Monday 28 January 2013

Refrigeration Oil Separators

Selecting, Installing Oil Separators 

Because refrigerants and refrigeration oils are miscible in one another, there will always be some oil that leaves the compressor with the refrigerant being circulated throughout the refrigeration system. Also, any time flooding or migration occurs, the crankcase oil is sure to be diluted with refrigerant. This will cause oil foaming at start-up. The crankcase will then build excessive pressures, often

forcing oil and refrigerant around the rings of the compressor's cylinders to be pumped into the discharge line. 

Oil separators remove 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. (See Figure 1.) Even though most oil separators are designed to be mounted vertically, there are some horizontal models available on the market. 

Oil separators are essential on low or ultra-low temperature refrigeration systems and on large air conditioning systems up to 150 tons. Most compressor manufacturers require oil separators on all two-stage compressors. Oil separators can also act as discharge mufflers to quiet compressor pulsation and vibration noises. 

Unusual system operating conditions often occur to compressors and rapid removal of oil from the compressor's crankcase happens. System flooding is an example. 

Sometimes these occurrences happen beyond the control of both the designer and installer. The velocity of the refrigerant flowing through the system should return oil to the compressor's crankcase. Even though proper refrigerant system piping designs maintain enough refrigerant velocity to ensure good oil return, sometimes this added pressure drop, which assists in getting the right refrigerant velocity for oil return, hampers the system's efficiencies. 

Sometimes a higher than normal pressure drop is intentionally designed into a system for better oil return. This, however, will cause higher compression ratios and low volumetric efficiencies that will lead to lower system capacities. 


Figure 1. A standard oil separator. 


Detrimental Effects Of Oil In A System

Oil that gets past the compressor and into the system not only robs the compressor crankcase of vital lubrication, it coats the walls of the condenser and evaporator. Oil film on the walls of these important heat exchangers will reduce their heat transfer abilities. The condenser will not be able to reject heat as efficiently as it should. Even though this oil film in the condenser will be hotter and thinner than if it were in the evaporator, system efficiencies will suffer. Head pressures will rise, causing higher compression ratios and lower volumetric efficiencies with lower than normal system capacities. 

Oil that coats the walls of the evaporator will decrease heat transfer to the refrigerant in the evaporator. A film of oil bubbles, which acts as a very good insulator, will form on the inside of the evaporator. The evaporator will now see a reduced heat load that will cause the suction pressure to be lower. Lower suction pressures cause higher compression ratios and lower volumetric efficiencies. The result is lower system capacity with much longer running times. 

Most control valves, including TXV and capillary tubes, will also experience inefficient performance due to the pressure of oil filming. Capillary tubes may experience wide variation in flow rates. Usually, reduced refrigerant flow rates with higher head pressures and lower suction pressures are experienced. TXV remote bulbs may not sense the correct refrigerant temperature at the evaporator outlet, causing improper superheat control. TXV hunting can also occur. 

If an oil separator isn't employed, the compressor often sees slugs of oil that are returning from the evaporator. The compressor's pistons can momentarily pump slugs of liquid oil that can build tremendous hydraulic forces because of the incompressibility of most liquids. Serious compressor valve and drive gear damage can result. 

How Oil Separators Work

Oil separators are almost always made of steel. As oil-laden discharge gas enters the oil separator's very large internal volume, it immediately slows down its velocity. This low velocity is the key to good oil separation. The oil is mixed with the discharge gas in the form of a fog. This refrigerant/oil fog now runs into internal baffling, which forces the fog mixture to change direction. At the same time, this fog mixture is slowing down rapidly on the surface of these baffles. Very fine oil particles collide with one another and form heavier particles. Finally, fine mesh screens separate the oil and refrigerant even farther, causing larger oil droplets to form and drop to the bottom of the separator. Often, a magnet is connected to the bottom of the oil sump to collect any metallic particles. 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. The pressure difference between the high and low sides of the refrigeration or air conditioning system is the driving force for the oil to travel from the oil separator to the crankcase. 

