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13-05-2004, 09:14 PM
A Floating Head Pressures Technology without a loss in TEV or EEV Capacity by Marc L O’Brien.
Definitions
EEV = Electronic Expansion Valve.
TEV = Thermostatic Expansion Valve.
dT= Temperature Change
dP= Pressure Change
As head pressure is dropped:
The compressor would consume considerably less power because:
Reduced compression ratios means for reduced power consumption per unit weight refrigerant circulated.
The compressor unloads because the weight of refrigerant circulated has reduced due to increased refrigerant specific refrigeration effect.
Although the compressor would tend to unload proportional to increased volumetric efficiency, this would not result in any marked power reduction since power is proportional to swept volume and the modified “required” swept volume is assumed to remain constant.
The compressor would tend to unload because:
Liquid temperature hence liquid enthalpy reduces giving increased refrigerant specific refrigeration effect allowing a reduction in mass flow.
Compressors swept volume would increase due to increased volumetric efficiency brought about by reduced compression ratios (Clearance Pocket).
Changes affected by thermophysical dynamics.
Altered condenser refrigerant charge.
There are two forces by which condensed liquid is encouraged to leave an air-cooled condensers horizontal tubes. The first force is “tube pressure drop” related to flow. The second force is “gravity flow” related to the inner tube liquid level. Whenever there is a reduction in mass flow by compressor unloading there is a corresponding squared reduction in tube pressure drop. If the compressor unloads to 50% of full load then the tube pressure drop is reduced to 25% of the original full load value. With this drastic reduction in tube pressure drop there is an associated and marked increase in tube liquid level as tube drainage becomes more gravity dependent. The result is an increase in condenser operating refrigerant charge.
With the drop in condenser operating pressure there is an accompanying vapour density drop, when reducing saturated condensing temperatures from 43oC to 20°C we witness a density reduction of 46%. This density change tends to increase liquid line mass. However, with the coinciding reduced liquid temperatures there is an increase in liquid density of 7% acting to free up liquid line volume, this effect somewhat accommodates the above-mentioned increased mass there.
Altered evaporator refrigerant charge.
Any time there is a reduction in evaporator flash gas there will be an increase in evaporator operating refrigerant charge. Variables affecting a reduction in evaporator flash gas are a reduction in refrigerant mass flow occurring when the compressor is unloaded due to reduced loads and/or reduced liquid enthalpies.
An analogy here would be a pot of water on the stove. Consider the water mass required to maintain a constant liquid level at varying thermal conditions. When the water is boiled, meaning water vapour forms at, and rises from, the bottom, we have a larger vapour to liquid ratio giving the combined water mass a low density. As you reduce the boiling, the liquid level will drop, as the waters density increases. If we wished to maintain the wet level but reduce the boiling then clearly we would need to add water to the pot.
Resulting altered TEV and EEV capacities.
This low liquid enthalpy induced refrigerant redistribution results in both EEV and TEV starvation at a time when their capacities are already cut somewhat by reduced pressure drops.
Overall system refrigerant mass redistribution, by the described increase in both evaporator and condenser operating charge, most often affects a reduction in liquid line mass to the point where all expansion devices are liquid starved. Any expansion valve, EEV or TEV, has its capacity considerably reduced when starved of refrigerant, when not being fed by a solid liquid seal.
Comparing EEV and TEV when floating head pressure.
With the speculated saturated condensing pressure drop from 43oC to 20oC there is a reduction in available TEV pressure drop, an increase in available liquid density and a reduction in liquid enthalpy. With these three changed aspects considered, and assuming we have solid liquid available, it could be estimated that:
EEV capacity loss may be in the region of 20%.
TEV capacity loss may be in the region of 28%.
With both TEV and EEV type expansion devices, capacity is subject to available liquid quality. It has to be guaranteed that there is a solid liquid head available at the EEV or TEV inlet.
The occurrence of liquid line vapour is especially increased with liquid line component pressure drop or line lift, while also, ambient or solar thermal gains will contribute.
