Low/High pressure control setting
Setting a pressure control requires you know the minimum and maximum design operating pressures of a system. Yet this must be aligned to the correct refrigerant used.
For example a coolroom.
If on a 6K evaporator design , room temp at 2.c - 6K evaporator TD giving you a -4 Saturated evaporator temperature. This means the room needs to be operating at -4.c
Head pressure , well this needs the condenser temperature difference chart but in short should never exceed AS1677 on pressure testing , which stated at a 43.c ambient , you should not exceed 59.c condensing temperature.
Setting your head pressure to 59.c maximum is key but lower is also ok especially if operating a freezer room.
The range of the control is the actual cut in pressure , the differential is the cut out, where range minus differential pressure = cut out pressure.
This too confusing for many, so look at it this way.
If you need to cut in below -4 S.E.T the set you pressure control range to a pressure before -4 S.E.T for the type of refrigerant.
Now the cut out pressure has been so ignored over the years especially with coolrooms. Many set the cut out to near 0kpa gauge or atmospheric pressure and this is not right. Assuming a pump down arrangement . If you have a gas leak or low capacity , the unit can operate at such a low pressure that any refrigerant in the coil left will likely dehydrate foods in the room , even at partial capacity due to the wide evaporator temperature difference from a loss of refrigerant. Its best not to exceed the minimum design saturated evaporator temperature too low. so eg at -4 S.E.T you cut it out at -10 S.E.T
Range pressure(cut in) - differential pressure = cut out pressure. Best way is to know how to set your cut out pressure is ......
Take your cut out pressure from your design cut in pressure and that leaves the required differential setting.
Set the differential to that setting.
Most High pressure cut-outs have internal differential reset pressures. But please just do not set a high pressure at 2500kpa as standard etc . For instance R134a hits 59.c at 1600 Kpa
Always check your refrigerant pressure temperature charts.
As a guide
Coolrooms should operate to a design S.E.T of -4 unless they have been re-engineered for higher humidity.
Freezer Rooms to - 24 S.E.T
Air Conditioners to 4.C S.E.T to 0.c S.E.T
Head pressures should not exceed 59.c based on HFC/HCFC refrigerants
Setting a pressure control requires you know the minimum and maximum design operating pressures of a system. Yet this must be aligned to the correct refrigerant used.
For example a coolroom.
If on a 6K evaporator design , room temp at 2.c - 6K evaporator TD giving you a -4 Saturated evaporator temperature. This means the room needs to be operating at -4.c
Head pressure , well this needs the condenser temperature difference chart but in short should never exceed AS1677 on pressure testing , which stated at a 43.c ambient , you should not exceed 59.c condensing temperature.
Setting your head pressure to 59.c maximum is key but lower is also ok especially if operating a freezer room.
The range of the control is the actual cut in pressure , the differential is the cut out, where range minus differential pressure = cut out pressure.
This too confusing for many, so look at it this way.
If you need to cut in below -4 S.E.T the set you pressure control range to a pressure before -4 S.E.T for the type of refrigerant.
Now the cut out pressure has been so ignored over the years especially with coolrooms. Many set the cut out to near 0kpa gauge or atmospheric pressure and this is not right. Assuming a pump down arrangement . If you have a gas leak or low capacity , the unit can operate at such a low pressure that any refrigerant in the coil left will likely dehydrate foods in the room , even at partial capacity due to the wide evaporator temperature difference from a loss of refrigerant. Its best not to exceed the minimum design saturated evaporator temperature too low. so eg at -4 S.E.T you cut it out at -10 S.E.T
Range pressure(cut in) - differential pressure = cut out pressure. Best way is to know how to set your cut out pressure is ......
Take your cut out pressure from your design cut in pressure and that leaves the required differential setting.
Set the differential to that setting.
Most High pressure cut-outs have internal differential reset pressures. But please just do not set a high pressure at 2500kpa as standard etc . For instance R134a hits 59.c at 1600 Kpa
Always check your refrigerant pressure temperature charts.