The oil separator is in the high side of the system and the compressor crankcase in the low side. This float-operated oil return needle valve is located high enough in the oil sump to allow clean oil to be automatically returned to the crankcase. Only a small amount of oil is needed to actuate the float mechanism. This ensures that only a small amount of oil is ever absent from the crankcase at any one given time. When the oil level in the sump of the oil separator drops to a certain level, the float will force the needle valve closed.

The oil return line from the oil separator to the crankcase should be just above room (ambient) temperature. This is caused from heat conduction to the line from the hot oil separator. If the oil return line is cool or cold to the touch, there may be liquid refrigerant vaporising in it as it passes oil. This problem can result from the oil separator's shell being uninsulated or poorly insulated and becoming too cool. If the shell is too cool, it can cool discharge gasses too much, resulting in condensed (liquid) refrigerant in the bottom of the oil separator. This will cause the float to rise too often because of increased levels of an oil and liquid refrigerant mixture in the bottom of the separator. 

Once the float rises and the orifice opens, the mixture of liquid refrigerant and oil passes through the oil return line. The liquid refrigerant will vapourise from the sudden pressure drop and cause the cool temperatures in the return line. 

Helical Oil Separators

Helical oil separators offer 99 percent to 100 percent efficiency in oil separation with low pressure drop. Upon entering the oil separator, refrigerant gas and oil fog mixture encounters the leading edge of a helical flighting. (See Figure 2.) The gas/oil mixture is centrifugally forced along the spiral path of the helix, causing the heavier oil particles to spin to the perimeter, where impingement with a screen layer occurs. This screen layer serves as an oil stripping and draining medium. The separated oil now flows downward along the boundary of the shell through a baffle and into an oil collection area at the bottom of the separator. 

The specially designed baffle insulates the oil collection and eliminates oil re-entrainment by preventing turbulence. Virtually oil-free refrigerant gas exits the separator through a fitting just below the lower edge of the helical flighting. A float-activated oil return valve allows the captured oil to return to the crankcase or oil reservoir. 


Figure 2. A helical oil separator. 

Selecting An Oil Separator

Although oil separator catalogues show capacity in tons or horsepower, the actual tonnage or Btu capacity may vary widely from the horsepower size of the compressor. Actual capacity of compressors is dependent on suction pressures, discharge pressures, liquid temperatures, rpm, and the density of the suction gases. 

The larger the capacity of the compressor, the larger the separator's volume must be, regardless of the piping size connections of the separator. The separator must be large enough to match the compressor, and the connection sizes must be the same, or larger, than the discharge line size of the system. This allows the discharge gases in the separator to be near the same pressure as the discharge line because of minimal pressure drop within the oil separator. Do not ever under size an oil separator. It will lose its ability to return oil and will cause high pressure drop, causing system inefficiencies. 

When adding an oil separator to an existing system, care must be taken to check the system frequently after start-up. Literally gallons of oil may be trapped in a system and may gradually return to the compressor crankcase and overfill it. 

Installing An Oil Separator 

The oil separator should be rigidly supported by a solid surface. It should never be supported by the discharge line alone. Its own weight plus the weight of the oil in the separator will stress the discharge line and may cause failure. A vibration eliminator should always be installed in the discharge line before the oil separator. 

The separator should then be supported rigidly to a solid surface such as a concrete wall or floor, steel post, or to the same base as the compressor. Simple lead anchors and bolts are not good for vibrating equipment because they usually work themselves loose. Studs should be anchored in the floor when it is poured. Special anchors guaranteed and designed to handle vibration should be used on an existing floor. 

On new installations, the compressor and separator should be mounted in their final position, and then the interconnecting piping installed while making sure alignment is perfect. There should not be any long, vertical discharge line. If there is a vertical discharge line of more than 10 to 12 feet, it is a good practice to install a drop leg trap to the floor just before the riser. This will catch any oil left in the vertical discharge line and prevent it from draining back to the compressor's discharge. 

