CHAPTER 3: COMPRESSOR COMPRESSOR
ệ Lecturer : ThS.Nguyễn Duy Tuệ
g y
y
1
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Chapter 3 : Compressor
OBJECTIVES
components components and and
g co d o s o U de s a d e e ec o o
Student can: operation operation Understand - Understand principles of some kinds of refrigerant compressor - Understand the effect of working conditions on compressor’s efficiency
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REFRERENCE
[1] Trane document Compressor [1]. Trane document - Compressor [2]. Industrial refrigeration handbook – McGraw- Hill ( Chapter 4,5 ) Hill ( Chapter 4 5 )
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CONTENTS
COMPRESSORS TYPES COMPRESSORS TYPES
RECIPROCATING COMPRESSOR
SREW COMPRESSOR
SCROLL COMPRESSOR
CENTRIFUGAL COMPRESSOR CENTRIFUGAL COMPRESSOR
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COMPRESSOR TYPES
g ,[
g y
]) Reference :(Page 1,[1]) ( The purpose of the compressor in a refrigeration p system is: 1/ to raise the pressure of the refrigerant vapor from evaporator pressure to condensing p pressure. 2/ to remain the pressure in evaporator p p
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COMPRESSOR TYPES
g y ,
g A review of the refrigeration cycle, using the pressure–enthalpy chart, will help to illustrate this p point.
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COMPRESSOR TYPES
p yp p y
y ( ), g , There are primarily four types of compressors used in the air-conditioning industry: reciprocating, scroll, helical-rotary (or screw), and centrifugal.
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COMPRESSOR TYPES
p y
yp We also classify to three types of compressor: + Hermetic type compressor :
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COMPRESSOR TYPES
+ Open type compressor : yp p p
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COMPRESSOR TYPES
+ Semi-Hermetic type compressor : yp p
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RECIPROCATING COMPRESSOR
(p g ]) Reference (page 4, [1]) , [
g y ,
p g g p
1. Construction : The refrigerant vapor is compressed by a piston that is located inside a cylinder, similar to the engine in an automobile. A fine layer of oil prevents the g refrigerant vapor from escaping through the mating g surfaces.
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p y
y
p g y , The piston is connected to the crankshaft by a rod. As the crankshaft rotates, it causes the piston to travel back and forth inside the cylinder. This motion is used to draw refrigerant vapor into the cylinder, compress it, and discharge it from the cylinder.
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RECIPROCATING COMPRESSOR
Crankshaft, connecting rod and oil ring : g g ,
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p , A pair of valves,
p p g y the suction valve and the discharge valve, are used to trap the refrigerant vapor within the cylinder during this process.
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RECIPROCATING COMPRESSOR
Cylindrical valve
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RECIPROCATING COMPRESSOR
g p ,
g
During the intake stroke of the compressor, the piston travels away from the discharge valve and p , creates a vacuum effect, reducing the pressure within the cylinder to below suction pressure.
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g , During the compression stroke, p
, g g p p g
p y
pp g g y p the piston reverses its direction and travels toward the discharge valve, compressing the refrigerant vapor and increasing the pressure within the cylinder. When the pressure inside the cylinder exceeds the suction pressure, the suction valve is forced closed, p trapping the refrigerant vapor inside the cylinder.
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a - The p possesses p compressor
p p p
p g certain refrigerating capacity in kW (tons of refrigeration) p g means that the compressor is capable of pumping the flow rate of refrigerant that will provide the p stated refrigeration capacity at the evaporator.
g p g y
requirement of
y - The influences of evaporating and condensing p temperatures on the refrigerating capacity and power the compressor must be understood
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p g p
2. Effect of volumetric efficiency: the evaporating temperature on (page 97, [2])
p y The volumetric efficiency of a compressor, ην in , ην
percent, is defined by the equation:
The displacement rate is the volume rate swept g g p y through by the pistons during their suction strokes.
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RECIPROCATING COMPRESSOR
The volumetric efficiency is less than 100% y
because of such factors: p g p
g - Leakage past the piston rings - Pressure drop through the suction and
g discharge valves
- Heating of the suction gas when it enters the
y y y cylinder by the warm cylinder walls
- The reexpansion of gas remaining in the
g g y cylinder following discharge
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RECIPROCATING COMPRESSOR
Please observe this chart :
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RECIPROCATING COMPRESSOR
yp p
p p p Discus type of Copeland compressor which hasn’t got clearance volume that makes the re- expansion of vapour
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y p
p p g
( Example 4.1: What is the volumetric efficiency of an eight-cylinder Vilter 458XL ammonia compressor operating at 1200 rpm when the saturated suction temperature is -1°C (30.2°F) and the condensing ) temperature is 30°C (86°F)?
p The bore and stroke of y y ( )
the compressor are g 114.3 by 114.3 mm (4-1/2 by 4-1/2 in). The catalog lists the refrigerating capacity at this condition as ) 603.1 kW (171.5 tons). (
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+ Solution: The volume swept by one piston during a stroke
is:
p The displacement p
p y rate is the displacement volume of one cylinder multiplied by the number of cylinders and the number of strokes per second:
s.z.n. π.