Some EEV manufacturers suggest the inclusion of a control characteristic to cycle condenser fans in the event of low load and/or low ambient EEV liquid starvation occurring as a result of liquid line vapour.
Subcooling to eliminate liquid line vapour.
Studying any refrigerants saturation curve, we can see that liquid saturated dT/dP ratios increase considerably at the lower 20oC conditions, such that for any given liquid line static lift or component frictional pressure drop an increase in subcool by some 56% is required as compared to the design 43°C liquid temperature. Even with the reduced mass flows resulting from increased specific refrigeration effect, this factor must still be considered. This increased dT/dP effect on liquid line lift is not changed by reduced mass flow following increased refrigeration effect, in fact it is worsened by liquid density increase. Further, with increased compressor COP comes reduced condenser TD and therefore a reduced available subcool margin.
Increasing system refrigerant charge to overcome the above described redistribution and subsequently reduced available subcool problems will typically result in unwanted condenser flooding during high loads at higher designed ambients effectively raising head pressures above design. Considering the higher saturated dP/dT ratios at higher saturated conditions we can understand the higher compression ratio penalties paid for slight increases in saturated condensing temperatures. Although compressor COP may be somewhat maintained by the fact that the increased charge acts to increase summer subcooling bringing liquid temperatures closer to ambient, there is still the increased rate of compressor wear to consider. Also, summer condenser fan operation is extended increasing overall system power consumption.
The reduced liquid to suction line temperature difference reduces suction/liquid heat interchanger capacity. This is especially so if it is intended that saturated refrigerant pressures are dropped even further below the already speculated 20oC, which is most often the case. Optional heat interchangers required for these low liquid to suction temperature differences may be so large as to make their use wholly impractical.
Elevated Condensing Units
One partial solution to the above charge redistribution effecting TEV and EEV capacity by starvation is to elevate the condensing unit and receiver (if used) and allow for a liquid line drop sufficient to overcome liquid line component pressure drops and so by gravity develop a healthy full liquid head at the valve inlet. However, even with this configuration, EEV and TEV capacities are still subject to a reduction when floating head pressures due to the reduction in operation pressure drops. Depending on the application, trial and error EEV size selection may be made to work with elevated condensing units and floating head pressures. Some commercial refrigeration equipment manufacturers are using elevations of 10 meters and more. If insufficient lift is available then we find complete system hunt i.e. valves being alternatively fed and starved causing compressors to load all the way up before completely unloading and then the condenser fans follow. This de-optimises the energy savings potential, increases system component wear and results in very poor oil circulation control.
Liquid Pressure Amplification.
Primarily, LPA is about achieving floating head pressures. On DX systems, a liquid line pump is added to the liquid line in order to somewhat maintain expansion device design operating pressure difference while head pressures are permitted to float with colder ambients. The pump also provides added subcool, by pressure amplification, needed to match the increased saturated refrigerant dT/dP ratio.
Of course, with LPA, a small additional charge quantity may be necessary to ensure at least 1K subcooling at the pump inlet. However, this charge addition is far less than the extra charge needed to achieve required subcool for the same liquid line pressure losses without the use of a pump. A relatively small flow-through receiver, containing saturated liquid, situated just above the pump, is all that may be required to maximize the lowering of head pressures without flooding the condenser in summer.
The advantage here then is that, in many instances, the cheaper and more proven TEV could be used in preference to the EEV. Also, the pump can provide for re-condensing of the liquid line vapour portions mentioned earlier in this article. When design valve pressure drops are still not being achieved a carefully sized capillary can be installed parallel to the TEV to supplement TEV orifice size. This has the added advantage of stabalising TEV control at times of low liquid enthalpy which otherwise reduces stability.
Even when floating head pressures, there are still often head pressure constraints requiring head pressure control, only at some lower limit. Considering the colder liquids increased saturated dT/dP ratio, head pressure controls cycling in and out can cause a “frothing” of the liquid line again reducing EEV and TEV capacity. The liquid pump will most often clear this liquid line frothing.
