variable expansion valve

[Lc_shi]

Hi sirs
Who has used the variable expansion valve as attached drawing?
It's said to keep zero superheat for evaporator.

rgds
LC

[US Iceman]

Hi LC,

Another interesting topic. I have not seen this before.

One comment I would offer in regards to zero superheat. This may sound like a way to improve the system efficiency however, if the system has capacity control I can see some potential problems.

If the compressor unloads, the suction line after the heat exchanger could approach the saturation temperature of the suction gas. If this occurs the suction gas can begin to condense.

When the compressor again loads up the liquid flies back to the compressor.

This would require special precautions with the installation to ensure the suction line is not trapped in any manner.

Look forward to hearing comments of others on this device.

[Peter_1]

With zero superheat, you will never fall within the specs of the compressor manufacturer.

[Brian_UK]

Yes, maybe zero superheat at the evaporator but is the larger diameter pipe section supposed to be something like an accumulator? Or is it just the scale of the drawing?

There must be some superheat effect otherwise there wouldn't be any vapour returning ??

[Dan]

I don't understand the drawing. If the flow control rod is experiencing colder temperatures... in other words if the liquid is exposed to suction temperatures, such as in a heat exchanger, I would expect it to INCREASE the flow through the orifice as it reduces in size. This is opposite of what is needed. Anyone have any theories on what the flow control rod is doing?

[wambat]

This sounds like US Patent # 7611911 which says
The invention provides for the control of an expansion valve without relying on measuring temperature at the suction side of a compressor. In particular, the control of the expansion valve is premised on a computation of discharge superheat using a mathematical algorithm based upon the current capacity of one or more activated compressors. The computation of the discharge superheat is preferably based on sensed suction and discharge pressures for the one or more compressors. The computed discharge superheat is compared with an actual discharge superheat that is based on a sensed discharge gas temperature. The comparison preferably permits the actual discharge superheat to be within a prescribed amount of the computed discharge superheat. This computational process has a much lower likelihood of error when contrasted with a computation based on sensing suction temperature. In this regard, when the compressor or compressors operate in the so called "flooded condition" (no suction superheat), the measurement of conditions of the refrigerant in an evaporator leaving section or compressor entering section gives no idea about the refrigerant quality (quantity of liquid refrigerant in a mixture) entering the compressor. In reality, when the refrigerant entering the compressor is a saturated gas or mixture of the saturated gas and liquid, the refrigerant temperature is equal to refrigerant saturated temperature with suction superheat being equal to 0. It is impossible to make a distinction between acceptable, transient operation with some liquid droplets entering the compressor and an operation with large amount of liquid, which results in a very rapid compressor failure.

Computing superheat based on the conditions of the refrigerant at discharge from the compressor allows a control to clearly distinguish refrigerant quality (amount of liquid in a mixture) entering the compressor. Knowing the refrigerant quality while operating with minimal or no suction gas superheat allows for an appropriate control of the EXV opening in a transient, low suction superheat.

[Lc_shi]

I got the description for it:

The variable expansion valve is a patented device (US patents 5,819,548 and 5,913,891) which can provide zero-superheat operation of an evaporator with potentially less cost than a thermostatic expansion valve. The valve and its principle of operation are illustrated in Figure (attached in above post).
Two concentric tubes and a rod form a liquid-suction heat exchanger at the inlet and exit of an evaporator. The warm liquid travels down the inner tube, around the outside of the rod. The refrigerant exiting the vaporator passes between the concentric tubes. The rod is made
of a material with a low coefficient of thermal expansion, and the tubes are made of a material with a high coefficient of thermal expansion, preferably aluminum. The evaporator end of the rod pushes up against a flow control orifice. As the liquid fraction exiting the vaporator
increases, the tubes are cooled, thus causing them to contract, resulting in more throttling at the orifice. When the liquid fraction decreases, the opposite occurs. In this way, the variable expansion valve can maintain evaporator operation with zero superheat. The interchange heat transfer provided by the tube assembly assures evaporation of refrigerant to maximize system
efficiency and protect the compressors from liquid slugging.

Hope it's clear.

rgds
LC

[Dan]

As the liquid fraction exiting the vaporator
increases, the tubes are cooled, thus causing them to contract, resulting in more throttling at the orifice. When the liquid fraction decreases, the opposite occurs. In this way, the variable expansion valve can maintain evaporator operation with zero superheat. The interchange heat transfer provided by the tube assembly assures evaporation of refrigerant to maximize system
efficiency and protect the compressors from liquid slugging.

Lc_shi, If I understand you, the rod is the constant and the tubing is the variable? Thus reducing length and causing the rod to throttle because it does not change it's length proportionately?

The evaporator end of the rod pushes up against a flow control orifice. As the liquid fraction exiting the vaporator

I picture the rod anchored at the top to the liquid line, permitting flow around it. I then perceive that the rod is not pushing against the flow control orifice, but more likely the orifice is pushing against the rod as the liquid line is contracting toward the rod as a result of the suction superheat reducing.

Is my understanding consistent with your thinking?

[Dan]

I have to add that if I picture this correctly, we do indeed have a thermostatic expansion valve that looks simple in design and inexpensive to manufacture. But oh, what research and devolopment would be necessary to mass produce. It has a theory I can picture working. I am dubious of the practicality. Metallurgy comes to mind. Reaction speed becomes an important consideration. Hysteresis (over and underfeeding under variable load conditions, just in case I used the wrong word) effects will require incredible fine-tuning. Parabolic valve seating would assist, etc.

To justify what I see as the expense of developing this device, I picture mass produced refrigeration cases or air-conditioners or automobile air-conditioners. All of which have different solutions for changing load conditions. How well it competes with electronic expansion devices, capillary tubing and fixed orifices will be the test of this design.

On the other hand, if I entirely misundertood your explanation, never mind what I just said. But it still beats the heck out of US Patent # 7611911

[Samarjit Sen]

Dear LC,

To be honest I have never heard about variable expansion valve. However I agree with Peter 1 that with zero superheat, what would be the characteristic of the compressor. Could you please provide further details on it.