V lt = lt
2d 4
Displacement rate:
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p y
py g
py p g )
g The mass flow rate can be computed by dividing the refrigerating capacity by the refrigerating effect. The enthalpy of ammonia leaving the condenser and entering the evaporator is 342.0 kJ/kg (138.7 Btu/lb) and the enthalpy leaving the evaporator is 1460.8 kJ/kg (619.6 Btu/lb). The mass flow rate m
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p g
g The specific volume of the refrigerant entering is 0.3007 m3/kg, so the actual
the compressor volume flow rate is:
Equation 4.1 can now be applied to compute : pp p q
Vtt = λ. Vlt lt
tt
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Note : Volume efficiency depend on : ratio pressure,
8
Band of volumetric efficiency of an 108L cylinder Sabroe ammonia compressor operating at 1170 rpm
manufacturer. Please observe this chart
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3. Influence of evaporating temperature on
refrigerating capacity :
p g p ( page 101, [2]) p q g , requirement, - Along with the power
the refrigerating capacity is a key characteristic of a compressor.
p - For a compressor p g p g y
g p p
g p to possess a certain refrigerating capacity means that the compressor is capable of compressing the flow rate of refrigerant from its suction pressure to its discharge pressure that will provide the specified heat-transfer rate at ) the evaporator (cooling load). (
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The overall equation that expresses the p q
q
V V
.
=
r
d
. h h Δ Δ ev
η 1η v v . 100 v
s
refrigeration rate is:
d
p ,
g g p where: qr = refrigeration rate, kW Vd = displacement rate, m3/s ην = actual volumetric efficiency, percent νs = specific volume of gas entering the s
compressor, m3/kg
ev
g g , g ∆hev = refrigerating effect, kJ/kg
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The volume rate of flow is available from a partp
of Eq.
flow measured at the
where V = volume rate of compressor suction, (m3/s)
j
g p g
The next objective will be to show the trend in the mass rate of flow m (kg/s), which can be done by introducing the specific volume of the suction gas y vs (m3/kg)
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p p
p p y
p g Effect of evaporating temperature on volume rate of flow measured at the compressor suction of an 8-cylinder compressor with a displacement rate of 0.123 m3/s (260 cfm) operating with a condensing temperature of 30°C
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p g p g p p
pp p p y
, compressors, the p p
An immediate observation from above is that the refrigerating capacity always decreases as the g evaporating temperature drops. At high evaporating temperatures the decrease in refrigeration capacity g is approximately 4% per °C and at low evaporating temperatures, near the maximum pressure ratios of g reciprocating in decrease refrigerating capacity is approximately 9% per °C
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4. Influence of condensing temperature on
refrigerating capacity :
q
V
.
=
r
d
. h Δ ev
q - Equation
s
p g ( page 105, [2]) η 1η 1 v . 100 v
p
p
once again becomes the tool, and all the terms g change as the condensing temperature varies with g the exception of the specific volume entering the compressor,(νs); which is a function of ,( s); the evaporating temperature only
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y y p g g
p p
g g p y
g
g entering volume p specific the on
The refrigerating capacity always decreases as the condensing temperature increases. Compared , the evaporating temperature, g to the influence of each degree change in the condensing temperature affects the refrigerating capacity to a lesser extent than a degree change in evaporating temperature. The reason for this difference is that changes in the evaporating temperature also exert a considerable the effect compressor, while the condensing temperature does not.
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The effect of changing condensing temperature g g g p
on refrigerant capacity
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5. Power q required a g reciprocating p
compressor: y by ( page 106, [2])
g p q p y Power required by a reciprocating compressor :
P = Power required if the compression is p q
adiabatic and frictionless, kW
ideal
p ,
p g g p
∆hideal = ideal work of compression, kJ/kg g This is one way to approach the question of how the evaporating and condensing temperatures affect the power requirement is to apply the equation
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C B , (
ideal
g) ∆hideal =hC-hB , (kJ/kg)
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g p ,
p the work until
p
g With a given condensing temperature, the ideal work of compression decreases as the evaporating , increases, of temperature compression shrinks to zero when the evaporating temperature reaches the same value as the condensing temperature.
