glossary of common refrigeration terms - pennine env of... · · 2016-01-13glossary of common...

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Glossary of Common Refrigeration Terms

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GLOSSARY Air-Cooled Condenser A condenser cooled by natural or forced flow of air. Ambient Temperature The prevailing temperature of the atmosphere surrounding the

component under consideration. Atmospheric Pressure The pressure exerted by the column of air in the atmosphere

above the reference point. Capacity Control Variation in the quantity of refrigerant circulated in order to vary the

refrigerant capacity. Compressor A machine for mechanically increasing the pressure of a gas. Condenser A heat exchanger in which a vapour is liquified by the removal of

heat. Condensing Pressure The pressure at which a vapour changes into a liquid at a specific

temperature. Defrost Elimination of an ice deposit from the surface of an evaporator. Desuperheat Removal of part or all of the superheat in a gas. Discharge Pressure The pressure of the compressed fluid discharged from a

compressor. Discharge Temperature The temperature of the compressed fluid discharged from a

compressor. Drier A device for removing moisture from the refrigerant. Evaporating Pressure The pressure at which a fluid vaporises within an evaporator at a

specific temperature. Evaporating Temperature The temperature at which a fluid vaporises within an evaporator at

a specific temperature. Evaporator A heat exchanger in which a liquid is vaporised to produce

refrigeration. Fin block A group of tubes which have been expanded into fins to form a

heat exchanger. Heat Exchanger A device designed to transfer heat between two physically

separated fluids. High-Pressure Switch A switch designed to stop the compressor motor should the

discharge pressure reach a predetermined maximum value. Liquid Back A situation where refrigerant liquid returns to the compressor. Low-Pressure Switch A switch designed to stop the compressor should the evaporating

pressure reach a predetermined minimum value.


Oil Pressure Switch A switch designed to stop the compressor should the oil pressure

drop to a predetermined minimum level. Oil Separator A device for separating oil from refrigerant vapour. Open Compressor A compressor driven by an external power unit. Requiring a shaft seal. Operating Conditions The conditions under which a refrigeration system works, including

the evaporating pressure and condensing pressure. Pressure Relief Valve A mechanical safety device to relieve excessive pressure in the

system. Reciprocating Compressor A positive displacement compressor with piston(s) moving linearly

and alternately in opposite directions in the cylinder(s). Refrigeration Capacity The quantity of heat which a refrigeration plant is capable of

extracting under specified conditions of time and temperature. Saturation A condition at which liquid and vapour may exist when in contact

with each other. Semi-Hermetic A compressor directly coupled to an electric motor and Compressor contained within a gas-tight bolted housing. Shut-off Valve A valve used to isolate particular items of equipment. Sight Glass A device which allows visual inspection of the level of a liquid

within a pressurised chamber. Sub cooled Liquid A liquid whose temperature is low than the condensing

temperature at its given pressure. Suction The low pressure section of a compressor into which gas is drawn

from the system. Suction Accumulator A vessel in the suction line designed to prevent slugs or liquid

refrigerant entering the compressor. Suction Return The temperature at which refrigerant gas enters the compressor. Temperature Superheat The quantity of heat added to dry saturated vapour to raise it from

its saturation temperature to a higher temperature. Temperature difference The difference in temperature between two substances, surfaces

or environments involving transfer of heat. Thermostat A automatic switch which is responsive to temperature.


Thermostatic expansion A valve which automatically regulates the flow of liquid Valve refrigerant in to the evaporator to maintain within close limits the C degree of superheat of the vapour leaving the evaporator. Ventstream Air Chiller A Pennine designed bespoke system for the chick chilling of

eviscerated poultry. Ventstream Duct The means of introducing refrigerated air, into the cavity of

eviscerated poultry, in a Pennine Ventstream to reduce the cooling period.

