Phụ kiện máy lạnh (Refrigeration Accessories) - Chương 10

CHAPTER 10: REFRIGERATION ACCESSORIES ACCESSORIES

Lecturer: ME.Nguyễn Duy Tuệ

Chapter 10: Refrigeration Accessories ThS.Nguyễn Duy Tuệ

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OBJECTIVES

thi t d h t In this chapter, student can : I

i ht

i f it t

t - How to calculatate and know installation criterion for refrigerant piping i i f i - Understand the function of some vessels and other accessories such as : sight glass, solenoid id valve….

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REFERENCES

[1]. [1] Refrigerant R f i t Industrial I d t i l Handbook-The H db k Th

McGraw-Hill Companies.

[2]. Trane, Danfoss technical document [2] T t t h i D f l d

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CONTENT

REFRIGERANT PIPING REFRIGERANT PIPING

VESSELS

OTHER EQUIPMENTS OTHER EQUIPMENTS

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REFRIGERANT PIPING

1. Design principles: - Must supply sufficient t ffi l i liquid flowrate to li id fl t t

M t evaporator

- Pressure drop must be low - Have enough velocity to draw oil back to

compressor

g - Don’t let refrigerant come back to compressor - Piping must be dry and clean

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REFRIGERANT PIPING

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REFRIGERANT PIPING

2. Suction pipe: - Undersizing the suction line

reduces compressor capacity by forcing it to operate at a lower suction pressure to maintain the desired evaporator temperature.

result

Oversizing the suction line increases initial in insufficient costs and may from the

- project refrigerant gas velocity to move oil evaporator to the compressor

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REFRIGERANT PIPING

Pressure drop arcording to saturate

temperature reduction by ASHRAE standard:

Refrigerant velocity in suction pipe should be 7~12m/s. Refrigerant velocity should higher 3m/s than 7m/s in for horizontal pipe and higher vertical pipe

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REFRIGERANT PIPING

- While operating, the suction line is filled with

superheated refrigerant vapor and oil. The oil flows on the bottom of the pipe and is moved along by the refrigerant gas flowing above it.

- When the system stops, the refrigerant may condense in the pipe depending on the ambient conditions. This may result in slugging if the liquid refrigerant is drawn into the compressor when the system restarts.

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REFRIGERANT PIPING

- To promote good oil

return, suction lines should be pitched 1/8 inch per foot (10.4 mm/m) in the direction of refrigerant flow.

- Evaporator connections require special care because the evaporator has the potential to contain a large volume of condensed refrigerant during off cycles.

- To minimize slugging of condensed refrigerant, the evaporators should be isolated from the suction line with an inverted trap as shown

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REFRIGERANT PIPING

The trap should extend above the top of the

evaporator before leading to the compressor.

- With multiple evaporators, the suction piping should be designed so that the pressure drops are from one coil equal and the refrigerant and oil cannot flow into another coil.

- Traps may be used at the bottom of risers to catch condensed refrigerant before it flows to the compressor. Intermediate traps are unnecessary in a properly sized riser as they contribute to pressure drop.

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REFRIGERANT PIPING

- Usually with commercially produced air the compressors are conditioning equipment, “prepiped” to a common connection on the side of the unit.

the refrigerant before enters it

- Suction line filter driers are available to help the clean compressor. Because they represent a significant pressure drop -> be added in some circumstances such as after compressor burnout.

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REFRIGERANT PIPING

Observe some diagrams:

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REFRIGERANT PIPING

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REFRIGERANT PIPING

+ Double suction riser: - In a double suction riser at full capacity, the flow passes through both risers with

refrigerant enough velocity to move the oil. - At minimum capacity, oil

in the riser flows backward and fills the trap at the bottom. Once the trap is full of oil, refrigerant flow through the large diameter riser is cut off and only refrigerant gas flows through the smaller diameter riser.

- The sum of

the two risers is sized for full capacity. The smaller diameter riser is sized for minimum capacity.

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REFRIGERANT PIPING

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REFRIGERANT PIPING

3. Discharge pipe: - Undersizing discharge lines will

the project and may result

reduce compressor capacity and increase compressor work. Over sizing discharge lines increases the in initial cost of insufficient refrigerant gas velocity to carry oil back to the compressor.

- Discharge lines should be sized for no more than 2 to 3°F (1.1 to 1.7°C) pressure loss. The actual pressure drop in PSI will depend upon the refrigerant.