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g ,
ideal, Now this figure shows the trends of m, ∆hideal, and the power as the evaporating temperature g changes while the condensing temperature remains constant.
of
Effect of evaporating temperature the ideal flow, flow the ideal on the mass rate of on the mass rate of work the and compression compressor power requirement. The condensing temperature is 30 C condensing temperature is 30°C
p g
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y p g q
g p g p
- Someone analyzing the power requirement of a reciprocating compressor for the first time may expect that raising the suction pressure will lighten the load on the compressor and lower the draw of p power.
- The range of pressure ratios against which y typically yp p g p
p reciprocating compressors operate is between about 2.5 and 8 or 9.
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g p
p
refrigeration plant,
g p , p
p precipitated p increase an y by is
p p
g
p - In this range of pressure ratios the power required by the compressor increases as the suction p pressure and temperature increase. This trend appears in the industrial for example, if the refrigeration load on the evaporator increases. The increase in refrigeration load almost y certainly in temperature of the product being cooled, which in , g turn raises the evaporating temperature. As a result, the power requirement of the compressor increases, often resulting in overload of the motor that drives the compressor.
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p g q
requirements of , p q g p
- Above figure shows the power requirement of a compressor if the compression were ideal. Under the figure, presents the power actual compressor. These trends derive directly from g catalog data.
p g
- One of the conclusions from an examination of p this figure is that the power increases toward a peak as the evaporating temperature increases.
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Actual power requirement of an 8-cylinder Sabroe 108L ammonia
compressor operating at 1170 rpm. compressor operating at 1170 rpm.
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5. Adiabatic compression p y efficiency :
( page 105, [2])
q p
ideal
p
( )
y Equation P=m.∆hideal presented a specially- ideal p defined compressor power requirement using the ideal work of compression ∆hideal. This ideal work of compression applies to a process which is both adiabatic (no transfer of heat) and frictionless. The actual work of compression, ∆hcomp, can be q calculate from the equation : P=m. ∆hcomp comp
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- The ratio of the ideal to the actual work of adiabatic the as is
p compression defined y, ηc compression efficiency, ηc:
η η
=c
idealh Δ comph Δ
g p
p - Such factors as the friction due to the mechanical rubbing of metal parts and the friction of the flow of refrigerant are losses that reduce the y compression efficiency.
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p p g The value of ηc drops at higher compression ηc
, g p
p
ideal
p
ratios because of increased forces of the rubbing g p parts, such as shafts on bearing and piston rings on low cylinders. There is also a dropoff of ηc at y compression ratios and this reduced efficiency is In fact, at a probably due to flow friction. , compression ratio of 1.0 the value of ∆hideal is zero, so any actual work, even though small, drives ηc to zero.
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p y
The adiabatic compression efficiency can best the compression ratio, as g y be correlated by demonstrated by this figure
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g - Knowledge of p
g ,
the adiabatic compression important uses. In the first efficiency has several p in comparing the ηc p place, the value of ηc is a tool effectiveness of two different compressors.
g
g p
y ηc and 80% at
p is - The trend shown in above figure that applicable to a specific compressor is fairly typical of , most good compressors, namely ηc is about 70% at high low ratios compression compression ratios.
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6. Effect evaporator g condensing and of
p temperature on system efficiency :( page 105, [2])
p g
The COP always increases with an increase in p evaporating temperature and decreases with an increase in condensing temperature.
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7. Discharge temperatures and water-cooled p g
heads
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g g
( p )
p g
the compressor inefficiencies of
g g
p y ,
Above figure shows some adiabatic discharge temperatures that would occur with ammonia and R- 22 were the compressions ideal (frictionless) and with no transfer of heat. The actual discharge temperatures would be higher than those shown if because of negligible heat is lost to the ambient. Because the cylinders and heads are hot, there is natural convection of heat to air, but particularly in the case of ammonia, more intensive cooling is needed.
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, , p
g
to prolong their of p breakdown the
oil p
g p
It is standard, then, for ammonia compressors to be equipped with water-cooled heads, thereby p g life and keeping valves cooler preventing high at p temperatures. Sometimes R- 22 compressors are equipped with water-cooled heads. Manufacturers recommend that discharge temperatures not exceed a temperature of approximately 135°C
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p
g y
Don’t let the out let temperature decrease too much, because refrigerant may be condensed. Flowrate can be regulated by control valve that remain water higher than condensing temperature
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q g p y p
g p p
p
p p p y
p q
7. Lubridation and oil cooling :g Small compressors may be able to achieve adequate lubrication of the moving parts by splash lubrication, virtually all reciprocating compressors used in industrial refrigeration practice are provided with forced lubrication. A positive-displacement p pump draws oil from the crankcase and delivers the oil to bearings, cylinder walls, and to the shaft seal on many compressors. Most pumps are driven off the compressor shaft and some are nonreversible, which fixes the required direction of compressor rotation.