Interleaved conveyor A means of using small conveyor wheels set in between larger

wheels to reduce the footprint of a Ventstream Air Chiller ‘X’ Stream A Low Temperature Impingement Freezer designed by Pennine. Crust Freezer A generic name for the Pennine ‘X’Stream impingement crust

freezer. ‘Flo-Stream’ The name of the Pennine designed system of enhancing liquid

refrigerant flow. Impingemen: The application of ultra high velocity air to a product in a Pennine ‘X’ Stream to shorten the cooling or freezing time. Displacement Cooling A means by which Pennine maintain a ‘bath’ of draught free

temperate air in an occupied processing room. Occupied zone The region between 2.0m and the floor of a production area being

the temperature controlled by Pennine in their Displacement Ventilation systems.

Air displacement terminal The ultra low velocity air diffusion unit designed by Pennine to

distribute air in the occupied zone of their Displacement Ventilation systems.

High hygiene cooler A bespoke Pennine designed stainless steel cooling unit which has

no horizontal surfaces. Suitable for cooked product blast chilling. Cleanable ceiling mounted cooler A unit cooler having features designed by Pennine in 1988 which

has been adopted as an industry standard.


SAFETY DEVICES COMPRESSOR Each compressor is protected by a series of safety devices that will not allow the compressor to run should a fault condition arise, they are as follows: High Pressure Switch stops the compressor if the discharge pressure rises above a pre set value – this switch is manually reset. Low Pressure Switch stops the compressor if the suction pressure drops below a pre set value – this is also manually reset. Oil Flow Switch stops the compressor if the oil flow drops too low. Oil Pressure Differential Switch stops the compressor if the oil pressure from the pump drops below a pre-determined pressure above suction pressure. High Discharge Temperature Thermistor stops the compressor if the discharge gas temperature exceeds a pre set level. Manual reset. Thermistor (hermetic/semi hermetic compressors and motors to driven compressors) stops the compressor if winding temperature exceeds a pre set level. Resets on temperature drop below pre set level and reset button pressed if fitted. CONDENSER / RECEIVER Pressure Relief Valve discharges gas from system if pressure exceeds a pre set level. Closes automatically on pressure drop below pre set level. Change this if it blows. High Pressure Switch see above. Frost Protection Thermostat (Evaporative Condensers) stops water pump when coolant temperature drops below pre set level. SURGE DRUM & PUMPS High Level Switch switches off pump(s) if level reaches/exceeds a pre set level. Pump Differential Switch switches off pump if differential pressure across the pump falls below pre set level for pre set length of time. Pressure Relief Valve see above. Pump Motor Thermistor see above. EVAPORATOR Defrost Termination stops defrost sequence after pre set temperature is reached. Defrost Termination Safety Timer stops defrost after pre set time if termination temperature is not reached within that time.


HIGH SIDE PLANT The ring main high side comprises of direct driven screw compressors, air cooled condenser and liquid receiver. Refrigerant suction gas is drawn into the compressor where it is compressed and consequently rises in temperature. The heat energy contained must be removed before the refrigerant can be re-used. The high pressure gas then passes to the air cooled condenser. In the air cooled condenser the hot gas passes through a coil matrix where the residual heat is extracted by air passing over the coil.

The refrigerant which has now condensed into a saturated liquid, drains into the receiver where it is stored until required. The receiver includes sight glasses from which the refrigerant level can be determined, and two pressure relief valves which in an emergency will relieve pressure in the system if it exceeds a pre set level. As required the liquid leaves the receiver where it passes through a liquid filter drier, sight glass (which shows if the refrigerant contains moisture) and solenoid valve (which is de-energised to pump down the system. Also should the compressors fail the solenoid valve is usually de-energised by means of safety relay to arrest the flow of refrigerant). After leaving the solenoid valve the saturated liquid passes through to the system. Capacity control is usually maintained by a neutral zone pressure switch which senses suction pressure and signals a step controller which energises or de-energises the compressor loading solenoids to demand. Head pressure is maintained within a set band by sensing discharge pressure and controlling the air cooled condenser fans accordingly.


