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A.Mortezania
04-02-2017, 11:46 AM
Dear All,

I have a question. In DX evaporators we can control temperature of evaporator and flow of refrigerant by expansion valve, Regarding flooded evaporators, How we can control temperature of inside of evaporator ?

Sincerely yours.

Glenn Moore
04-02-2017, 12:57 PM
An expansion valve is purely a superheat controller, it has no other function and cannot control an evaporators temperature. The compressor controls the suction pressure and therefore the evaporating temperature.
The same with a flooded evaporator, the liquid level controller only controls the level of liquid in the evaporator to cover the tubes , but again the evaporator temperature is controlled by the compressor suction pressure control
The use of back pressure valves , hot gas dump valves, speed controlled compressors or any other capacity control will control your evaporating temperature, but your thermostatic expansion valve or liquid level controller has very little to do with your evaporating temperature inside the evaporator.
The TEV and liquid level control simply give the system a controlled liquid flow into the evaporator be it superheat control ( DX ) or liquid level ( Flooded )

A.Mortezania
04-02-2017, 02:32 PM
An expansion valve is purely a superheat controller, it has no other function and cannot control an evaporators temperature. The compressor controls the suction pressure and therefore the evaporating temperature.
The same with a flooded evaporator, the liquid level controller only controls the level of liquid in the evaporator to cover the tubes , but again the evaporator temperature is controlled by the compressor suction pressure control
The use of back pressure valves , hot gas dump valves, speed controlled compressors or any other capacity control will control your evaporating temperature, but your thermostatic expansion valve or liquid level controller has very little to do with your evaporating temperature inside the evaporator.
The TEV and liquid level control simply give the system a controlled liquid flow into the evaporator be it superheat control ( DX ) or liquid level ( Flooded )

Dear Glenn,

Yes you are right, Suction of compressor causes decreasing pressure inside of evaporator to reach to demand temperature. Your explanation is correct and also thanks for your explanation about flooded evaporators.
But please consider if expansion valves do not control mass flow of refrigerant you can not reach to request temperature on evaporator.
Yes, Suction cases evaporating but beside of expansion valve that help to control refrigerant mass flow, Consider if you put away expansion valve, Suction of compressor can not evaporate refrigerant inside of evaporator.

Sincerely yours.

Glenn Moore
04-02-2017, 03:04 PM
Controlling mass flow is a design function of a tev, which is why you select a tev against design system capacity (kw) , evaporating temperature, delta p (condensing p minus evaporating p) and refrigerant type.Selecting the tev by capacity and evaporating temperature information confirms that the tev is correct size for the design system condition. So provided the tev has been selected correctly the valve will control the flow of refrigerant purely by the valves superheat setting, the evap T is constantly changing during the pull down and the superheat will modulate the valve as required not the evap T.
Rule 1 An Expansion Valve is a SUPERHEAT CONTROLLER
If the Tev is to small by design error you will operate with too high superheat and too low evaporating temp
If the Tev is to big the superheat will 'hunt' causing large superheat swings up and down with potential liquid exiting the evaporator and damaging the compressor.

A.Mortezania
04-02-2017, 03:22 PM
Dear Glenn,

Please see my photo below.

Expansion device is mass flow controller thereby superheat controller …… .





14617
Sincerely yours.

Glenn Moore
04-02-2017, 06:19 PM
Wiki is wrong, superheat demand, controls the refrigerant flow through the valve, and not the flow controls the superheat
The TEV will only allow refrigerant to flow if the superheat derived by the evaporator is sufficiently high to drive the TEV to some sort of opening degree. If the superheat is to low the valve will close ie no flow.
The Wiki should read ' The valve controls the superheat at the outlet of the evaporator, by modulating the refrigerant flow via the information received from its 3 control signals.
The primary difference between a Thermostatic expansion Valve and an Electronic Expansion valve is that the Thermostatic valve is controlled by the evaporator , whereas the Evaporator is controlled by an Electronic valve.
The valve shown in the Wiki drawing is a Danfoss PHT valve which is a servo assisted valve and does not work as described in the Wiki scribe and the gas in the sensing bulb is not similar to the gas in the as its a mixture to try to simulate the refrigerant pressure/temperature characteristic across the valves evaporating temperature range with the same static superheat setting across its range

cduque
04-02-2017, 10:35 PM
Expansion valves are not used in flooded evaporators since the pressure in the separator is already near the evaporating temperature of the evaporator. The refrigeration fluid that leaves the evaporator have simultaneously liquid and gas so we do not have any super-heat.
What usually is used is a regulating valve to have an adjustment of pressure to near the evaporating temperature.

PaulZ
05-02-2017, 11:46 PM
Hi Mortezania
Wikipedia is not always right, if you want to know how TX valves work go to the companies who make them, Danfoss, Emerson, Parker etc.
Regards
Paul

RANGER1
06-02-2017, 01:13 AM
http://files.danfoss.com/TechnicalInfo/Dila/01/PDGA0A102_TEVA.pdf

This one does not control superheat, but liquid level which can be used on flooded evaporator with surge drum etc.
Sporlan level master same thing.