EEV Vs TEV

[dchatta]

can some body explanin to me how does an EEv differ from an TEV : BOTH ARE METERING ( refrigerant metering) devices basically?

how does it control the super heat ? :(

[Mark]

Hi,dchatta :)

Welcome to RE :) ,The EEV/TEV comparison is discussed in various forums on this site, see supermarket refrigeration and industrial.If you need further advice ,plenty of us are glad to help.

Best regards

Mark

[Peter_1]

http://www.refrigeration-engineer.com/forums/showthread.php?t=1717

[dchatta]

After going through a lot of references regarding the EEV and TEV, what I understand is that no body can say surely that it (EEV) can lead to energy savings( saving power consumption).

However, almost every body agrees to the fact that in bigger systems it does prove to be effective in saving the power bill.

One question however still hovers around in my mind how does a compressor (talking about a recip. one) consume variable power while delivering different capacities (talking about single cylinder hermetically sealed compressors, say MT series from DANFOSS)?

To this question I see following reason , as follows( please correct me if I am wrong !) :
Assuming a hypothetical data only for purpose of understanding the concept:

Suppose 10,000 molecules / sec. are required to cater to a evaporator heat load of 10 kW; at fixed evaporation pressure of 50 Psi.

When the heat load increases to say 20 kW (assuming it comes under the range of the compressor) the TEV will “meter-in” 20000 molecules / sec.now to maintain the “evaporation pressure” of 50 Psi. due to increased “rate of evaporation” inside the evaporation.

This increased rate of evaporation increases the “density” ( no of molecules “sucked – in” by the compressor ;a compressor possesses a fixed displacement) at that instance, of the refrigerant inside the cylinder.
Now the compressor being forced to compress more no of molecules per stroke, extracts more power from supply line.





So , we understand the following :

1 ) compressor power consumption varies at different capacities due to “density changes” in the “ sucked – in” refrigerant per stroke of the cylinder.
2) “ thus if we are able to “ meter- in” less dense (optimum density) refrigerant into the compressor displacement for every changed condition of load we can keep the compressor to its “ most optimal” power consumption mode or may be the “lowest power” consumption mode.
However, one thing that remains unexplained is how then does the decrease in ambient lower power consumption ( talking of reciprocating and not a centrifugal compressor – which changes its throw with alteration in speed of rotation)

This ( understanding the fact : how does the compressor consume power)
I think will bring me closer to understanding the difference between AKV and TEV !

Please correct me if my line of thinking is faulty and suggest needful.

[shogun7]

The function of the thermostatic expansion valve (TXV or TEV) is to hold a constant evaporator superheat. When set and operating properly, the TXV will keep the evaporator active throughout its entire length. The conventional TXV is controlled by springs, bellows, and push rods.

The electronic expansion valve (EEV) operates with a much more sophisticated design. EEVs control the flow of refrigerant entering a direct expansion evaporator. They do this in response to signals sent to them by an electronic controller.[A miro computer]
A small motor is used to open and close the valve port. The motor is called a step or stepper motor. Step motors do not rotate continuously. They are controlled by an electronic controller and rotate a fraction of a revolution for each signal sent to them by the electronic controller. The electronic signals sent by the controller to the EEV are usually done by a thermistor connected to discharge airflow in the refrigerated case.

A thermistor is nothing but a resistor that changes its resistance as its temperature changes. Other sensors are often located at the evaporator inlet and outlet to sense evaporator superheat. This protects the compressor from any liquid floodback under low superheat conditions.
Balanced, ported TEVs have a much greater ability to follow load than conventional TEVs, but still not to the extent that electric expansion valves (EEVs) can.
The resolution of an EEV is governed by the stroke and number of steps in that stroke.So the closer you can follow the load the better the overall efficiency Think about it like the offset you have with a thermostat. The closer to set point(RESOLUTION) the better the efficiency (energy)

[Peter_1]

A small motor is used to open and close the valve port. The motor is called a step or stepper motor. Step motors do not rotate continuously. They are controlled by an electronic controller and rotate a fraction of a revolution for each signal sent to them by the electronic controller. The electronic signals sent by the controller to the EEV are usually done by a thermistor connected to discharge airflow in the refrigerated case.

Don't forget the PWM operated ones.