Marc O’Brien
Applications & Field Technician
Fridgetech.Com Ltd
Marc@Fridgetech.com
Fridgetech.Com
Definitions
EEV = Electronic Expansion Valve.
TEV = Thermostatic Expansion Valve.
dT= Temperature Change
dP= Pressure Change
As head pressure is dropped:
The compressor would consume considerably less power because:
Reduced compression ratios means for reduced power consumption per unit weight refrigerant circulated.
The compressor unloads because the weight of refrigerant circulated has reduced due to increased refrigerant specific refrigeration effect.
Although the compressor would tend to unload proportional to increased volumetric efficiency, this would not result in any marked power reduction since power is proportional to swept volume and the modified “required” swept volume is assumed to remain constant.
The compressor would tend to unload because:
Liquid temperature hence liquid enthalpy reduces giving increased refrigerant specific refrigeration effect allowing a reduction in mass flow.
Compressors swept volume would increase due to increased volumetric efficiency brought about by reduced compression ratios (Clearance Pocket).
Changes affected by thermophysical dynamics.
Altered condenser refrigerant charge.
There are two forces by which condensed liquid is encouraged to leave an air-cooled condensers horizontal tubes. The first force is “tube pressure drop” related to flow. The second force is “gravity flow” related to the inner tube liquid level. Whenever there is a reduction in mass flow by compressor unloading there is a corresponding squared reduction in tube pressure drop. If the compressor unloads to 50% of full load then the tube pressure drop is reduced to 25% of the original full load value. With this drastic reduction in tube pressure drop there is an associated and marked increase in tube liquid level as tube drainage becomes more gravity dependent. The result is an increase in condenser operating refrigerant charge.
With the drop in condenser operating pressure there is an accompanying vapour density drop, when reducing saturated condensing temperatures from 43oC to 20°C we witness a density reduction of 46%. This density change tends to increase liquid line mass. However, with the coinciding reduced liquid temperatures there is an increase in liquid density of 7% acting to free up liquid line volume, this effect somewhat accommodates the above-mentioned increased mass there.
Altered evaporator refrigerant charge.
Any time there is a reduction in evaporator flash gas there will be an increase in evaporator operating refrigerant charge. Variables affecting a reduction in evaporator flash gas are a reduction in refrigerant mass flow occurring when the compressor is unloaded due to reduced loads and/or reduced liquid enthalpies.
An analogy here would be a pot of water on the stove. Consider the water mass required to maintain a constant liquid level at varying thermal conditions. When the water is boiled, meaning water vapour forms at, and rises from, the bottom, we have a larger vapour to liquid ratio giving the combined water mass a low density. As you reduce the boiling, the liquid level will drop, as the waters density increases. If we wished to maintain the wet level but reduce the boiling then clearly we would need to add water to the pot.
Resulting altered TEV and EEV capacities.
This low liquid enthalpy induced refrigerant redistribution results in both EEV and TEV starvation at a time when their capacities are already cut somewhat by reduced pressure drops.
Overall system refrigerant mass redistribution, by the described increase in both evaporator and condenser operating charge, most often affects a reduction in liquid line mass to the point where all expansion devices are liquid starved. Any expansion valve, EEV or TEV, has its capacity considerably reduced when starved of refrigerant, when not being fed by a solid liquid seal.
Comparing EEV and TEV when floating head pressure.
With the speculated saturated condensing pressure drop from 43oC to 20oC there is a reduction in available TEV pressure drop, an increase in available liquid density and a reduction in liquid enthalpy. With these three changed aspects considered, and assuming we have solid liquid available, it could be estimated that:
EEV capacity loss may be in the region of 20%.
TEV capacity loss may be in the region of 28%.
With both TEV and EEV type expansion devices, capacity is subject to available liquid quality. It has to be guaranteed that there is a solid liquid head available at the EEV or TEV inlet.
The occurrence of liquid line vapour is especially increased with liquid line component pressure drop or line lift, while also, ambient or solar thermal gains will contribute.
Some EEV manufacturers suggest the inclusion of a control characteristic to cycle condenser fans in the event of low load and/or low ambient EEV liquid starvation occurring as a result of liquid line vapour.