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REFRIGERANT PIPING

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REFRIGERANT PIPING

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REFRIGERANT PIPING

g g p

p p g g y ,

p y - Discharge lines carry both refrigerant vapor and oil. Since refrigerant may condense during the off cycle, the piping should be designed to avoid liquid refrigerant and oil from flowing back into the compressor.

g backward before into it

p - Traps can be added to the bottom of risers to catch oil and condensed refrigerant during off cycles, the flows compressor.

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REFRIGERANT PIPING

g p p

- Discharge lines should be pitched 1/8 inch per foot (10.4 mm/m) in the direction of refrigerant flow towards the condenser

, p

backwards into

- Whenever a condenser is located above the p compressor, an inverted trap or check valve should be installed at the condenser inlet to prevent liquid g from flowing g refrigerant the compressor during off cycles. p g g p ,

( )

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- With reciprocating compressors, a discharge muffler is installed in the discharge line to minimize y is easily p pulsations (that cause vibration). Oil trapped in a discharge muffler, so it should be placed in the horizontal or downflow portion of the

REFRIGERANT PIPING

p g p ,

p ) (

- With some reciprocating compressors, a discharge muffler is installed in the discharge line to is minimize pulsations (that cause vibration). Oil easily trapped in a discharge muffler, so it should be p placed in the horizontal or downflow portion of the piping, as close to the compressor as possible.

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REFRIGERANT PIPING

Discharge Line Piping Details

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REFRIGERANT PIPING

We could design double pipe same as suction p p g

pipe, if some problem occure in oil carrying

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REFRIGERANT PIPING

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REFRIGERANT PIPING

4. Liquid pipe:

- Liquid lines connect

the condenser to the to the TX

evaporator and carry liquid refrigerant valve.

- If the refrigerant in the liquid line flashes to a gas because the pressure drops too low or because of an increase in elevation, then the refrigeration system will operate poorly. Liquid sub-cooling is the only method that prevents refrigerant flashing to gas due to pressure drops in the line.

- The actual line size should provide no more

than a 2 to 3°F (1.1 to 1.7°C) pressure drop.

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REFRIGERANT PIPING

- The actual pressure drop in PSI (kPa) will

depend on the refrigerant.

significantly

- Oversizing liquid lines is discouraged because increase system will the in turn, affects the oil

it refrigerantcharge. This, charge.

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REFRIGERANT PIPING

This figure shows the condenser below the g

evaporator.

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REFRIGERANT PIPING

q - As the liquid refrigerant g

is lifted from the the refrigerant

to the evaporator, g

condenser p pressure is lowered. Different refrigerants will have different pressure changes based on elevation.

- Pressure drop in the liquid line is the sum of the liquid

g p the friction loss, plus the weight of refrigerant column in the riser.

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REFRIGERANT PIPING

d d

V 4.V 4. vπ .

Where:

- V=G:δ refrigerant volume flowrate(m3/s) G : mass flowrate of refrigerant (kg/s) - G : mass flowrate of refrigerant (kg/s) - v : refrigerant velocity (m/s) - d : diameter , (m) d : diameter , (m)

p p g 5. Refrigerant piping calculation : Step 1: Diameter calculation:

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REFRIGERANT PIPING

p p

(

)

P Δ =

. λ

. . δ ζ δ

L v L v . d 2

v v 2

Step 2 : Pressure drop calculation : + Method 1 : Use Darcy – Weisbach fomular :

y ,( g g

L : piping length, ( m ) λ : friction factor, (dimensionless) ) , ( v : refrigerant velocity, (m/s) ρ : refrigerant density ,(kg/m3) ) ρ ζ: pressure-drop coefficient for fitting

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REFRIGERANT PIPING

y y

The friction factor f is a function of the Reynolds number and the roughness of the pipe, and can be , read from a Moody chart .The Reynolds number, Re, is a dimensionless group

if R 2100 d t b l l i i

The flow is laminar if Re<2100 and turbulent if Th fl t if Re>3000

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REFRIGERANT PIPING

q The other quantity needed to determine the

g ,

y friction factor from Moody chart is the relative g roughness, ε/D. The roughness ε of the surfaces of two different materials used for refrigerant pipe and tubes are:

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REFRIGERANT PIPING

Viscosities of several refrigerants g in their

saturated liquid and saturated vapor states.

