaporating water into a
moving air stream. The effectiveness of this evaporative cooling process
depends upon the wet bulb temperature of the air entering the unit, the volume
of air flow and the efficiency of the air/water interface.
Evaporative condensers use water sprays and air flow to condense refrigerant
vapors inside the tubes. The condensed refrigerant drains into a tank called a
liquid receiver. Refrigerant subcooling can be accomplished by piping the
liquid from the receiver back through the water sump where additional cooling
reduces the liquid temperature even further.
Cooling towers are essentially large evaporative coolers where the cooled
water is circulated to a remote shell and tube refrigerant condenser. Notice
the cooling water is circulating through the tubes while refrigerant vapor
condenses and gathers in the lower region of the heat exchanger. Notice also
that this area "subcools" the refrigerant below the temperature of
condensation by bringing the coldest cooling tower water into this area of the
condenser. The warmed cooling water is sprayed over a fill material in the
tower. Some of it evaporates in the moving air stream. The evaporative process
cools the remaining water.
The volume of water used by both evaporative condensers and cooling towers
is significant. Not only does water evaporate just to reject the heat, but
water must be added to avoid the buildup of dissolved solids in the basins of
the evaporative condensers or cooling towers. If these solids build up to the
point that they foul the condenser surfaces, the performance of the unit can be
greatly reduced.
Defrosting
Defrosting is a procedure, performed periodically on
refrigerators
and
freezers
to maintain their operating efficiency. Over time
water
vapour in the air condenses on the cooling elements within the cabinet. It
also refers to leaving frozen food at a higher temperature prior to cooking.
Defrosting a freezer
The resulting
ice
inhibits heat transfer out of the cabinet increasing running costs. Furthermore
as the ice builds up it takes increasing space from within the cabinet -
reducing the space available for food storage. Defrosting the unit is achieved
by:-
- Temporarily removing all food from the cabinet.
- Turning off power to the unit.
- Leaving the doors to the unit open
- Waiting for the ice to melt and draining it appropriately.
Using a towel is advisable when completing this step.
The process may be sped up by mechanical removal of ice, or the introduction
of gentle heat into the cabinet. Placing a pan of hot water in the cabinet and
closing it is an effective method. Using a fan to blow in room temperature air
will also greatly speed up the melting process as well as help to evaporate the
damp surfaces. Note that the fastest manual way is to use a vacuum cleaner:
simply insert the hose into the exhaust port (nearly all are designed for
this), and use the wand to blow on the coils; this method is much faster than
any other.
Any mechanical removal of ice should be done gently so that the equipment is
not damaged.
It is generally recommended that defrosting should be done annually.
Many newer units employ
automatic defrosting (often called
"frost-free" or "no frost") and do not require manual
defrosting in normal use.
AC Compressor
Air conditioning is the cooling and
air for comfort, the term can refer to any form of cooling, heating or
ventilation that modifies the condition of air. An air conditioner is an
instrument, system, or machinery designed to calm down the air temperature and
humidity within an region, typically using a refrigeration cycle but sometimes
using evaporation, commonly for comfort cooling in buildings and motor
vehicles.
Humidity control
Air conditioning tools usually reduces the humidity of the air. From the
processed air the coil evaporator condenses the water vapor, (much like an
ice-cold drink will condense water on the outside of a glass), sending the
water to a drain and removing water vapor from the cooled space and reducing
the relative humidity. Since the human perspire to make himself cool by the
evaporation of perspiration from the skin, drier air (up to a point) improves the
comfort provided. The comfort air conditioner is designed to create a 40% to
60% relative humidity in the occupied space.
Relative Humidity
The amount of water vapor in the air
at any given time is usually less than that required to saturate the air. The
relative humidity is the percent of saturation humidity, generally calculated in relation to saturated vapor
density.
The most common units for vapor density are gm/m3. For example, if the actual
vapor density is 10 g/m3 at 20°C compared to the saturation vapor density at that temperature of 17.3 g/m3 , then the relative
humidity is
What is Humidification?
It is the artificial regulation of
humidity in home environments, industrial environments, and health care
applications such as artificial respiration. To be comfortable, people require a certain amount of
ambient humidity -- not too high, and not too low. Adequate humidification in a manufacturing environment stabilizes moisture in wood,
paper, and textiles, while preventing warping in glue joints. In all
environments, humidification
reduces fire risk and static electricity while making the area feel
comfortable.