Subcooling to eliminate liquid line vapour.
Studying any refrigerants saturation curve, we can see that liquid saturated dT/dP ratios increase considerably at the lower 20oC conditions, such that for any given liquid line static lift or component frictional pressure drop an increase in subcool by some 56% is required as compared to the design 43°C liquid temperature. Even with the reduced mass flows resulting from increased specific refrigeration effect, this factor must still be considered. This increased dT/dP effect on liquid line lift is not changed by reduced mass flow following increased refrigeration effect, in fact it is worsened by liquid density increase. Further, with increased compressor COP comes reduced condenser TD and therefore a reduced available subcool margin.
Increasing system refrigerant charge to overcome the above described redistribution and subsequently reduced available subcool problems will typically result in unwanted condenser flooding during high loads at higher designed ambients effectively raising head pressures above design. Considering the higher saturated dP/dT ratios at higher saturated conditions we can understand the higher compression ratio penalties paid for slight increases in saturated condensing temperatures. Although compressor COP may be somewhat maintained by the fact that the increased charge acts to increase summer subcooling bringing liquid temperatures closer to ambient, there is still the increased rate of compressor wear to consider. Also, summer condenser fan operation is extended increasing overall system power consumption.
The reduced liquid to suction line temperature difference reduces suction/liquid heat interchanger capacity. This is especially so if it is intended that saturated refrigerant pressures are dropped even further below the already speculated 20oC, which is most often the case. Optional heat interchangers required for these low liquid to suction temperature differences may be so large as to make their use wholly impractical.
Elevated Condensing Units
One partial solution to the above charge redistribution effecting TEV and EEV capacity by starvation is to elevate the condensing unit and receiver (if used) and allow for a liquid line drop sufficient to overcome liquid line component pressure drops and so by gravity develop a healthy full liquid head at the valve inlet. However, even with this configuration, EEV and TEV capacities are still subject to a reduction when floating head pressures due to the reduction in operation pressure drops. Depending on the application, trial and error EEV size selection may be made to work with elevated condensing units and floating head pressures. Some commercial refrigeration equipment manufacturers are using elevations of 10 meters and more. If insufficient lift is available then we find complete system hunt i.e. valves being alternatively fed and starved causing compressors to load all the way up before completely unloading and then the condenser fans follow. This de-optimises the energy savings potential, increases system component wear and results in very poor oil circulation control.
Liquid Pressure Amplification.
Primarily, LPA is about achieving floating head pressures. On DX systems, a liquid line pump is added to the liquid line in order to somewhat maintain expansion device design operating pressure difference while head pressures are permitted to float with colder ambients. The pump also provides added subcool, by pressure amplification, needed to match the increased saturated refrigerant dT/dP ratio.
Of course, with LPA, a small additional charge quantity may be necessary to ensure at least 1K subcooling at the pump inlet. However, this charge addition is far less than the extra charge needed to achieve required subcool for the same liquid line pressure losses without the use of a pump. A relatively small flow-through receiver, containing saturated liquid, situated just above the pump, is all that may be required to maximize the lowering of head pressures without flooding the condenser in summer.
The advantage here then is that, in many instances, the cheaper and more proven TEV could be used in preference to the EEV. Also, the pump can provide for re-condensing of the liquid line vapour portions mentioned earlier in this article. When design valve pressure drops are still not being achieved a carefully sized capillary can be installed parallel to the TEV to supplement TEV orifice size. This has the added advantage of stabalising TEV control at times of low liquid enthalpy which otherwise reduces stability.
Even when floating head pressures, there are still often head pressure constraints requiring head pressure control, only at some lower limit. Considering the colder liquids increased saturated dT/dP ratio, head pressure controls cycling in and out can cause a “frothing” of the liquid line again reducing EEV and TEV capacity. The liquid pump will most often clear this liquid line frothing.
Marc O’Brien
Applications & Field Technician
Fridgetech.Com Ltd
Marc@Fridgetech.com
Fridgetech.Com