HIGH SIDE PLANT The ring main high side comprises of direct driven screw compressors, evaporative condenser, de-super heater and liquid receiver. Refrigerant suction gas is drawn into the compressor where it is compressed and consequently rises in temperature. The heat energy contained must be removed before the refrigerant can be reused. The high pressure gas then passes to the evaporative condenser through two hot gas/water de-super heaters where some of the heat is taken out of the hot gas and is used to heat water. In the evaporative condenser the hot gas passes through a coil matrix where the residual heat is extracted by water passing over the coil. Air is also blown up through the coil, accelerating evaporation of the water and therefore the condensing effect (hence the term Evaporative Condenser).

The refrigerant that has now condensed into a saturated liquid drains into the receiver where it is stored until required. The receiver includes sight glasses from which the refrigerant level can be determined, and two pressure relief valves which in an emergency will relieve pressure in the system if it exceeds a pre set level. As required the liquid leaves the receiver where it passes through a liquid filter drier, sight glass (which shows if the refrigerant contains moisture) and solenoid valve (which is de-energised to pump down the system). Also, should the compressors fail the solenoid valve is de-energised by means of safety relay to arrest the flow of refrigerant. After leaving the solenoid valve the saturated liquid passes through to the direct expansion evaporators and the vent stream surge drum.


Capacity control is maintained by a neutral zone pressure switch that senses suction pressure and signals a step controller that energises or de-energises the compressor loading solenoids to demand. Head pressure is maintained by a proportional fan speed controller which senses discharge pressure and modulates the speed of the evaporative condenser fan to slow if discharge pressure is low and quicken if discharge pressure is high. An ambient thermostat will operate the water pump irrespective of load if the ambient temperature should fall below a pre set level.


DX SYSTEM HIGH SIDE PLANT The high side refrigeration plant basically comprises of one or more compressors, one or more condensers and a high pressure liquid receiver. Refrigerant suction gas is drawn into the compressor where it is compressed and consequently rises in pressure and temperature. The heat energy contained must be dissipated before the refrigerant can be reused. The high pressure gas passes from the compressor to the coil matrix of the condenser. In an air-cooled condenser air is drawn through the coil matrix in order to reduce the temperature of the discharge gas to a point at which it condenses. (In an evaporative condenser, re-circulated water is also sprayed on the coil to further aid heat dissipation.)

The refrigerant that has now condensed into a saturated liquid drains into the high pressure receiver where it is stored until required in the system. The refrigerant leaves the receiver through a filter drier (to ensure cleanliness), sight glass and solenoid valve. The solenoid valve is closed to pump down the system. In the event of a failure of the compressor, the solenoid will be de-energised automatically to arrest the flow of refrigerant. The liquid refrigerant is then distributed to the thermostatic expansion valves, which meter the flow of refrigerant into the room evaporators. As the refrigerant picks up heat from the room, its state changes to superheated gas, it is then drawn back to the compressors. During a fluctuating load, a steady suction pressure is maintained by electrically selecting and loading solenoids in the compressor. These effectively negate some of the compressive effect and are generally known as “unloading” or “capacity control”.


As the load fluctuates thereby causing the compressors to load and unload, the condenser capacity varies proportionally. In order to maintain a condensing pressure within a control band, the fans are simply switched on and off. With an evaporative condenser the air stream running from the centrifugal fans is dampered at the demand of a stepless proportional controller. When using some screw compressors it is necessary to maintain a minimum discharge pressure at the oil separator to circulate the oil. This is achieved by fitting a pressure regulating valve in the discharge line. This valve would also be used to maintain a discharge pressure if hot gas defrosting of evaporators is utilised.


THE OPERATING PRINCIPLE OF A DX AIR HANDLING UNIT (WITHOUT DEFROST) Air is drawn into the air handling unit through a direct expansion evaporator at the rear. The cool air is then drawn into the centrifugal fan and discharged into the air discharge ductwork. The air is distributed within the conditioned air by means of low velocity multidirectional air distribution grilles or socks. The sock disperses air at a very low velocity into the area thereby reducing the probability of draughts. Refrigerant liquid is metered into the evaporator coil circuitry by the thermostatic expansion valve and is distributed throughout the circuits by the “spider”. The refrigerant, which is now at a low pressure, absorbs heat from the surrounding area and “boils”. The resulting vapour is decanted from the evaporator into the suction line by pressure differential. The vapour should have a superheat of about 6°K when it enters the suction line. The suction vapour then returns to the compressor where the cycle recommences. The on/off operation is controlled by an electronic thermostat which monitors the temperature of the air returning to the air handling unit. The thermostat would normally be set with a differential of 2°C i.e, for a room temperature of 10°C, on at 11°C – off at 9°C.