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REFRIGERANT PIPING

p Example 1 : What is the pressure drop per m of tube length -6.7°C flows p g

when 1.51 kg/s of R-22 vapor at through copper tubing that is 74.8 mm ID?

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REFRIGERANT PIPING

+ Solution :

From the Moody chart, at the above values of Re

, and ε/D, the friction factor f is found to be 0.0108.

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REFRIGERANT PIPING

Finally substitute applicable terms into the

pressure drop equation

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REFRIGERANT PIPING

Example 2 : Calcualte friction factor of steel pipe when carrying amonia liquid at 35oC and velocity at 0,51 m/s in 51mm ID ?

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REFRIGERANT PIPING

+ Solution :

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REFRIGERANT PIPING

+ Pressure drop coefficient for fitting, TL[1]:

We have to add weight of refrigerant column: We have to add weight of refrigerant column: ΔPcđ =h.g.ρ ,(Pa)

h : refrigerant column of riser, (m) h : refrigerant column of riser (m) g : acceleration, (m/s2) ρ : refrigerant density, (kg/m3) : refrigerant density (kg/m3)

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REFRIGERANT PIPING

We could use this weight of column riser:

+ Note : we have to add pressure for downward

pipe

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REFRIGERANT PIPING

+ Method 2 : Using equivalant length:

Many designers find it convenient to express fittings and open valves as the pressure drop of equivalent length of straight pipe. This equivalent length, Ltđ, can then simply be added to the actual length of straight pipe to determine the total pressure drop.

Set : Ltđ = (ζ.d)/λ ,(m)

P

l (

l

).

g

(

Pa

)

+=Δ

+

. h . ρ

ρλ λ v . . d 2

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REFRIGERANT PIPING

A typical value of the friction factor λ (or f) for

refrigeration applications is approximately 0.02.

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VESSELS

1. Oil seperator : - Oil remove separators oil

from the compressor's discharge gas, temporarily store the oil, and then return it to the compressor's crankcase. Oil separators are located close to the compressor in the discharge line

- Oil separators are essential on low or ultra-low temperature refrigeration systems and on large air conditioning systems up to 150 tons.

- Oil separators can also act as discharge mufflers to quiet compressor pulsation and vibration noises.

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VESSELS

+ Oil separator principles: - Reduce mixture of oil-vapour immediately - Change mixture’s direction by internal baffle - Use the fine mesh to filt oil -> Oil droplets to form and drop to the bottom of the separator. When the level of oil gets high enough to raise a float, an oil return needle is opened and the oil is returned to the compressor crankcase through a small return line connected to the compressor crankcase.

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VESSELS

The pressure difference between the high and the refrigeration or air conditioning low sides of system is the driving force for the oil to travel from the oil separator to the crankcase.

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VESSELS

+ Oil separator structure :

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VESSELS

2. High pressure receiver :

Sized to accept the full pumpdown volume of refrigerant in the entire system at 80% full. On large systems, it is sized to supply ½ hour of liquid supply buffer to the low side. There are two kinds of high pressure receiver : vertical or horizontal vessel

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VESSELS

3. Low pressure receiver :

This vessel is one that performs both the role of liquid/vapor separation and liquid storage at low pressure. It contain low temperature liquid after coming out expansion valve to supply to evaporator equally

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VESSELS

Liquid level in low pressure receiver :

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VESSELS

g p p

g - The surge volume serves the purpose of accommodating liquid that might be forced out of evaporators during defrost.

g p

g g

pp y q

p - The reason for needing this ballast volume is p p that during startup or resumption of operation of one the pump may withdraw or more evaporators, refrigerant from the vessel at a greater rate than is supplied at that moment by the combination of the controlled liquid supply and from the return from the liquid/vapor line

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VESSELS

+ Computing the ballast volume: p g

p q interval p y

p p q ,

the The ballast volume is provided to permit p p pumps to draw liquid from the low-pressure receiver for a short to bring the liquid content of g evaporators up to the steady-state amount following a shutdown. Typically, a five-minute time period is assumed adequate for this purpose, so the ballast volume is the design pump flow rate in volume flow p per minute multiplied by 5. p y

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VESSELS

+ Computing the surge volume: p g g

Two major contributors to momentary excess

p flow into the low-pressure receiver are:

flooding of abnormal rates of liquid out of the evaporator due to defrost or to sudden increases of refrigeration load

q p -

p p q the liquid in the liquid/vapor return line that rains back to the low-pressure receiver in the event p of pump or power failures.