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y p g the oil , Particularly on large compressors,
q
p g )
), p y (
g g y
g p p is passed through a watercooled heat exchanger that cools the oil. The rate of water flow required for cooling is of the order of 10 liters/min (several g gallons per minute). Another guideline is to set the the leaving water water-flow rate such that temperature is about 45°C (113°F), and then rely on the compressor manufacturer to have provided a cooler large enough to maintain a satisfactory oil temperature with this flow rate. A typical oil temperature during operation is 50°C
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y
Crankcase heaters automatically come into is
g
the
p, g p
p service during compressor shutdown. If the oil , p permitted to become cool during shutdown, the refrigerant—particularly halocarbons—will dissolve in the oil. Upon startup, the refrigerant boils foaming and possible oil carryout off, causing oil from the compressor.
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p yp y
g g
p y
The type of oil separator traditionally found in the discharge line of reciprocating compressor is a p small vessel using abrupt changes of direction of the oil-laden refrigerant to separate oil droplets that p then periodically are returned to the compressor crankcase.
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Typical safety cutouts associated with the oil yp y
system are those that shut off the compressor if:
g temperature or a low oil pressure p p - A high oil
occur.
y p
y g (
p
p p - The oil pressure cutout usually senses the the pump, which pressure differential across yp typically must be higher than about 100 kPa (15 psi). The cutout could be set to shut down the low compressor after a 90-second duration of pressure. This time delay permits the compressor a p time interval to build up the oil pressure on startup.
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,[
1. Construction : - ]) (page 19,[1]) (p g The helical-rotary compressor
g y g p
p y
traps the y refrigerant vapor and compresses it by gradually p the refrigerant. This shrinking the volume of g p particular helical-rotary compressor design uses two mating screw-like rotors to perform the compression p process.
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p g p
y
g - Refrigerant vapor enters the compressor housing through the intake port and fills the pockets , formed by the lobes of the rotors. As the rotors turn, they push these pockets of refrigerant toward the p discharge end of the compressor.
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p
g g p g p
p p
, y In this example helical-rotary p compressor, is drawn into the compressor refrigerant vapor through the suction opening and passes through the motor, cooling it. The refrigerant vapor is drawn into the compressor rotors where it is compressed and discharged out of the compressor. (watch the clip)
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p p
g p y y ,
p g y
g (p g ,[ g , The suction vapor enters the top of the rotors, and as the rotors turn a cavity appears at 1. Cavity 2 is continuing to fill, and cavity 3 is completely filled. Cavity 4 has now trapped gas between its threads and the housing. Cavity 5 is in the compression process with the volume shrinking as the cavity ]) bears against the end of the housing. (page126,[2])
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p p
p ,
p y , ,
Because the screw compressor completes its expulsion of gas with virtually no volume remaining, there is no clearance volume to reexpand, as is the case with the reciprocating compressor. It would be expected, then, that the volumetric efficiency and refrigerating capacity drop off less as the pressure ratio increases.
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Table shows the comparison of g p
g evaporating the as p p
g refrigerating capacity and power of a screw and reciprocating p compressor temperature changes.
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SREW COMPRESSOR
g g p
p compressor the as p p
Indeed at the higher condensing temperature of 35°C there is a greater dropoff in capacity of the g evaporating g reciprocating temperature decreases.
g p But at
p p y the lower condensing temperature of the percentage reduction in 20°C (68°F), F), capacity is about the same for the two compressors.
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y p p
y p g g p
,
p y
p 2. Capacity control and part-load performance: The most common device for achieving a variation in refrigerating capacity with a screw compressor is the slide valve. The slide valve is cradled between the rotors and consists of two members, one fixed and the other movable. The compressor develops p full capacity when the movable portion bears on the fixed member.
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The slide valve permits a smooth, continuous
to 10% of full
modulation of capacity from full capacity.
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g p y
p The percentage of full power always exceeds the percentage of full capacity.
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y p p
from compressor varies
g g p
p y The percent capacity reduction does not vary the slide valve. The linearly with the motion of p p precise to relation compressor, but the general curve is as shown in figure. The relationship shows that small changes of position of the slide valve at high capacity have a y dominant influence on the capacity.