In some applications there is a requirement for heating (Monday morning start for instance). When this applies the heaters are controlled either with a separate thermostat, which would normally be set to bring the heaters on at a temperature of about +6°C and switch them off at about +8°C. It must be noted that the thermostat that controls the heaters should not overlap the cooling thermostat. Where heating/cooling control is required, a neutral zone thermostat is usually employed. On applications where equipment cooling production areas is connected to chill ring mains, an evaporating pressure regulator is required. This will maintain an artificially high pressure within the cooler (normally between 3.8 bar and 4.0 bar for R22 and 4.8 bar and 5.0 bar for R404A). In maintaining an equivalent temperature within the cooler of approximately –1°C the cooler will not ice up and therefore will not require defrosting. (If the cooler does ice up there is a problem.)


THE OPERATING PRINCIPLE OF A DX COOLER (WITHOUT DEFROST) Refrigerant liquid is metered into the evaporator coil circuitry by the thermostatic expansion valve and is distributed throughout the circuits by the “spider”. The refrigerant, which is now at a low pressure, absorbs heat from the surrounding area and “boils”. The resulting vapour is decanted from the evaporator into the suction line by pressure differential. The vapour should have a superheat of about 6°K when it enters the suction line. The suction line vapour then returns to the compressor where the cycle recommences. The on/off operation is controlled by an electronic thermostat, which monitors the temperature of the air returning to the cooler. The thermostat would normally be set with a differential of 2°C e.g, on at +1°C/off at –1°C.

On application where equipment cooling production areas is connected to chill ring mains, an evaporating pressure regulator is required. This will maintain an artificially high pressure within the cooler (normally between 3.8 bar and 4.0 bar for R22 and 4.8 bar and 5.0 bar for R404A). In maintaining an equivalent temperature within the cooler of approximately –1°C the coil block will not ice up and therefore will not require defrosting. (If the cooler does ice up there is a fault.)


THE OPERATING PRINCIPLE OF ELECTRIC DEFROST At a predetermined time the controller will shut off the liquid line solenoid which will stop the flow of refrigerant to the cooler. As part of the same sequence the electric heaters come on and begin to warm the coil block. By the time the heat begins to permeate the coil block, the residue of liquid in the cooler tubes will have boiled off thereby allowing the coil block temperature to rise above the evaporating temperature. (If for any reason liquid passes into the cooler during defrost the coil will not clear and the fault should be corrected.) The coil block temperature will begin to rise until the ice has melted. At a pre-set temperature (usually around 10°C) a defrost termination thermostat will send a signal that will de-energise the defrost heaters. In certain circumstances there may be a drip down period (about 2 minutes) although usually the liquid line solenoid is energised without running the cooler fans to “snap freeze” any remaining droplets of water onto the tubes of the cooling coil. This is called the “fan delay time” and is usually about 2 minutes in duration. After the “fan delay time” the fans become operational and normal cooling is resumed.

In order to provide a safety backup there is a timer in circuit that terminates the defrost heating after about 30 minutes whether or not the defrost termination thermostat has operated. This is to stop the cooler from “cooking” if the defrost termination set up is faulty. The probe of the defrost termination thermostat should be positioned where the coil has ice last. This is usually at the bottom of the block or on the distributor.


THE OPERATING PRINCIPLE OF HOT GAS DEFROST ON A DX COIL At a predetermined time the controller will shut off the liquid line solenoid ‘A’ that will stop the flow of refrigerant to the cooler. The sequence will then initiate the de-energising of the suction line solenoid ‘B’ and the energising of the hot gas inlet solenoid ‘C.' It is not usually necessary to achieve a pumpdown before a hot gas defrost – if in doubt, check. NB The suction line solenoid must be closed BEFORE the opening of the hot gas line. When hot gas is called for it is normal for a valve to close in the discharge line that will maintain an artificially high discharge pressure to force gas down the hot gas line to the evaporator. The hot gas will flow into the cooler through the suction leader and will condense within the tubes. The filling of the tubes will cause the pressure to rise within the coil matrix until it is in excess of the main line liquid pressure. Consequently, the condensed liquid will flow out of the coil through the “defrost return line” past the check valve ‘D’ into the main liquid line. Until the pressure in the coil block is well above the equivalent of 0°C, defrost will not commence, as the refrigerant will not be warm enough to heat the tubes.

