DESIGN OF REFRIGERATION AND AIR
CONDITION SYSTEM (BME 475)
• Topic: Refrigeration Equipment & Controls
Patricia Kwakye-Boateng (Meng)
Vapor Compression System Components
Compressor
Evaporator
Hot Gas Valve
Condenser
Thermal Expansion Valve
Vapor Compression System Components
Evaporator
Evaporator
EVAPORATOR CAPACITY
• The amount of heat absorbed over a given period of time.
• Heat transfer capacity of an evaporator:
• Q= UA(T2 -T1) ..... W or J/s
• U = Overall heat transfer coefficient
• A = Area of evaporator surface
• T2 = Temperature of medium to be cooled
• T1 = Saturation temperature of refrigerant at evaporator pressure
FACTORS AFFECTING THE HEAT TRANSFER CAPACITY OF
AN EVAPORATOR
1.MATERIAL
2.TEMPERATURE DIFFERENCE
3.VELOCITY OF REFRIGERENT
4.THICKNESS OF THE EVAPORATOR COIL WALL
5.CONTACT OF SURFACE AREA
TYPES OF EVAPORATORS
According to type of construction
1.Bare tube coil evaporator
2.Finned tube evaporator
3.Plate evaporator
4.Shell and tube evaporator
Bare tube coil evaporator
Finned tube evaporator
(Prime surface evaporator)
• Over the bare tube metal fins are
fastened.
• Easy to clean and defrost
• Used natural convection to produce • Shape, size, spacing can be adapted
for better rate of heat transfer.
the necessary air motion.
• Fans were installed to increase • Adding the fins to the pipe increases
the effective surface area, leading to
efficiency and lower costs, the coil
increased
heat
transfer
and
size was reduced.
evaporator capacity, but at the same
time creating a greater air flow
restriction.
Plate evaporator
•The bare coils are either welded on the
plate or between the two plates (usually
made of aluminum because it conducts
heat well) which are welded together.
• Liquid refrigerant flows through a tube
going through the plates.
• The plate type was designed to improve
the efficiency of the bare tube design.
• This design may be used in a home
refrigerator where food would actually
be put on the evaporator. By doing this,
it would increase the efficiency of the
heat transfer rate.
Shell and tube evaporator
•Can handle fluids at high temperatures
and pressures.
•Available in flooded as well as dry
expansion type.
•Baffle plates are provided for good
turbulence of liquid; Cap. 2-250TR
Shell and tube evaporator
Baffles
serve
two
functions;
• Strategic
positioning
supports the tubes,
preventing
vibration
and sagging.
• Channel the fluid in the
shell side, resulting in
more efficient heat
transfer.
TYPES OF EVAPORATORS Contd
According to the manner in which liquid refrigerant is fed
1.Flooded evaporator
2.Dry expansion evaporator
Dry Expansion Evaporator
Liquid refrigerant is generally fed by an
expansion valve.
• Control of the rate of flow of refrigerant to
the evaporator in such a way that all the
liquid is vaporized and the vapor is also
superheated to a limited extent by the time
it reaches the outlet end.
• evaporator in its length is filled with a
varying proportion of liquid and vapor. The
amount of liquid in the evaporator will vary
with the load on the evaporator.
• The inside of the evaporator is far from
‘dry’ but wetted with liquid; the refrigerant
reaches the evaporator outlet it is no more
wet (no liquid) but dry (superheated) vapor.
•
Flooded Evaporator
• Certain flooded evaporators can be fitted with a device
(sometimes called a surge chamber) that prevents
liquid from going back to the compressor and return
liquid from going back to the evaporator.
• In larger systems, commonly found on centrifugal
chillers.
• The flooded evaporator (cooler), contains tubes (the
number varies greatly with design and capacity).
• The refrigerant circulates around the tubes and is
called the primary refrigerant.
• The secondary refrigerant is R-718, more commonly
known as water. This water is sent through the tubes
and then circulated to various chilled-water coils to
remove heat.