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VESSELS

q g p

q p p g

The fraction of liquid in the coil during operation is top or bottom fed. depends on whether the coil For a top-fed coil the percentage of liquid is often assumed to be about 30%. For bottom-fed coils the p percentage is sometimes chosen as high as 80%. g g

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VESSELS

p p , q

4. Suction line trap or Accumlator: - The purpose of the suction line accumulator is g the compressor against an accidental p p to protect return of in its liquid phase, by the refrigerant suction pipes, which could cause a “liquid hammer” the with a risk of mechanical destruction of compressor.

g, p ,

y substantially y efficiency the of p

p - By adding a heat exchanger in the suction accumulator, this improves the subcooling, reduces the risks of the remaining refrigerant in liquid and it the improves installation.

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VESSELS

+ Accumulator structure : performance priciple is p p p

same as oil separator

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VESSELS

+ Accumulator position : p

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VESSELS

5. Interstage desuperheater g p ( ( flash tank or

p subcooler ): p p The purpose of

q g

is to desuperheat the vessel discharge vapor from the low-stage compressor by g through the liquid in the vessel or bubbling it subcooling liquid with a heat exchanger

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VESSELS

+ Flash tank with and without subcooling :g

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VESSELS

+A with p

p y g ( ) q p g y

( ) (a) flash-tank/desuperheater desuperheating by bubbling vapor through liquid and (b) desuperheating by spraying liquid into vapor.

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VESSELS

5. Thermosyphon receiver : - A small but important

vessel g for cooling concept type of oil

is the p thermosyphon receiver which is an integral part of that screw compressors.

p - The key requirement is that

it provides a reserve for five minutes of flow to the oil cooler if the supply of liquid from the condenser is interrupted.

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VESSELS

5. Thermosyphon receiver : yp

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VESSELS

g - Some designers and contractors combine the thermosyphon receiver and the system receiver as shown in this figure.

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VESSELS

p 6. Oil pot : - A small vessel frequently installed beneath the p low-pressure receiver and other larger vessels is an oil pot whose purpose is to accumulate oil to g facilitate periodic draining.

- It’s used for Amonia which is immiscible with

oil

- Since the oil pot is usually classified as a

p y , vessel, it must be protected by relief valves.

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VESSELS

1 1

+ Oil pot structure: + Oil pot structure:

1. Sight glass, 2. Pressure gauge, 3. Relief valve, 4. Connected to suction pipe, 5. From other vessels 4. Connected to suction pipe, 5. From other vessels

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VESSELS

7 Liquid-vapour suction heat exchanger : 7. Liquid vapour suction heat exchanger : Objectives of that : Increasing the system performance - Increasing the system performance - Subcooling liquid refrigerant to prevent flash

gas formation at inlets to expansion devices gas formation at inlets to expansion devices

- Fully evaporating any residual

liquid that may remain in the liquid suction prior to reaching the remain in the liquid-suction prior to reaching the compressor

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VESSELS

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VESSELS

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VESSELS

g g y 8. Purging system :

Air in the condenser will raise head pressure, g p p y mainly due to its insulating properties. Air molecules in the gas from the compressor will be blown to the q quiet end of the condenser. This air accumulates on the heat transfer surfaces.

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VESSELS

+ How can air come in ? - By leaking through condenser seals and valve below p pressure suction is g

p packings when atmospheric conditions

y , repairs, coil p - When the system is open for

p cleaning, equipment additions…

g g y g ;

g - When charging by refrigerant trucks; adding oil - By the breakdown of refrigerant or lubricating

g p ; oil; or from impurities in the refrigerant.

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VESSELS

To determine the amount of air in a refrigeration g

system, consider these symptoms:

p g

g ; - High discharge temperatures; g - High condensing (head) pressures; g; - High condensing subcooling; g - Slightly vibration of high pressure gauge - High amp draws. + Principle of air purging equipment : - We condense refrigerant

g p in mixture of air+refrigerant, because refrigerant have higher boiling temperature than air

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VESSELS

+ Air purging structure:

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OTHER EQUIPMENTS

1. Solenoid valve : A solenoid valve is used to stop the flow of refrigerant within the system. These valves are magnetically operated, and an electric winding controls the opening and closing of the valve. The valve is typically a normally-closed type of valve so that it is closed when it is deenergized.

