Is a capillary tube considered to be a modulating mtering device, as a TXV is.

[TopMechanic]

<i>Sorry for the double post, please remove th eother if it is a problem.</i>

I have often pondered this query, and have found many different answers to it

What do you think it is?

modulating metering device or not ?

I have oftened wondered this because the tube responds to subcooling unlike a fixed oriface would, and more like a TXV does.

[Robearbam]

I always thought cap tube and fixed were the same....this is what I found;

Fixed ( non-moving ) metering devices, such as capillary tubes, orifices, etc., never adjust to system conditions, therfor standard fluid dynamics come into play, where a higher pressure pushing in ( head pressure ), in relation to a lower resisting pressure on the other side ( back or evaporator pressure ) means more ***** flow. Lower head pressures mean less flow. Thus, there is no active control of superheat. If you were to charge a system of this type 'to a full glass' under most conditions, you would be incorrectly charged ( generally overcharged, but actually undercharged in some cold weather conditions ). Thus, having a sight glass would be misleading, as it would be telling you a fact that is not relevent to correct charge.

[wambat]

<i>Sorry for the double post, please remove th eother if it is a problem.</i>

I have often pondered this query, and have found many different answers to it

What do you think it is?

modulating metering device or not ?

I have oftened wondered this because the tube responds to subcooling unlike a fixed oriface would, and more like a TXV does.

Of course the cap tube is a metering device, however it’s not controlled by superheat like a TXV is but even with no moving parts, the capillary tube will vary flow rate as system pressures change in the evaporator, in the condenser, or both. In fact, it can only reach its best efficiency at one set of high- and low-side pressures. This is because the capillary tube works off of the pressure difference between the high and low sides of the refrigeration system. As the pressure difference between the high and low sides of the system becomes greater, the flow rate of refrigerant increases.

[US Iceman]

I think the key term here is "modulating". Cap tubes, orifices, & TXV's are all metering devices.

With orifices or cap tubes you get what you get depending on charge and the info wambat provided.

The TXV's however will modulate to control superheat.

To do that with a cap tube or orifice you would have to continually vary the refrigerant charge right?

That makes the cap tubes and orifices non-modulating the way I see it.

[wambat]

I think the key term here is "modulating". Cap tubes, orifices, & TXV's are all metering devices.

With orifices or cap tubes you get what you get depending on charge and the info wambat provided.

The TXV's however will modulate to control superheat.

To do that with a cap tube or orifice you would have to continually vary the refrigerant charge right?

That makes the cap tubes and orifices non-modulating the way I see it.

Yes but as I said It will "vary the flow" is that not 'modulating" ?The dictionarysays,
(v) change the key of, in music; "modulate the melody"
(v) of one''s speech, varying the pitch
(v) fix or adjust the time, amount, degree, or rate of; "regulate the temperature"; "modulate the pitch"
(v) adjust the pitch, tone, or volume of
(v) vary the frequency, amplitude, phase, or other characteristic of (electromagnetic waves)

[Robearbam]

I guess one could say that a TXV is temperature modulating (hence the word thermostatic) and cap tube is pressure modulating. PRV (Pressure Regulated Valve) ???

[US Iceman]

OK, so we're both right.

My thoughts were the TXV is capable of adjusting to various operating conditions. In other words it has some means of self-control as a response to a change, i.e., independent control.

On the other hand, the cap tube is dependent on a change in the operating conditions to cause a variation in flow.

Of course the question was not which one is dependent or independent of system changes. So I think there is where is I'll stop.

[davidk]

Regarding capilary metered systems, do you consider that is efective to control the compressor speed to adjust the cooling capacity (ie with an inverter)?

[wambat]

Regarding capilary metered systems, do you consider that is efective to control the compressor speed to adjust the cooling capacity (ie with an inverter)?

In a word NO! The cap tube is most efficient at only one set of conditions

[wambat]

OK, so we're both right.

My thoughts were the TXV is capable of adjusting to various operating conditions. In other words it has some means of self-control as a response to a change, i.e., independent control.

On the other hand, the cap tube is dependent on a change in the operating conditions to cause a variation in flow.

Of course the question was not which one is dependent or independent of system changes. So I think there is where is I'll stop.

AH it's always a pleasure to talk with you as opposed to the other chap

[US Iceman]

Don't go all squishy on me. I still have a reputation to protect.

Aren't discussions more fun when their civil?