As a guide
Coolrooms should operate to a design S.E.T of -4 unless they have been re-engineered for higher humidity.
Freezer Rooms to - 24 S.E.T
Air Conditioners to 4.C S.E.T to 0.c S.E.T
Head pressures should not exceed 59.c based on HFC/HCFC refrigerants
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Reversing Valves
Advantages
1 - adds heat of compression into the indoor unit which increases the co-efficient of performance.
2 - In Australia's climate, the most effective for heating for kW of energy in to kW of refrigeration out. So puts out more heating energy than cooling.
Disadvantages
1 - The pressure drop it creates in the suction line partially de-rates the units total cooling capacity. This is why cooling only units always seem to have more cooling capacity than the equivalent heat pump alternative
(but inverter models can ramp up frequency and compensate depending on the manufacturers limitations).
2 - Defrosts are going to occur at 6.c ambients or lower, sub zero climates may require electric heater boosting (Australia not likley)
3 - Higher condensing temperature differences require capacity control, as such the unit accelerates performance over 19.c air on. Most splits terminate capacity at 65.c condensing temperatures. Airflow is critical.
The irony..............
A reversing valve is controlled by a reversing valve... smile emoticon yes seriously , the pilot electrical valve is a mini reversing valve.
In countries like Australia we tend to energise the solenoid on heating where as colder countries energise the solenoid for cooling . This is so we utilise the units majority needs without adding extra wattage for a magnetic coil. Makes sense.
Whats the difference?
Well 180 degrees of installation so be careful how you install them after repairs.
PS ...NEVER>>> vapour charge a system with a reversing valve from evacuation. If your unlucky the partial gas pressure changeover will create small gate change and align suction and discharge ports in the valve creating unloading and impossible addition of further gas charge. Always liquid charge to near weight.
Inverters will ramp up to minimum frequency speeds to allow positive higher pressure changeover of the gateway before changing modes. ------clever.....
See this excellent animation from Danfoss animations link below, It explains all and makes the story complete.
Reversing valve image from Actrol catalogue volume 9
http://www.ra.danfoss.com/…/Danfoss%20Saginomiya%204%20-%20…
Advantages
1 - adds heat of compression into the indoor unit which increases the co-efficient of performance.
2 - In Australia's climate, the most effective for heating for kW of energy in to kW of refrigeration out. So puts out more heating energy than cooling.
Disadvantages
1 - The pressure drop it creates in the suction line partially de-rates the units total cooling capacity. This is why cooling only units always seem to have more cooling capacity than the equivalent heat pump alternative
(but inverter models can ramp up frequency and compensate depending on the manufacturers limitations).
2 - Defrosts are going to occur at 6.c ambients or lower, sub zero climates may require electric heater boosting (Australia not likley)
3 - Higher condensing temperature differences require capacity control, as such the unit accelerates performance over 19.c air on. Most splits terminate capacity at 65.c condensing temperatures. Airflow is critical.
The irony..............
A reversing valve is controlled by a reversing valve... smile emoticon yes seriously , the pilot electrical valve is a mini reversing valve.
In countries like Australia we tend to energise the solenoid on heating where as colder countries energise the solenoid for cooling . This is so we utilise the units majority needs without adding extra wattage for a magnetic coil. Makes sense.
Whats the difference?
Well 180 degrees of installation so be careful how you install them after repairs.
PS ...NEVER>>> vapour charge a system with a reversing valve from evacuation. If your unlucky the partial gas pressure changeover will create small gate change and align suction and discharge ports in the valve creating unloading and impossible addition of further gas charge. Always liquid charge to near weight.
Inverters will ramp up to minimum frequency speeds to allow positive higher pressure changeover of the gateway before changing modes. ------clever.....
See this excellent animation from Danfoss animations link below, It explains all and makes the story complete.
Reversing valve image from Actrol catalogue volume 9
http://www.ra.danfoss.com/…/Danfoss%20Saginomiya%204%20-%20…
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Electronic vs Thermostatic Expansion Devices...