In humidification, two quantities are commonly used. Absolute
humidification is
expressed in grams of moisture per cubic volume of air, while the more commonly
used relative humidification is expressed as a ratio between the amount of moisture
currently in the air and the maximum moisture the air could hold before condensation occurs. A typical comfortable level of relative humidification is between 35% and 50%. Excess humidity can cause the
growth of mold or fungus. Too little humidity can cause static discharge or the
accumulation of unwanted dust, contributing to allergies.
Many humidifiers are cheap and
require little maintenance. In industrial settings, they are often hung from
the ceiling among duct work. Humidification is intimately tied to heating and cooling systems. The
level of humidity in the air is also a function of the temperature. Therefore, humidity control systems are often integrated with cooling systems.
Dehumidifier
A dehumidifier is mostly a household appliance that reduces the level of humidity in the air, usually for health reasons, as humid air can
cause mold and mildew
to grow inside homes, which has various health risks. Relative humidity is
preferably 30 to 50%.[1] Very high humidity levels are also unpleasant for human
beings, can cause condensation and can make it hard to dry laundry or sleep. Higher
humidity is also preferred by most insects, including clothes moths, fleas
and cockroaches. Dehumidifiers are used in industrial climatic chambers for
keeping certain level of humidity.
Dew point Control
Dew Point Control, LLC
(DPC) is an equipment
leasing company that provides an array of choices for hydrocarbon dew point
control. Working within an industry that is in constant flux, DPC strives to
match each customer's needs with the most cost efficient technology.
·
Meet
transporting pipeline hydrocarbon dew point specifications.
·
Capture
NGL liquid upgrade income.
·
Capture
additional income by removing the "crude" component prior to NGL
processing to avoid processing, transportation, fractionation and marketing
fees.
·
Controlling
gathering system liquid drip for efficiency and safety reasons.
·
Improve
measurement volumes recorded by orifice custody transfer meters by removing the
liquid buildup on the orifice plates.
·
Prove
the potential of new gathering systems prior to the capital commitment of the
deep liquid NGL recovery facility.
·
Short
term lease to move gas to market during the construction phase of a deep liquid
recovery NGL facility.
·
To
provide additional short term capacity to a deep NGL liquid recovery facility
by conditioning gas that bypasses the existing facility or leaning the NGL
component of the gas entering the NGL liquid recovery facility.
Types of air conditioner equipment
Window and through-wall units
Room air conditioners come in two
forms: unitary and packaged terminal PTAC
systems. Unitary systems, the common one room air conditioners, sit in a window
or wall opening, with interior controls. Interior air is cooled as a fan blows
it over the evaporator. On the exterior the air is heated as a second fan blows
it over the condenser. In this process, heat is drawn from the room and
discharged to the environment. A large house or building may have several such
units, permitting each room be cooled separately. PTAC systems are also known
as wall split air conditioning systems or ductless systems.[5] These PTAC systems which are frequently used in hotels have
two separate units (terminal packages), the evaportive unit on the exterior and
the condensing unit on the interior, with tubing passing through the wall and
connecting them. This minimizes the interior system footprint and allows each room
to be adjusted independently. PTAC systems may be adapted to provide heating in
cold weather, either directly by using an electric strip, gas or other heater,
or by reversing the refrigerant flow to heat the interior and draw heat from
the exterior air, converting the air conditioner into a heat pump. While room
air conditioning provides maximum flexibility, when cooling many rooms it is
generally more expensive than central air conditioning.
Evaporative coolers
In very dry climates, evaporative
coolers are popular for improving comfort during hot weather. This type of
cooler is the dominant cooler used in Iran,
which has the largest number of these units of any country in the world,
causing some to referring to these units as "Persian coolers." An evaporative cooler is a device that draws
outside air through a wet pad, such as a large sponge soaked with water. The sensible heat of the incoming air, as measured by a dry bulb
thermometer,
is reduced. The total heat (sensible heat plus latent heat) of the entering air is
unchanged. Some of the sensible heat of the entering air is converted to latent
heat by the evaporation of water in the wet cooler pads. If the entering air is
dry enough, the results can be quite comfortable; evaporative coolers tend to
feel as if they are not working during times of high humidity, when there is
not much dry air with which the coolers can work to make the air as cool as
possible for dwelling occupants. Unlike air conditioners, evaporative coolers
rely on the outside air to be channeled through cooler pads that cool the air
before it reaches the inside of a house through its air duct system; this
cooled outside air must be allowed to push the warmer air within the house out
through an exhaust opening such as a open door or window.[7]
These coolers cost less and are
mechanically simple to understand and maintain.