The oil return line, which runs from the oil separator to the crankcase, is usually 3/8 SAE flare. Sometimes, there is a chance that there may be some liquid refrigerant that condensed in the separator during the off cycle. You do not want to return this mixture directly back to the crankcase. Or, if the float and needle assembly in the separator are stuck open, hot gas would blow back directly into the crankcase and cause severe overheating of the oil. In the case of the semi-hermetic compressor, oil normally returns to the compressor with the suction gas through the end bell of the compressor. This gas has to flow through the motor compartment and through an oil return check valve in the crankcase. 

This will help vapourise any condensed refrigerant in the oil before it reaches the crankcase. The tap on the compressor's suction housing makes an excellent return point for oil coming from an oil separator. Any liquid refrigerant mixed with the oil will be vaporise by the motor heat. Any hot gas blown back will be mixed with cool suction gas and stands a better chance of not harming the compressor if an oil fill hole is used directly to the crankcase. 

Some oil separator manufacturers can provide a wraparound heater of about 50 watts to help prevent refrigerant condensing in the coil separator during the off-cycle. Most oil separators must be insulated to keep them hot during the on and off cycles. This will prevent refrigerant from condensing in them and mixing with the oil in their sumps.

Thursday 24 January 2013

Evaporators

Evaporators 
The refrigerant undergoes various changes throughout the vapor compression cycle and it is in the evaporator where it actually produces the cooling effect. The evaporator is usually a closed insulated space where the refrigerant absorbs heat from the substance or food to be cooled.

The cooling effect is produced by the refrigerant rotating continuously in the refrigerating or vapor compression cycle. The refrigerant gets ompressed and superheated in the compressor and then loses heat in the condenser. In thethrottling valve the pressure and temperature of the refrigerant is reduced suddenly and drastically. The low temperature liquid refrigerant enters the evaporator and produces the chilling effect for a refrigerator and cooling effect for an air-conditioner. 


The space comprising the evaporator is an enclosed space. For instance, in the case of a household refrigerator, the small enclosed freezer section has an evaporator embedded into it. In the case of the deep freezer the evaporator is enclosed in the space where ice or ice cream is to be made. The evaporator section of refrigerators is usually insulated by using insulating materials like polyurethane foam (PUF).


The low temperature refrigerant flowing through the evaporator absorbs heat from the food, substance or any other enclosed space and gets converted into a gaseous state as its temperature rises. This is then sucked by thecompressor, which compresses it, keeping the cycle of refrigerant continuous.

In the case of air-conditioners the evaporator is also called the cooling coil. Usually the fan would pass the hot room air over the evaporator coil, which is chilled, hence the air gets cooled. This air is then supplied into the room, where it creates the cooling effect by absorbing heat from the room.

Evaporators are of various types. Evaporators used for industrial refrigeration and air-conditioning purposes are very large and also called chillers. They are usually made in the form of shell and tube types with two possible arrangements: namely, dry expansion evaporators and flooded evaporators. In dry expansion evaporators the refrigerant usually flows through the tube side while the liquid to be chilled flows through the shell side. The flooded system is used where large quantities of liquids have to be cooled to extremely low temperatures. Since the load in such cases is very high, a large quantity of refrigerant flows through these evaporators. In flooded evaporators the refrigerant will usually pass through the shell side while the liquid to be chilled will pass through the tube side.
For smaller and home purposes there are three types of evaporators: bare-tube type, plate-surface type and finned evaporators:

In bare-tube evaporators the refrigerant flows through the bare-tube and the fluid to be chilled flows directly over it.

Plate-surface evaporators are used in household refrigerators. These evaporators are formed by welding together two plates that have grooves on their surface. When they are welded, the closed grooves form a sort of the tubing through which the refrigerant flows.