Well, the short answer is that the electronic stepper type motorised valves win the efficiency dividends with increased surface area of evaporator used by lower superheats and being able to work with floating the head pressure much lower.
Yet they demand a logical driver system only as good as the logic embedded into the CPU. Well , they use the pressure enthalpy characteristics of the refrigerant as an operating guide which seems floorless. ( 0 to 0.5K superheat)
The thermostatic expansion valve cant react as quickly and requires a larger index of superheat to prevent hunting. It also requires a balanced port arrangement , and sufficient pressure drop to operate correctly. Yet I cant ignore the fact that I feel more in control using these valves and being forced to set the ideal standard superheat conditions. (4 to 6K superheat)
Air conditioning is already at the top of the efficiency chain with much lower compression ratio's and greater mass flow of refrigerant for the volume used. All because they work at the highest saturated evaporation temperatures! And mostly all split systems now employ the use of Electronic stepper motors to squeeze the performance even more, combined with inverter technology that see's savings in energy input at part load conditions. This is logical, since from 2004 air conditioning sales in Australia doubled , and we need to find more efficiency dividends for power input.
Our refrigeration systems tend to only use electronic stepper control in much more larger systems with much less volumetric efficiency , trying to float the head pressure and sometimes use liquid pumps to keep compression ratio's much lower. Mostly we are still using thermostatic expansion valves for nominal control. I guess most houses dont have a coolroom or freezer room. Most condensing units are not inverter driven. Yet we need ideal temperature differences in cool rooms to keep the humidity correct for stored product.
Our domestic market which in some cases could see more domestic class fridges in a household than an air-conditioners do not use stepper type expansion. We are still using capillary tube systems with much higher superheats (25 to 30K superheat).
Yet the market has forced hydrocarbon refrigerants and linear compressors, inverter drive in an effort to reduce power consumption and refrigerant weight. Design has changed to reduce air change loads by using smaller access doors in refrigerators rather than open the main door. Domestic fridges still work on much higher superheats but use less power than before??
It all comes down to costs at the end of the day. The costs are based on economies of scale and what regulatory influences we have on the designs. People want more for less and that is also in line with the current sustainability needs we look for with energy consumption. Currently we have found new ways to reduce the costs of producing energy both fiscally and environmentally but have we really found the ways to use less of it?
The expansion devices below are different and produce very different efficiency's They work so very differently but they have one thing in common. They work on superheat.
Superheat has a direct impact on power consumption
The air conditioning industry is really a "hands-off" approach so that manufacturers can embed effective control in their systems that constantly self adjust. Always trying to reach the ideal EER and COP. Nothing is serviceable , just repairable. Refrigeration allows us to set and tune the devices according to design evaporator K TD's. WHY??
The Airconditioning market is heavily regulated with minimum energy performance conditions needed on classes of equipment.
Maybe that explains the differences?
Well, the short answer is that the electronic stepper type motorised valves win the efficiency dividends with increased surface area of evaporator used by lower superheats and being able to work with floating the head pressure much lower.
Yet they demand a logical driver system only as good as the logic embedded into the CPU. Well , they use the pressure enthalpy characteristics of the refrigerant as an operating guide which seems floorless. ( 0 to 0.5K superheat)
The thermostatic expansion valve cant react as quickly and requires a larger index of superheat to prevent hunting. It also requires a balanced port arrangement , and sufficient pressure drop to operate correctly. Yet I cant ignore the fact that I feel more in control using these valves and being forced to set the ideal standard superheat conditions. (4 to 6K superheat)
Air conditioning is already at the top of the efficiency chain with much lower compression ratio's and greater mass flow of refrigerant for the volume used. All because they work at the highest saturated evaporation temperatures! And mostly all split systems now employ the use of Electronic stepper motors to squeeze the performance even more, combined with inverter technology that see's savings in energy input at part load conditions. This is logical, since from 2004 air conditioning sales in Australia doubled , and we need to find more efficiency dividends for power input.