• https://www.youtube.com/watch?v=G
EX-skmDnuI
TYPES OF EVAPORATORS
According to the mode of heat transfer
1.Natural convection evaporator
2.Forced convection evaporator
Natural convection Evaporators
••Low velocity and min. hydration is
require.
•Velocity of air depends upon temp.
difference.
•Circulation of air around coil depends
upon its size shape and location.
•The coil should occupy 2/3rdof width
of the path & 3/4ththe length of the
box.
Forced convection
•Air is forced over refrigerant coils
•Fins are provided to increase heat
transfer rate
•More
efficient
than
natural
convection evaporators
•Require less cooling surface and high
evaporator pressure can be used which
save power input to the compressor
TYPES OF EVAPORATORS
According to operating condition
1.Frosting evaporator
2.Non-frosting evaporator
3.Defrosting evaporator
Frosting evaporators
•Operates below 0°C
•The frost forms on the evaporator
comes from the moisture of the air.
•Cooling efficiency is decreases until
the ice and frost is removed.
Non-frosting evaporators
•Operates above 0°C therefore frost
does not forms on evaporators.
•Temp. close to cooling 0.6°C to 1°C.
•RH from 75-80 % in the cabinet
•This keeps the food fresh & stops
shrinking in weight.
http://www.appliance411.com/faq/defrostproblem.shtml
DEFROSTING EVAPORATORS
•Frost creates on the coils when the
compressor is running & melts after
the compressor shuts off.
•Temp. of about -7°C to -6°C
•It also keeps high RH of about 90% to
95%.
http://www.appliance411.com/faq/howdefrostworks.shtml
Vapor Compression System Components
Compressor
“Heart of
the system”
Compressor Functions
Increases pressure
Increases temperature
60º
125º
So ambient seems cool
Pumps
Refrigerant
To condense vapor
CONDENSER
Providing liquid to
metering device
AMBIENT AIR 95o
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Classification of compressors
◉ Positive displacement
• ○ Positive displacement compressors have chambers which decrease in
• volume during compression.
◉ Non-positive displacement (centrifugal).
• ○ Non-positive displacement compressors have fixed-volume chambers.
◉ Beyond this distinction, each type differs based on its
specific
• mechanism for fluid compression.
Classification of compressors
◉ Open types have a separate housing for the compressor and the
motor.
◉ They rely on lubricant in the system to splash on pump
components and seals.
◉ If not operated frequently, the system can leak its operating
gases.
◉ Open compressors can be driven by non-electric power sources
such as combustion engines.
Classification of compressors
◉ Hermetic types seal the compressor and motor together in the same
housing.
○ These compressors are leak-free and can sit for long periods unused,
• but cannot be maintained or repaired.
◉ Semi-hermetic types also contain the motor and compressor in one
housing, but instead of a one-piece housing they incorporate gasketed/bolted
covers.
○ These can be removed for maintenance and repair of the compressor or motor.
Suction Cooled
Hermetic Compressor
“Hermetically
Sealed”
Discharge Hot Gas
Suction Vapor
Welded Shell
Motor
Spring mounted
Pistons & Valves
Crankshaft
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Water & Air Cooled
IN
OUT
KW Series
KA Series
Water Cooled
Air Cooled
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Copelametic® Semi-Hermetic Compressor
Overload
Head
Motor Terminals
Valve Plate
Piston
Motor:
Rotor and Stator
Crankshaft
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Types of comprssors
◉ There are basically 5 types of air conditioner compressors
that are commonly used in the HVAC industry:
• Reciprocating
• Scroll
• Screw
• Rotary
• Centrifugal
Reciprocating/Piston Compressors
◉ Piston and cylinder arrangement to
provide compressive force - like IC
engines.
◉ Reciprocating motion of the piston due
to external power compresses the
refrigerant inside the cylinder.
◉ Low initial cost and a simple, easy to
install design.
◉ Large power output range - can reach
extremely high pressures. However,
maintenance costs are high, potential
vibrational issues.
◉ Not typically designed to run
continuously at full capacity.