[frank]

The electronic signals sent by the controller to the EEV are usually done by a thermistor connected to discharge airflow in the refrigerated case.

Hi Shogun

That may be the case with refrigerated cases but with air conditioning the electronics normally looks at much more than just the discharge airflow. Lots of sensor readings are considered before signals are sent to the EEV and with the Daikin range of VRV they can control the space down to 0.5C of set point.

Couple this close control of metering refrigerant into the evaporator with a speed controllable compressor and you have a system that will be far more energy efficient than a standard on/off compressor and TEV set up. :)

[bernard]

Hi I am having trouble with flood back from 8 freezer coldrooms.Two have tev with TD of about 8 degrees these are fine.The other six have zinger valves and have a very low T.D,some times negative.All probes work RMS screw compressors,coils not iced.Why am I getting negative T.D.


Thankyou

Bernard

[Andy]

Hi Bernard :)
could be any number of things, but I would strip the valves and check them for wear/debri stopping them from closing properly. Then move on to the family of controller faults, anything from the wrong refrigerant set to a faulty controller.
Maybe just a faulty probe, if you have a logging ability on the control system, log your probes over a period of time and compare a number of them ie air off inlet and outlet and see if anything strange shows up.
Working thru the problem systematically will find out the fault.
Kind Regards. Andy.
:)

[dallan]

Bernard

The first thing I would check is the evaporator in probe location, make sure your not getting a false reading.

[shogun7]

Sorry FRANK, I failed to mention electronically derived, pressure-temperature superheat requiring the use of a pressure transducer, a temperature sensor, and a pressure-temperature table or equation and a simpler but less accurate measure of superheat that is the two-temperature method. In the two-temperature method, the temperature is sensed at the inlet and at the outlet of the evaporator. The difference in temperatures is then assumed to be superheat. The problem is that refrigerants or blends with temperature glides may affect two-temperature superheat control. Ordinarily, superheat setpoint must be higher to overcome the effects of glide However, an advantage to two-temperature superheat is cost; pressure transducers are far more expensive than thermistors. Additionally, two-temperature superheat works with any refrigerant without reprogramming. The temperature difference between the two sensors will indicate superheat no matter what the pressure-temperature relationship of the refrigerant. The main disadvantage of the two-temperature method is the uncertainty that the inlet sensor is located properly. For the two-temperature superheat method to be accurate, the inlet sensor must be located in a position that has saturated refrigerant present at all times. Often, only flow testing of individual evaporators will provide the information necessary to establish the correct location. Failure to find, or use, the proper location can lead to poor control or compressor damage. :o

[Andy]

Hi Dallan :)
if the evaporator inlet was faulty, which quite often happens, the faulty probe can be seen crossing the outlet probe on graph after defrost, which is physically impossible. A faulty inlet probe leads to low evaporator utilization ie a starved evaporator, in this case the evaportor is flooding back.

Kind Regards. Andy :)

[bernard]

hi had a further look a problem today.although I de iced some of the evaps two days ago.they were iced again today found problem to be two suction valves not closing during hot gas defrost.i will sort this out and see if there are any other problems


thankyou


bernard

[Latte]

Sorry to sound biased.


LONG LIVE THE TEV :D

Regards

Raymond

[dchatta]

dear Mr. shougun,

wat was bothering me was not asto wat is the working principle of TEV(discrete control.. stepped... not closely spaced).... or that of Electronic exp valve ( FUZZY control... very close ... fine control over refrigerant flow...!)

although EEV gives "fine" control over Refrigerant control... wat I was curious to know was how much effeciency increse is achieved over a TEV...?
actually the orifiece of TEV meant for 5 TR plant is so small ...

Any ways thanks !
in actual wat caused my interest in EEV's was a new system :TURBOCOR -compressor ..
My company ( i'm just fresh sales Er. there) is planning to install two of these systems here in India.. we have the marketing rights in india for those systems...

It was for first time that i had heard of EEV ... (EEV's r not used in INDIA ... very costly... have to be imported)
and NOW I understand how the TURBO COR is able to achieve IPLV of 0.4 Kw/TR in their mACHINES...they have VFD + EEV ...and they have eliminated the "BEARINGS" and "oil"...

any ways thanks again .. it was good to discuss thing with you all . you have been very helpful...

regards
Dchatta