Our refrigeration systems tend to only use electronic stepper control in much more larger systems with much less volumetric efficiency , trying to float the head pressure and sometimes use liquid pumps to keep compression ratio's much lower. Mostly we are still using thermostatic expansion valves for nominal control. I guess most houses dont have a coolroom or freezer room. Most condensing units are not inverter driven. Yet we need ideal temperature differences in cool rooms to keep the humidity correct for stored product.
Our domestic market which in some cases could see more domestic class fridges in a household than an air-conditioners do not use stepper type expansion. We are still using capillary tube systems with much higher superheats (25 to 30K superheat).
Yet the market has forced hydrocarbon refrigerants and linear compressors, inverter drive in an effort to reduce power consumption and refrigerant weight. Design has changed to reduce air change loads by using smaller access doors in refrigerators rather than open the main door. Domestic fridges still work on much higher superheats but use less power than before??
It all comes down to costs at the end of the day. The costs are based on economies of scale and what regulatory influences we have on the designs. People want more for less and that is also in line with the current sustainability needs we look for with energy consumption. Currently we have found new ways to reduce the costs of producing energy both fiscally and environmentally but have we really found the ways to use less of it?
The expansion devices below are different and produce very different efficiency's They work so very differently but they have one thing in common. They work on superheat.
Superheat has a direct impact on power consumption
The air conditioning industry is really a "hands-off" approach so that manufacturers can embed effective control in their systems that constantly self adjust. Always trying to reach the ideal EER and COP. Nothing is serviceable , just repairable. Refrigeration allows us to set and tune the devices according to design evaporator K TD's. WHY??
The Airconditioning market is heavily regulated with minimum energy performance conditions needed on classes of equipment.
Maybe that explains the differences?
Electronic stepper valves have very little moving parts with simple construction. The can bi-flow easily and work with very little port pressure as long as it has subcooled liquid. These have been a major factor in efficiency increases to domestic and commercial air-conditioning systems. They allow better COP/EER through effecting good flow capacity and lower superheats at varying ambient conditions.
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Danfoss T2 series expansion valves , one of the industry's best performing valves for both internal and external equalisation types. The new stainless laser welding technology and stainless power elements/capillary/bulb have really extended valve life and performance.
image source www.danfoss.com |
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Electronic expansion valves
Advantages * Very precise low superheat control * Can work with less subcooled liquid pressure at the valve port * Less mechanical restriction and can Bi-flow which is handy for heat pump air conditioner applications. Disadvantages. * Requires a complex driver system to control the valves opening/closing degree * Loses memory on degree position on a power loss so requires resetting on every power up. This is not an issue , just a fact. * Needs a power source to operate, not self maintaining like a thermostatic expansion valve. The driving magnetic motor is called a stepping motor. It does not rotate at speed but by steps. These steps are part of design opening to maximum converting open and closed in thousands of possible positions. This is how precise control can be maintained. Effectively the driving motor pulses a forward/reverse magnetic field to a permanent magnet rotor. The rotor acts on a needle valve which creates the pressure drop and flow. They typically In AC use a 5 or 6 wire circuit with 4 winding taps for north south polarity forward/reverse. They either use two or one common winding connection. Split system AC units employ the expansion devices in the outdoor unit and provide a saturated liquid line to the indoor system. While this has been done with capillary tubes previously the Electronic expansion valve (EEV) can be more effective at part load conditions both indoor and outdoor and maintain low superheat. The benefit of controlling a successive pressure drop from the outdoor unit to the indoor unit by pre expansion allows for smaller compressors to be used to pump liquid at volume and height/distance. This reduces both cost and power footprint. In the case of multi split AC units with more than one indoor off the outdoor. Each EEV can attempt to unify the return vapour gas temperature to the compressor which will assist in capacity control and displaced volume from the compressor. Most faults with EEV valves come from two typical problems. Contaminated systems or particulate blockages. EEV motorised head failure which is rare given they are mostly 12Vdc. It usually is supply based from the driver circuit pcb inclusive of wiring faults and corrosion of terminations. Indirect failures are the result of logic malfunction or thermistor sensor damage/calibration. |
Image source from www,danfoss.com
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