At a pre-set temperature (usually around 10°C) the coil block is generally free of ice. The defrost termination thermostat senses this from its usual probe position either on the “spider” or on a return bend at the bottom of the coil block. In any case the probe should be positioned at the last point where the ice melts. At Defrost termination the hot gas solenoid ‘C’ is de-energised and the cooler usually will stand for a period with all valves closed. At the end of this “drip time” the suction solenoid ‘B’ will open which will relieve the pressure in the cooling coil back to suction. It maybe that on larger coils a pilot solenoid is opened first to relieve the pressure otherwise there could be a rush of refrigerant back to the compressors (and a sound like the clap of doom).


Bear in mind that at the end of defrost the coil is virtually full of high pressure liquid and as soon as pressure is relieved (when the suction line opens) the liquid will boil furiously At the compressors there should be a knock out pot or suction accumulator that will stop this surge of liquid entering the compressor, provided that defrosts on different coils are not too close together. There should be a period after the suction line solenoid ‘B’ opens and before the liquid line solenoid energises. This period should be long enough to boil off the greater proportion of refrigerant liquid in the cooling coil (DX coolers normally run 25% wet). When the liquid line solenoid ‘A’ is energised, normal cooling operation is re-established. If the cooler is not clear of ice in 15 minutes after the hot gas solenoid ‘C’ is energised, there is a fault. The total defrost period, including rests should be 30 minutes.


THE OPERATING PRINCIPLE OF A REFRIGERANT LIQUID SUBCOOLER The refrigerant liquid enters and leaves the receiver at saturation point. This is likely to be between 35°C and 40°C. If at this condition the refrigerant liquid is subject to a swift pressure reduction or a high heat flux (for example a hot roof void) it will boil within the pipework causing “flash gas”. As “flash gas” can cause problems elsewhere in the system (not least with controls) it is not desirable.

A way to overcome this phenomenon is to cool the refrigerant liquid to a temperature that is below its saturation or boiling point. This will compensate for heat pick up or excessive pressure drop.


COMPRESSORS WITH ECONOMISERS Some compressors are fitted with an extra suction connection that is normally termed an “economiser port”. With the economiser port connected (and support equipment installed) the refrigerant capacity and system efficiency can be improved over normal single stage plants. The advantages become more apparent with the high pressure ratios of low temperature systems.

The characteristics of the screw compressor enables the introduction of more refrigerant suction gas some way down the rotors than can be initially introduced through the dry suction connection. (The pressure at this point is similar to that of the intermediate pressure of a two-stage system). The additional mass flow of gas through the compressor provides the additional capacity but at a greatly reduced power penalty due to the now increased suction pressure condition. (While the power demand goes up, it only slightly increases in comparison to greatly increased capacity). In addition to the increased performance of the compressor the system efficiency can be increased by means of subcooling the refrigerant supply to the coolers.


OIL RECOVERY FOR PUMPED REFRIGERATION SYSTEMS A sample of refrigerant is taken from the discharge side of the refrigerant pump and this is passed through a heat exchanger in the compressor discharge line via an expansion device.

The “oil rich” mixture boils and the refrigerant element turns to superheated gas, the gas /oil mix is returned to the dry suction line which feeds directly back to the compressor. The superheat measurement of the gas/oil mix should be in the region of 10 - 20°K.