Finned evaporators are commonly used in window, split and packaged air-conditioners. They are in the form of a copper coil over which several fins are welded to increase the cooling area of the evaporator. Hot air passes over this evaporator and gets chilled as it enters the room.

Sunday 20 January 2013

Throttling or Expansion Devices

EXPANSION DEVICES

There are different types of expansion or throttling devices. The most commonly used are: 
(a) Capillary tube, 
(b) Float valves, 
(c) Thermostatic expansion valve.

1 Capillary Tube


Instead of an orifice, a length of a small diameter tube can offer the same restrictive effect. A small diameter tubing is called ‘capillary tube’, meaning ‘hair-like’. The inside diameter of the capillary used in refrigeration is generally about 0.5 to 2.28 mm (0.020 to 0.090’). The longer the capillary tube and/or the smaller the inside diameter of the tube, greater is the pressure drop it can create in the refrigerant flow; or in other words, greater will be the pressure difference needed between the high side and low side to establish a given flow rate of the 
refrigerant.
The length of the capillary tube of a particular diameter required for an application is first roughly determined by empirical calculations. It is then further correctly established by experiments. The capillary tube is not self-adjusting. If the conditions change, such as an increase in the discharge/condenser pressure due to a rise in the ambient temperature, reduction in evaporator pressure, etc. the refrigerant flow-rate will also change. Therefore a capillary tube, selected for a particular set of conditions and load will operate somewhat less efficiently at other conditions. However if properly selected, the capillary tube can work satisfactorily over a reasonable range of conditions.
As soon as the plant stops, the high and low sides equalise through the capillary tube. For this reason, the refrigerant charge in a capillary tube system is critical and hence no receiver is used. If the refrigerant charge is more than the minimum needed for the system, the discharge pressure will go up while in operation. This can even lead to the overloading of the compressor motor. Further, during the off cycle of the unit, the excess amount will enter the cooling coil and this can cause liquid flood back to the compressor at the time of starting. Therefore, the refrigerant charge of the capillary tube system is critical. For this reason, a refrigerant liquid receiver cannot be used. The charge should be exactly the quantity as indicated by the manufacturer of the refrigeration unit. 
Since the capillary tube equalises the high side with the low side during the off cycle, the idle pressures at the discharge and suction of the compressor will be equal. Therefore at the time of starting, the compressor motor need not overcome the stress of the difference of pressure in the suction and the discharge sides. In other words the compressor is said to start unloaded. This is a great advantage as a low starting torque motor is sufficient for driving the compressor.
The capillary tube is quite a simple device and is also not costly. Its pressure equalisation property allows the use of a low starting torque motor. The liquid receiver is also eliminated in a capillary tube system because of the need to limit the refrigerant charge. All these factors help to reduce the cost of manufacture of the systems employing a capillary tube as the throttling device.The capillary tube is used in small hermetic units, such as domestic refrigerators, 
freezers and room air conditioners.

2 Float Valves

There are mainly two types of float valves- low side float valves and high side float valve. 