Compressor Operation:
Intake stroke
Piston drops
• Lowers pressure in cylinder
• Pulls intake valve open
• Vapor is sucked into cylinder
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Suction vapor “sucked”
into compressor
Valve opens
Piston
Downstroke
Piston Downstroke
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End of Intake Stroke
Piston at bottom
• Cylinder pressures equalize
• Intake valve closes
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Valve closes
Pressure equalized
Piston at bottom
Piston at bottom
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Compression Stroke
Piston rises
• Increases pressure in cylinder
• Rising to discharge pressure
• Discharge valve opens
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Pressure increased
Piston rises
Compression Stroke
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Valve opens
Discharge to
Vapor fully Condenser
compressed
Piston at Top
Top of Stroke
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Compressor Operation:
Compression Stroke
Piston at top
• Cylinder pressures equalize
• Discharge valve closes
• Small amount of discharge gas remains
(This is called “clearance volume”)
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Clearance volume
Valve closes
Suction valve
can now open
Hot gas re-expands
Start Downstroke
Start Downstroke
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Compressor Operation:
Downward Stroke
Piston drops
• Gas in clearance volume re-expands
• Cylinder pressure drops
Note, reciprocating compressor
inefficiency:
Compressor energy is wasted during reexpansion
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Valve closes
Compression stroke
starts next cycle
Piston at Bottom,
Cycle Complete
Cycle is done
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Complete Compressor Cycle
Each cycle:
Suction
• And re-expansion
Compression
•
And discharge
Following are 3 continuous cycles:
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Complete Cycle
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Cycle Complete
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Reciprocating Compressor – Volumetric Efficiency
Volumetric Efficiency :
Ratio of free air delivered to the displacement of the compressor.
6
3
2 Ratio of Effective Swept Volume to Swept Volume.
P2
Presence of Clearance Volume
 Volumetric Efficiency less than 1. ( 60 – 85 % )
PV n = C
5
4
P1
V3
Clearance Volume,
V3=Vc
V4
1
Effective Swept Volume
Volumetric Efficiency =
V1
Swept Volume
V1 – V4
=
Effective Swept Volume, V1-V4
Swept Volume, V1-V3=Vs
V1 – V3
Clearance Volume
Clearance Ratio =
Total Volume, V1
=
Swept Volume
Vc
= γ ( 4 – 10 % )
Vs
Reciprocating compressor action in VCR
Rotary Compressors
◉ Uses circular motion for compression
(two rotating elements).
◉ Rotating blade (vane) type
• Refrigerant is trapped by rotating vanes
• Refrigerant compresses as volume
decreases.
◉ Stationary blade (vane) type
• Equipped with only one blade or vane
Rotary Compressor by Tecumseh
Discharge out
Suction in
Rotary Vane
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Helical rotary (screw type compressors)
◉ Use a pair of helical rotors or screws
which mesh together to compress the
refrigerant between them.
◉ low to medium initial and maintenance
costs and few moving parts.
◉ A continuous, flowing output is produced
◉ Compressor capacity is controlled by a
slide valve
◉ high pressure for a small quantity of gas
and consume less power.
◉ difficulty in dirty environments, high
rotational speeds, and shorter life
expectancies than other designs.
• Scroll type Compressors
◉ Utilizes two identically machined
scrolls – one scroll is stationary, the other
orbits
◉ The nesting of the scrolls traps vapor
◉ Gas is introduced from the outer edge
refrigerant is discharged from the center
◉ Scroll compressors are quiet, smoothoperating units with few moving parts
◉ Highest efficiency ratio of all
compressor types.
◉ However, as fully hermetic designs,
they cannot be easily repaired. They also
typically cannot rotate in both directions.
Scroll Compressors
by Bristol
Discharge Gas
Suction
Courtesy of
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• Centrifugal type compressors
◉
Rely on centrifugal force - utilizes an
impeller
◉ No pistons, valves or cylinders
◉Do not operate on the
positive
displacement principle, but have fixed
volume chambers.
◉
Well suited to compressing large volumes
of refrigerant to relatively low pressures.
◉ Capacity is controlled by inlet vanes.