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SREW COMPRESSOR
3. Performance characteristic of a basic screw
compressor : ( page130, [2])
, p g
p g
A g before
i,
k=1.29 for NH3 K=1.18 for R22
, p In contrast to the reciprocating compressor, the screw compressor has no suction and discharge valves but accepts a certain volume of suction gas in a cavity and reduces this volume a specific fundamental discharge. amount characteristic of the basic screw compressor is its built-in volume ratio, vi, which is defined as follows:
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k=1.29 for NH3 K=1.18 for R22
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Note : The built in volume ratio is Vi is shown Note : The built-in volume ratio is Vi is shown in the capacity charts, catalogue and other materials. materials
L.M.H. represent the following : Volume ratio L = 2.6 M= 3.6 H=5.8 Volume ratio M= 3 6 H=5 8 L = 2 6
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If p g
p the , compressor, then p
y
g p g p
the pressure ratio against which the to that compressor pumps is precisely equal the developed within discharge port is uncovered at the instant that the g p pressure of the refrigerant in the cavity has been the discharge line, and the raised to that of compressed gas is expelled into the discharge line by the continued rotation of the screws.
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y p
, g
p y
g g y g The compressed refrigerant has not yet reached the dischargeline pressure when the discharge port is uncovered, so there is a sudden rush of gas from the discharge line into the compressor that almost , instantaneously increases the pressure. Thereafter, the continued rotation of the screws expels this gas as well as the refrigerant ready to be discharged
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, g ,
g g p The third situation, as shown in figure, occurs when the discharge-line pressure is lower than that p achieved within the compressor. At the instant the discharge port is uncovered there is a sudden rush of gas out of the compressor into the discharge line.
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p
p p
The use of a compressor whose volume ratio has not been matched with operational conditions is a waste power and does not provide efficient operation.
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SREW COMPRESSOR
will
p 4. Variable volume ratio compressors change Performance g p
let condition g condensing pressure change -> volum ratio must be changed following that -> variable volume used
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SREW COMPRESSOR
i
g
g p p ,
p p p The variable vi device of figure consists of two parts which can move independently. In this figure ( ) (a) the two parts have no gap between them, so no refrigerant vapor vents back to the suction and the y compressor operates at full capacity.
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SREW COMPRESSOR
If is to be increased but y full capacity p the vi i
maintained, both parts move to the right
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i
g ,
If the high value of vi is to be maintained, but the left member backs off
Note : The motion of the two members requires a complex control, and there are limitations in achieving the desired vi when the capacity must also be reduced. educed If the capacity has been reduced by as much as 50%, the variable vi portion of the control may no longer be able to the control may no longer be able to meet its requirements.
the capacity reduced, p which vents some vapor back to the suction
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j p
p p g
g, ( ) g ,
5. Oil injection and seperation : - The screw compressor is provided with oil to ( ) (1) sealing of internal serve three purposes: clearances between the two rotors and between the rotors and housing, (2) lubrication of bearings, and (3) actuation of the slide valve.
result - Excessive oil quantities will
p y j
p g (
q in undesirable hydraulic hammer. The system designer and operator may not need to know the injection oil flow rate. We can take it from catalog or about 0.065 to 0.11 L/min per kW of refrigeration (0.06 to 0.1 gpm/ton of refrigeration) for high-stage machines.
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y g
p g
g 6. Oil cooling methods: The injected oil that seals the clearances in the p compressor is intimately mixed with the refrigerant undergoing compression. The refrigerant vapor becomes hot during compression and transfers to the oil as it passes through the some heat p compressor. The oil must be cooled before reinjection, and four of the important methods of oil g cooling are:
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g +Four of the important methods of oil cooling p
are:
q g j
-Direct injection of liquid refrigerant -External cooling with a thermosyphon heat
exchangerg
g refrigerant q liquid into of -External cooling with cooling water or antifreeze p g -Pumping the
refrigerant/oil mixture as it leaves the compressor.
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g
The method of cooling oil with an external heat to cooling water or rejects heat that
exchanger antifreeze
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p Evaporation of p
p p
q this pumped liquid cools the p refrigerant/oil vapor mixture to the desired 49°C temperature before the mixture enters the separator. flowrate controled by temperature
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Cooling oil by direct injection of liquid refrigerant g q g y j
at an early stage of the compression process.
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g yp g
y
natural flows
+Oil cooling with thermosyphon heat exchanger: in oil cooling - The thermosyphon concept g g q achieves heat transfer by boiling liquid refrigerant at the condensing pressure. Furthermore, the boiling y g refrigerant convection by (thermosyphon effect) through the heat exchanger.
g y
- We have two different diagram: System receiver position is under heat exchanger or higher heat exchangerg
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This figure that the liquid level g q in the system y
q receiver is above that of the heat exchanger, a p requirement for the natural circulation to take place.