Low-side Float Valve

This is similar to the float valves used for water tanks. In a water tank the float valve is fixed at the outlet of the water supply pipe to the tank. When the water level is low in the tank, the float ball hangs down by its own weight and the float arm keeps the valve fully open to allow water flow into the tank. As the water level rises, the float ball (which is hollow) floats on the water and gradually rises according to the water level, throttling the water through the valve. Ultimately when the tank is full, the float valve completely closes the water supply. As the water from the tank is used, the water level falls down; the float ball also lowers down, opening the valve according to the level of water in the tank.
The low-side float valve also acts in the same way in a refrigeration system. As the name implies the float valve is located in the low pressure side of the system. It is fixed in a chamber (float chamber) which is connected to the evaporator. The valve assembly consists of a hollow ball, a float arm, needle valve and seat. The needle valve-seat combination provides the throttling effect similar to the expansion valve needle and seat. The movement of the float ball is transmitted to the needle valve by the float arm. The float ball being hollow floats on the liquid refrigerant. The needle valve and seat are located at the inlet of the float chamber. As the liquid 
refrigerant vaporises in the evaporator, its level falls down in the chamber. This causes the float ball to drop and pull the needle away from the seat, thereby allowing enough liquid refrigerant to flow into the chamber of the evaporator to make up for the amount of vaporisation. When enough liquid enters, the float ball rises and ultimately closes the needle valve when the 
desired liquid level is reached. The rate of vaporisation of liquid and consequent drop in the level of the liquid in the evaporator is dependent on the load. Thus the movement of the float ball and amount of opening of the float valve is according to the load on the evaporator. The float valve responds to liquid level changes only and acts to maintain a constant liquid level in the evaporator under any load without regard for the evaporator pressure and temperature.Like in the expansion valve, the capacity of the low-side float valve depends on the pressure difference across the orifice as well as the size of the orifice.
Low-side float valves are used for evaporators of the flooded-type system. In bigger capacity plants a small low-side float valve is used to pilot a liquid feed (and throttling) valve. According to the liquid level in the evaporator, the float valve transmits pressure signals to the main liquid feed valve to increase or decrease the extent of its opening. Thus the low-side float valve in such a system is called a ‘pilot’ and the liquid-feed valve is known as the pilot-operated liquid-feed valve.

High-side Float Valve


The high-side valve like the low-pressure float valve, is a liquid level sensing device and maintains a constant liquid level in the chamber in which it is fixed. However it differs from the low-side float valve in the following respects.

(a) The high-side float valve and its chamber are located at the high pressure side of the system, while the low-side float valve is located at the low-pressure side of the system.


(c) In the high-side float valve, the valve opens on a rise in the liquid level in the chamber, just the opposite action of the low-side float valve, which closes on a rise in liquid level in the chamber.


The high-side float chamber is located between the condenser and 
evaporator. The liquid condensed in the condenser flows down to the float 
chamber.

As the liquid level rises in the chamber, the float ball also rises, thereby opening the needle valve. As the liquid level falls in the chamber, the float valve tends to close the seat orifice. It is obvious that refrigerant vapour is condensed in the condenser at the same rate at which the liquid vaporises in the evaporator; the float chamber receives and feeds liquid to the evaporator 
at the same rate. Since the rate of vaporisation of the liquid in the evaporator is according to the load, the high-side float obviously works as per the load.
This type of float valve is generally used in centrifugal-refrigeration plants.Refrigerant feed/throttling devices for flooded chillers are usually the low side or high-side float valve. For example, in centrifugal plants, the chiller is of the flooded type and generally high-side float valves are used as throttling devices. In a flooded chiller working in conjunction with a reciprocating compressor, a low-side float valve is used as the throttling and refrigerant liquid flow control.

3 Thermo - static Expansion Valve

The name ‘thermostatic-expansion valve’ may give the impression that it is a temperature control device. It is not a temperature control device and it cannot be adjusted and used to vary evaporator temperature. Actually TEV is a throttling device which works automatically, maintaining proper and correct liquid flow as per the dictates of the load on the evaporator. Because of its adaptability to any type of dry expansion application, automatic operation, high efficiency and ability to prevent liquid flood backs, this valve is extensively used.The functions of the thermostatic-expansion valve are: 

(a) To reduce the pressure of the liquid from the condenser pressure to 
evaporator pressure, 

(b) To keep the evaporator fully active and 

(c) To modulate the flow of liquid to the evaporator according to the load 
requirements of the evaporator so as to prevent flood back of liquid 
refrigerant to the compressor.

It does the last two functions by maintaining a constant superheat of the refrigerant at the outlet of the evaporator. It would be more appropriate to call it a ‘constant superheat valve’.The important parts of the valve are:
Power element with a feeler bulb, valve seat and needle, and a superheat adjustment spring.55
Refrigeration