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OTHER EQUIPMENTS

Solenoid valve is used to control the flow of multi sections evaporator. In this application, a valve the is installed in the liquid line, upstream of expansion valve for each individually controlled section of the evaporator coil.

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OTHER EQUIPMENTS

Another common use of a solenoid valve is to the enable system pump-down and prevent refrigerant from migrating through the system when the compressor is shut off.

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OTHER EQUIPMENTS

2. Liquid line filter drier : - It is installed upstream of the solenoid valve

and the expansion valve.

- It prevents moisture (water) and foreign matter, from introduced during the installation process, entering the expansion valve and the solenoid valve.

- Realize, however, that there is no substitute for

cleanliness during system installation.

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OTHER EQUIPMENTS

- When water is mixed with refrigerant and oil, is added by the compressor, acids are

and heat formed that can damage the valves or compressor. Additionally, certain foreign materials such as copper and brass particles can act as a catalyst in in the formation of chemical reactions that result acids.

- These acids can corrode system components

and cause the oil to sludge.

- The filter drier should be installed close to the solenoid valve to provide the most protection for the solenoid and expansion valves.

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OTHER EQUIPMENTS

+ Position of filter :

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OTHER EQUIPMENTS

+ Structure of filter: - A typical liquid-line filter drier

includes a molded, porous core. The core has a high affinity for moisture and removes foreign matter from the refrigerant.

- The two common types of

filter driers are replaceable core and sealed. The replaceable core type allows the core to be easily replaced. The reducing the sealed type is completely closed, chances of refrigerant leaks.

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OTHER EQUIPMENTS

+ Structure of filter:

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OTHER EQUIPMENTS

3. Moister – indicator sight glass: - It is installed in the liquid line, upstream of the to to

expansion valve, and permits the operator observe the condition of the refrigerant prior entering the expansion valve.

- The value of the sight glass is in its moisture

indication ability

determine charge proper to

- The sight glass should not be used to determine system refrigerant charge or subcooling. Actual temperature and pressure measurements are required and subcooling.

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OTHER EQUIPMENTS

+ Installation position : With the sight glass installed directly ahead of the expansion valve, it can also be used to detect the presence of bubbles in the liquid line. This would indicate that some of flashed into vapor the liquid refrigerant has upstream of the expansion valve.

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OTHER EQUIPMENTS

4. Suction line filter : Similar to the liquid-line filter drier, the suction line filter performs the task of removing foreign matter from the refrigeration system. It is installed in the suction line, just upstream of the compressor.

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OTHER EQUIPMENTS

- The suction filter contains filter media to remove copper filings, flux, dirt, and other foreign matter that may have been introduced during the installation process or as the result of a compressor failure.

the compressor. Dirt

- It protects the compressor parts from the abrasive action that could result if these materials enter can obstruct oil robbing the compressor bearings of passages, lubrication.

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The two common types of suction line filters are replaceable core and sealed. The replaceable core type allows the core to be easily replaced. The sealed type is completely closed, reducing the chances of refrigerant leaks.

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- Replaceable core suction filters are commonly

installed after a compressor failure has occurred.

has from the removed been

- The core is replaced after the foreign matter or acid system. Additionally, suction filters should be installed in all field-assembled systems.

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5. Hot gas muffler: - The purpose of the hot gas muffler is to smooth the pulsations associated with the refrigerant out vapor being discharged from a reciprocating compressor, reducing noise and vibration.

the manner in which it

- The pressure of the refrigerant vapor leaving a reciprocating compressor fluctuates rapidly because of is compressed by the reciprocating pistons

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- The muffler contains a perforated tube inside a

shell.

the - The pressure peaks cause some of refrigerant vapor to pass from the perforated tube into the muffler shell.

- This shell is divided into chambers that allow it

to absorb these peaks.

characteristic pulsating in

In essence (in principle), the muffler shaves off the peaks of these pulsations and fills in the valleys, the the reducing discharge line.

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When used,

the hot gas muffler should be located in the discharge line, as close to the reciprocating compressor discharge as possible. This minimizes the sound emission from the unmuffled section of discharge line.

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6. Shut off valve: Shutoff valves are used to isolate one part of the refrigeration system from the rest. Additionally, they can be used to trap the refrigerant charge in one component of the system, the condenser for example, to permit service or repair to another part of the system.