THE PRINCIPLE OF A PUMPED FLOODED EVAPORATOR

The refrigerant is metered into the surge drum to maintain a level that is determined by an adjustable level control. (It would be usual to have the level at about ¼ of the way up the drum.) The flash gas resulting from the introduction of the liquid refrigerant passes immediately into the dry suction line back to the compressors. The liquid refrigerant passes down the drop leg of the surge drum into the inlet part of the refrigerant pump where it encounters an internal filter. (It is important that this filter is checked for debris as part of a regular maintenance schedule.)

The pump then pumps the liquid refrigerant into the pumped liquid line and out to the evaporator matrix where heat from the surrounding air vaporises some of the liquid refrigerant. This liquid/vapour combination rises up through the evaporator boiling as it goes and leaves the evaporator to return to the surge drum down the wet return pipe at a ratio of about 1.5 to 2 parts liquid to 1 part gas. It should be noted that the flow to each evaporator is regulated by a pressure reduction valve to ensure that the evaporators are evenly balanced. While the discharge pressure at the pump is in the order of 2.5 bar above suction gauge pressure this pressure advantage is lost as soon as the liquid begins to boil after it passes the pressure reduction valve. The liquid/gas mixture relies on pressure differential to return to the surge drum and this is likely to be about .1 bar. As the liquid/gas mixture enters the surge drum from the wet return line the liquid falls to the bottom of the surge drum to be taken up by the pump and the gas passes into the dry suction line to the compressor.


NORMAL OPERATION Liquid line solenoid ‘A’ is open along with wet return solenoid ‘B’ to allow constant flow of refrigerant through the evaporator causing boiling within the tubes and the cooling of the air that passes through the coil matrix. The hot gas solenoid ‘C’ is closed.


DEFROSTING At the start of defrost both solenoids ‘A’ and ‘B’ close thereby cutting off the evaporator from its supply of refrigerant. After a rest period of approximately 2 minutes, hot gas solenoid ‘C’ opens to provide high pressure hot gas from the compressors. The pressure in the evaporator quickly rises and the cold refrigerant is forced out through the defrost pressure relief valve ‘D.' Until the pressure in the evaporator rises sufficiently stop the hot gas condensing the evaporator tubes will not get hot enough to melt the ice. When the tubes have reached a temperature when all the ice has disappeared (8° to 10°) the hot gas valve ‘C’ will close and the evaporator will stand with all valves closed for about two minutes. At the end of this “driptime” a small pilot solenoid around ‘B’ will energise relieving the remaining pressure within the evaporator into the wet return line.

If for any reason this pilot valve should not operate, when the main valve ‘B’ opens two minutes later there would be a slug of liquid returning under comparatively high pressure back to the surge drum. This would dramatically increase the pressure in the drum (causing compressors to start unnecessarily) and raise the level. (There could also be a noise like the clap of doom.) About two minutes after the opening of main valve ‘B’ when normal boiling is restored the pumped liquid valve ‘A’ opens to permit the normal flow of refrigerant to the now defrosted evaporator. The hot gas defrost valve should only be open for about 10 to 15 minutes, any longer and there could be a problem.


THE OPERATING PRINCIPLE OF A GRAVITY FED FLOODED EVAPORATOR An electrical or mechanical level control valve will regulate the flow of refrigerant into a surge vessel. The flash gas, which forms immediately due to pressure reduction, is drawn out of the vessel back to the compressor through the dry suction line. Liquid refrigerant drops down the liquid supply leg to the evaporator where heat from the surrounding media vaporised some of the liquid. This liquid/vapour combination (being less dense than the column of liquid in the liquid supply leg) is pushed back to the surge drum by pressure/density difference. Back in the surge drum the liquid settles to the bottom of the drum and the vapour passes into the dry suction line. The weight of refrigerant that passes down the dry suction line is replaced via the control valve thereby maintaining a constant level in the surge vessel.


DEFROSTING THE FLOODED EVAPORATORS During normal operation the flow of refrigerant would be through the evaporator from liquid inlet to wet return however, during defrost the flow is reversed with the hot gas entering the wet return line and flowing through the evaporator coil matrix where it cools, giving up its heat to the surrounding metal. The resulting liquid condense leaves through the liquid line to finally pass back to the surge drum.

glossary of common refrigeration terms - pennine env of... · · 2016-01-13glossary of common...

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