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yp g
g q
A thermosyphon oil cooling installation where the level of the system receiver is at or below the g , level of the heat exchanger, requiring an additional receiver.
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g y
selecting yp size the of
includes yp
We have to design the thermosyphon system the which thermosyphon receiver and the sizes of three main lines. These pipes include:
g q p - The liquid/vapor line from the heat exchanger
to the receiver
q - The liquid line from the receiver to the heat
exchanger
p - The vapor line from the receiver to the header
carrying discharge vapor to the condenser(s).
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p p p y
The preliminary steps in the basic procedure of selecting the components in the system are to determine the flow rates.
Step 1: Determine the heat rejection rate at the
, qoc, oil cooler, qoc,
where qtot=refrigeration capacity+heat equivalent of p p p g ( y ) compressor power ( condensing capacity )
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Step 2 : Compute the evaporation rate p p p
, , g
Step 3 : Calculate the flow rate through the oil cooler, , assuming a recirculation ratio of 2:1 for R- 22 and 3:1 for ammonia
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p yp
Step 4 : Thermosyphon receiver. The size of
, thus , p
is expected that It p p
p p
p the receiver is chosen so that a , five minutes of operation, is reserve for available if the supply of liquid from the condenser is to the interrupted. the outlet in the system receiver is at about the midpoint , thermosiphon receiver. Thus, the thermosiphon receiver should be twice the size of the volume of g five minutes of refrigerant evporation.
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Step 5 : Liquid line from receiver to the heat p q
exchanger.
g
p p thermosiphon system y
yp y
pp
This section of line carries a flow rate greater than the rate evaporated, because a properly g operating circulates unevaporated liquid back to the receiver. Designers thermosyphon systems strive for a circulation of ratio of 3:1 for ammonia and 2:1 for R-22, where the circulation ratio means the rate supplied to the heat exchanger divided by the rate evaporated.
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q g y
p y
The following equations may be used to compute the required pipe size, D, (in) inches to g abide by the pressure gradients and circulation ratios specified above (22.6 Pa/m for ammonia and 113 Pa/m for R-22))
For ammonia:
For R22:
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Step 6 : Liquid/vapor line from heat exchanger p g q p
g
p y
to thermosiphon receiver. p The recommended pressure gradients for the liquid/vapor return line are 9.04 Pa/m for ammonia and 45.2 Pa/m for R-22. To abide by these pressure gradients, the required pipe sizes are given by the g q following equations: For ammonia:
For R-22:
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p p line from the receiver to the
Step 7:Vapor condenser header.
g g p q
A flow of refrigerant equal to passes through this the pressure in the than the g p
line. To motivate this flow, thermosiphon receiver must be higher entrance to the condenser.
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y g
p Flow-rate carrying capacities of various line sizes in the vent pipe between the receiver and the condenser.
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p p yp
p g p
p g
p
g
The thermosyphon concept operates because of the higher pressure developed down the liquid leg in comparison to the magnitude of pressure reduction of the less-dense mixture of liquid and vapor flowing upward in the line between the heat exchanger and the receiver. Since the pressure difference is p p proportional to the vertical distance over which this difference in density prevails, a certain minimum vertical distance should be provided between the in the thermosiphon receiver and the liquid level heat exchanger. Reference 11 recommends a minimum elevation difference of 1.8 m
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g p p
g
Example:p Design the thermosiphon oil-cooling system g serving an ammonia screw compressor operating with an evaporating temperature of -20°C and a p condensing temperature of 35°C. The full-load requirement at refrigerating capacity and power ( these conditions are 1025 kW (291.4 tons of refrigeration) and 342 kW (458.5 hp), respectively.
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Example:p
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7. Economizer circuit using a side port: p g
The refrigerant in Cavity 5, for example, is at p a pressure somewhere between suction and discharge.
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g g pp
p
- Refrigerant can be supplied through this opening at an intermediate pressure, and the p the compressor continues the compression of all refrigerant. p g, p ,
p g p - This opening, often called the side port, offers within one compressor some of the advantages of a , multiple-compressor, two-stage installation
p p
- Manufacturers of screw compressors are usually able to choose the position of the side port so that the desired intermediate pressure can be provided.
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y g p p
p
g
, - Additional refrigeration capacity is provided, however, because the liquid flowing to the py evaporators has been chilled and its enthalpy reduced. The power reqirement of the compressor increase because of the additional gas to be will compressed from the side-port pressure to the g p condensing pressure.