Portable air conditioners
Portable air conditioners (or PACs) are moveable units that can be used to
cool a specific room in a home and do not require permanent installation.
[8] Warm air in the room is drawn in
through inlets on the portable air conditioner. The air is circulated through
the unit and is cooled by evaporator coils with refrigerant running through
them and then blown out through the front. Remaining hot air in the unit is
expelled and vented through the back with an exhaust hose.
[9] All portable air conditioners
require exhaust hoses for venting.
Single Hosed Units
A single hosed unit has one hose that runs from the back of the portable air
conditioner to the vent kit where hot air can be released. A single hosed
portable air conditioner can cool a room that is 475 sq. ft. or smaller and has
at most a cooling power of 12,000 BTUs.
[10]
Dual Hosed Units
Dual hosed units are typically used in larger rooms. One hose is used as the
exhaust hose to vent hot air and the other as the intake hose to draw in
additional air (usually from the outside). These units generally have a cooler
power of 12,000-14,000 BTUs and cool rooms that are around 500 sq. ft.
[10] The reason an intake hose is
needed to draw in extra air is because with higher BTU units, air is cycled in
large amounts and hot air is expelled at a faster rate. This creates negative
air pressure in the room, and the intake hose stabilizes the room's air
pressure.
[9]
Split Units
Portable units are also available in split configuration, with the
compressor and evaporator located in a separate external package and the two
units connected via two detachable refrigerant pipes, as is the case with fixed
split systems.
Split
portable units are superior to both single and dual hosed mono-portable units
in that interior noise and size of the internal unit is greatly reduced due to
the external location of the compressor, and no water needs to be drained from
the internal unit due to the exterior location of the evaporator.
A drawback of split portable units compared with mono-portables is that a
surface exterior to the building, such as a balcony must be provided for the
external compressor unit to be located.
Heat and Cool Units
Some portable air conditioner units are also able to provide heat by
reversing the cooling process so that cool air is collected from a room and
warm air is released. These units are not meant to replace actual heaters
though and should not be used to cool rooms lower than 50 °F (10 °C).
Central air conditioning
Central air conditioning, commonly referred to as
central air (
U.S.)
or
air-con (
UK), is an air conditioning system which uses ducts
to distribute cooled and/or dehumidified air to more than one room, or uses
pipes to distribute chilled water to heat exchangers in more than one room, and
which is not plugged into a standard
electrical
outlet.
With a typical
split system, the condenser and compressor are located
in an outdoor unit; the evaporator is mounted in the
air handler
unit. With a
package system, all components are located in a single
outdoor unit that may be located on the ground or
roof.
Central air conditioning performs like a regular air conditioner but has
several added benefits:
- When the air handling unit turns on, room air is drawn in from
various parts of the building through return-air ducts. This air is pulled
through a filter
where airborne particles such as dust and lint are removed.
Sophisticated filters may remove microscopic pollutants as well. The filtered air is
routed to air supply ductwork that carries it back to rooms. Whenever the
air conditioner is running, this cycle repeats continually.
- Because the condenser unit (with its fan and the compressor) is
located outside the home, it offers a lower level of indoor noise than a
free-standing air conditioning unit.
Mini (Small) Duct, High Velocity
A central air conditioning system using high velocity air forced through
small ducts (also called mini-ducts), typically round, flexible hoses about
2 inches in diameter. Using the principle of aspiration, the higher
velocity air mixes more effectively with the room air, eliminating temperature
discrepancies and drafts. A high velocity system can be louder than a
conventional system if sound attenuators are not used, though they come
standard on most, if not all, systems.
[11]
The smaller, flexible tubing used for a mini-duct system allows it to be
more easily installed in historic buildings, and structures with solid walls,
such as
log
homes. These small ducts are also typically longer contiguous pieces, and
therefore less prone to leakage. Another added benefit of this type of ducting
is the prevention of foreign particle buildup within the ducts, due to a
combination of the higher velocity air, as well as the lack of hard corners.
[12]