◉ Simple design, few moving parts,
and
energy efficiency when operating multiple
stages.
◉ Typically used for very large applications
Lubrication
There MUST be an oil film on
every load bearing surface!!!!
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Lubrication of crankshaft and rods
Crankshaft:
Hollow center for oil supply
Holes in shaft lubricate bearing surfaces
Connecting rods:
Connect the crankshaft to the piston
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Crankshaft, Connecting
CrankshaftRods,
oiling and Pistons
Piston
Connecting Rod
OIL
Crankshaft
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Compressor Lubrication
To Cylinders
To Bearings
From
Oil
Pump
Back to
Crankcase
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Sight Glass
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Proper level of oil
• Crankcase must have enough oil
• Level of oil in sight glass must be visible
Too high: Can’t see top of oil
Too low: Can’t see any oil
Good: Anywhere in sight glass
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Oil Sight Glass
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Compressor Specifications
◉ There are a number of specifications to consider when selecting
compressors.
◉ These include
○ Compressor capacity
○ Condensing temperature
○ Evaporating temperature
○ Refrigerant flow rate
○ Power consumption
Compressor specifications: Capacity
◉ Capacity (kW/hr – BTU/hr) measures the ability of a refrigerant compressor
to remove heat from the refrigerant gas.
◉ Nominal capacity ratings are based on a standard set of conditions which
include condensing temperature (CT), evaporative temperature (ET),
refrigerant, and motor rpm.
◉ Typically, refrigeration compressors and air conditioning compressors can
run at many different values for these parameters, with corresponding
changes in their cooling capacity.
◉ Once in use, compressors can be tweaked and adjusted to the desired
capacity and operating conditions.
Compressor specifications: Other parameters
◉ Condensing temperature is the range of condensing
temperatures over which the compressor is rated to operate.
◉ Evaporating temperature is the range of evaporative
temperatures over which the compressor is rated to operate.
◉ Flow rate is the rate (by mass) at which the fluid is passed
through the compressor, measured in kilograms per hour (kg/hr).
◉ Power (W) is the input power required to run the compressor
motor at a specific operating point.
Compressor Features
◉ RAC compressors incorporate a number of special features:
• Low noise - compressor operation generates less noise for applications
where a quiet environment is desired.
• Light weight - compressor is compactly built or constructed with lowdensity materials for cooling systems which require low weight
components.
• Variable speed - compressor has speed adjustment for running at various
operating flow rates and conditions.
• Thermal shut off - compressor features controls which turn the
compressor off at high temperatures to prevent it from overheating.
They also can provide restart once the compressor has cooled down
below a certain temperature.
• Sealing - describes how the compressor and motor drive are situated in
relation to the gas or vapor being compressed.
Condenser
▪ Heat is rejected
▪ The heat can be reclaimed.
Various types:
▪ Air-cooled
▪ Water-cooled
▪ Evaporative
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CONDENSER
❖ Condenser Classifications
❖Based on the external fluid,
condensers can be classified
as:
a) Air cooled condensers
(natural convection type or
forced convection type)
b) Water cooled condensers
(Double pipe or tube-in-tube
type, Shell-and-coil type, Shelland-tube type)
c) Evaporative condensers
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CONDENSER TYPES
wire-and-tube type condenser
evaporative condenser
2-pass, shell-and-tube type condenser
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Expansion device
❖ Functions
▪ Such
as
thermostatic
expansion valve (TXV).
▪ Changes the refrigerant from
high temperature high.
▪ Reduces
pressure
and
temperature.
▪ Controlls the refrigerant flow.
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Expansion Device
❖Classifications
Fixed opening type or variable
opening type. Include;
1. Hand (manual) expansion valves
2. Capillary Tubes
3. Orifice
4. Constant pressure or Automatic
Expansion Valve (AEV)
5. Thermostatic Expansion Valve (TEV)
6. Float type Expansion Valve
a) High Side Float Valve
b) Low Side Float Valve
7. Electronic Expansion Valve
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Auxiliary Components
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Auxiliary Components cont’d
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Thank you…?