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Economizer cycle in its best operation is not p y
of
Comparison
the coefficients of performance of a two-stage ammonia g an with system economized single-stage compressor equipped with compressor equipped with a flash-type subcooler.
quite as efficient as two stage
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y
g y
One reason for the inability of the economized system using a side port to attain the efficiency of a two-stage system is illustrated . This unrestrained expansion consitutes a thermodynamic loss.
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y y p
q py p
g p
p the power q p
It can be inferred that the capacity of the system increase. This increase occurs, because the will g enthalpy of liquid reaching the expansion valve is reduced, even though the volume flow rate at the inlet to the compressor remains unchanged. Due to the admission of additional gas during the , compression process, p requirement increases.
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y
- The economizer cycle is most effective when refrigeration is operating at full
p the compressor y capacity.
, y g p
p p
p p ,
p - With compressors equipped with slide valves p for capacity control, the opening of the slide valve changes the pressure within the compressor at the side port. Because the start of compression is delayed, the pressure in the cavity is low when the side port is first uncovered. Thus, the pressure at the side port progressively drops as the slide valve opens.
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p p
p pp - Another potential application of the side port is to provide the suction for an intermediate- temperature evaporator.
p p
p industry p the
g
p
p - Here again there are limitations imposed by p the prospect of the drop in side-port pressure. In the intermediate-temperature food g p p evaporator is often serving spaces storing unfrozen food where the drop in evaporating temperatures g g much below freezing could damage products. A the side port offers attractive conclusion is that p possibilities, but it also has limitations. ,
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g p p Similar , to the reciprocating compressor,
p g
two scroll p compressor
p p p
the compressed refrigerant vapor p g
the scroll compressor works on the principle of trapping y the refrigerant vapor and compressing it by g p gradually shrinking the volume of the refrigerant. scroll uses The configurations, mated face-to-face, to perform this the scrolls are compression process. The tips of fitted with seals that, along with a fine layer of oil, p p from prevent escaping through the mating surfaces.
(p g , [ ]) Note : Reference (page 8, [1])
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, y pp
, y
g g
y the center of
, The upper scroll, called the stationary scroll, contains a discharge port. The lower scroll, called the driven scroll, is connected to a motor by a shaft and bearing assembly. The refrigerant vapor enters through the outer edge of the scroll assembly and the discharges through the port at y stationary scroll.
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g
p g
the motor shaft causes the scroll
j The center of the scroll journal bearing and the the motor shaft are offset. This offset center of imparts an orbiting motion to the driven scroll. Rotation of to orbit—not rotate—about the shaft center.
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g
,
g p p
p g g This orbiting motion causes the mated scrolls to form pockets of refrigerant vapor. As the orbiting motion continues, the relative movement between the orbiting scroll and the stationary scroll causes the pockets to move toward the discharge port at the center of the assembly, gradually decreasing the refrigerant volume and increasing the pressure.
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Three revolutions of the motor shaft are required q
to complete the compression process.
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g ,
g p p g
g
- During the first full revolution of the shaft, or the intake phase, the edges of the scrolls separate, allowing the refrigerant vapor to enter the space between the two scrolls. By the completion of first , g , revolution, the scrolls meet again, the edges of forming two closed pockets of refrigerant.
g revolution, or , - During the second full
p g
p produces revolution
the compression phase, the volume of each pocket is , y progressively reduced, p g increasing the pressure of the the trapped refrigerant vapor. Completion of nearmaximum second compression.
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, g g
g g p g
p
- During the third full revolution, or the discharge phase, the interior edges of the scrolls separate, releasing the compressed refrigerant through the discharge port. At the completion of the revolution, , is reduced to zero, the volume of each pocket forcing the remaining refrigerant vapor out of the scrolls.
- Notice that these three phases
, p
p g ,
intake, y g compression, and discharge occur simultaneously in these an ongoing sequence. While one pair of g is being p pockets is being formed, another pair compressed and a third pair is being discharged.
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, p g p
g p g g p
g g p
g y ,
y, p
y , ,
In this example scroll compressor, refrigerant vapor enters through the suction opening. The , refrigerant then passes through a gap in the motor, cooling the motor, before entering the compressor housing. The refrigerant vapor is drawn into the scroll assembly where it is compressed, discharged the into the dome, and finally discharged out of compressor through the discharge opening. In the g air-conditioning industry, scroll compressors are rooftop units, split widely used in heat pumps, systems, self-contained units, and even small water chillers.
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+ Advantages of scroll compressors: g p
Scroll
yp p
type compressors are inherently more p efficient compared to other types of compressors for many reasons:
p p g
g
- The absence of pistons for gas compression enables scroll compressors to reach nearly 100% gy leading to reduced energy y, volumetric efficiency, costs.
p yp
, - Re-expansion losses, a typical feature of each piston stroke encountered in reciprocating models, are eliminated.
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, (p )
, - In addition, valve (ports) losses are eliminated, since suction and discharge valves (ports) do not exist.
, g p
g yp p p
- Furthermore, due to the absence of several y p moving parts, scroll compressors are considerably quieter in operation compared to other types of , compressors, p like for example reciprocating type ones.
g p
y - Their weight and footprint are considerably types of compared to other bulkier
p y smaller compressors in use nowadays.
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p , - Gas pulsation is also minimised,
if not eliminated and consequently, they can operate with less vibration.
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+ Disadvantages of scroll compressors: p g
g y
- Being fully hermetic, perhaps the biggest p disadvantage of scroll compressors is that they are generally not easily repairable. They cannot be disassembled for maintenance.
g y
- Many reciprocating compressors are tolerant on rotating in both directions. This is usually not the case for scroll compressors.
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y,
g g [ ,
y g - In the air-conditioning industry, helical-rotary compressors are most commonly used in water chillers ranging from 70 to 450 tons [200 to 1,500 kW].
g p p
gy ,
the refrigerant. y) gy (
p p p - The centrifugal compressor uses the principle of dynamic compression, which involves converting energy from one form to another, to increase the pressure and temperature of It gy converts kinetic energy (velocity) to static energy (pressure). The core component of a centrifugal g compressor is the rotating impeller.
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, p y ,
g p
g , g p
- The center, or eye, of the impeller is fitted with into radial blades that draw refrigerant vapor y p passages that are internal to the impeller body. The rotation of the impeller causes the refrigerant vapor to accelerate within these passages, increasing its velocity and kinetic energy.
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p
g g
p
g ,
p g - The accelerated refrigerant vapor leaves the impeller and enters the diffuser passages. These p passages start out small and become larger as the refrigerant travels through them. As the size of the y, , diffuser passage increases, g the velocity, and the refrigerant therefore the kinetic energy, of y decreases. The first law of thermodynamics states that energy is not destroyed—only converted from one form to another. Thus, the refrigerant’s kinetic energy (velocity) is converted to static energy (or ) static pressure).
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, g g p ,
p
g , ,
- Refrigerant, now at a higher pressure, collects the in a larger space around the perimeter of compressor called the volute. The volute also becomes larger as the refrigerant travels through it. Again, as the size of the volute increases, the kinetic energy is converted to static pressure.
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p gy
p g This chart plots the conversion of energy that takes place as the refrigerant passes through the centrifugal compressor.
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g g p
y
gy, g
gy
p , In the radial passages of the rotating impeller, increasing its the refrigerant vapor accelerates, gy velocity and kinetic energy. As the area increases in the diffuser passages, the velocity, and therefore the the refrigerant decreases. This kinetic energy, of reduction in kinetic energy (velocity) is offset by an g increase in the refrigerant’s static energy or static the high-pressure refrigerant pressure. Finally, p collects in the volute around the perimeter of the compressor, where further energy conversion takes p place.
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Máy nén ly tâm
Cánh chỉnh tải
Dàn ngưng tụ
Bộ điều khiển
Bình bay hơi
g Centrifugal Chiller
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g g g
Following are the advantages and isadvantages of centrifugal compressors, over to the reciprocating compressors:
g g
p +Advantages : - High reliability, eliminating the need for standby and y, compressors installed
multiple y capacity. p
- For the same operating conditions, machine
g prices are lower for high volume flow rates. p
- Less plot area for installation for a given flow
rate.
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- Machine is small and light weight with respect g p g
to its flow rate capacity.
- Installation costs are lower due to smaller size
Low total maintenance costs
,
g q p ), g y (
- When a turbine is selected as a driver, the centrifugal compressor’s speed level allows direct drive (no gear unit), thereby minimizing equipment cost, reducing power requirements, and increasing unit reliability.y
- Flow control
is simple, continuous, and g y efficient over a relatively wide flow range.
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- No lube (or seal) oil contamination of process p ( )
gas.
y p p - Absence of any pressure pulsation above
surge point.
+ Disadvantages:g - efficiency than most
p yp types for
g p y positive Lower the same flow rate and displacement pressure ratio, especially for pressure ratios over 2. Due to recycle not efficient below the surge point
- Very sensitive to changes in gas properties,
p g y especially molecular weight
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g g
q g g
- Not effective for low molecular weight gases. The pressure ratio capability per stage is low, tending to require a large number of machine stages, hence mechanical complexity.
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