In The Presence Of
Gary Lloyd
of vapor, and you can't have superheat in the presence of liquid. But,
what does "in the presence of" mean?
Consider the following scenario:
You have a refrigerant tank that is half full (or half empty if you
prefer). At the top of the tank, you have a heat source. At the bottom
of the tank, you have a cooling source. Let's imagine that they are
equal so that the liquid level doesn't change.
The vapor at the top of the tank will be superheated!!!
The liquid at the bottom of the tank will be subcooled!!!
Saturation (and therefore the pressure/temperature relationship)
exists only where the vapor and the liquid are in direct contact.
This explains why the liquid in a liquid line can be subcooled even
while vapor is still present. Saturation exists only where the vapor
is contacting the liquid.
It also explains why the vapor in a suction line can be superheated
even while liquid is still present. Saturation exists only where the
liquid is contacting the vapor.
What do YOU think?
Gary
===================================================
Everything should be made as simple as possible, but not simpler.
-Albert Einstein-
Gary R. Lloyd CMS - TECH Method Training
HVACR Trouble Shooting Books/Software
Tel 313-671-0188 Fax 313-676-9262
http://www.gatecom.com/~tmethod
tme...@gatecom.com
tme...@aol.com
Gary Lloyd
>We have all been taught that you can't have subcooling in the presence
>of vapor, and you can't have superheat in the presence of liquid. But,
>what does "in the presence of" mean?
>Consider the following scenario:
>You have a refrigerant tank that is half full (or half empty if you
>prefer). At the top of the tank, you have a heat source. At the bottom
>of the tank, you have a cooling source. Let's imagine that they are
>equal so that the liquid level doesn't change.
>The vapor at the top of the tank will be superheated!!!
>The liquid at the bottom of the tank will be subcooled!!!
>Saturation (and therefore the pressure/temperature relationship)
>exists only where the vapor and the liquid are in direct contact.
>This explains why the liquid in a liquid line can be subcooled even
>while vapor is still present. Saturation exists only where the vapor
>is contacting the liquid.
>It also explains why the vapor in a suction line can be superheated
>even while liquid is still present. Saturation exists only where the
>liquid is contacting the vapor.
>What do YOU think?
>Gary
Despite the fact that, with the above statements, I have contradicted
evey textbook ever written, and in the process challenged the
foundation upon which our understanding of the refrigeration cycle is
built, no one seems to have taken the bait.
Let's raise the ante a little.
1. I have personally observed, in many systems, the sight glass
clearing at 10-15 degrees subcooling, measuring both temperature and
pressure at the outlet of the receiver (or the condenser in systems
without receivers). Try it.
2. Here's an experiment you may (or may not) want to try. Reduce the
superheat at the inlet of the compressor to 1 degree. Then explain to
me (and your customer) why the compressor no longer has any suction
valves.
paul milligan
tme...@gatecom.com (Gary Lloyd) wrote:
>We have all been taught that you can't have subcooling in the presence
>of vapor, and you can't have superheat in the presence of liquid. But,
>what does "in the presence of" mean?
At an equalized condition, where all adjustments to temp and
pressure have already occurred, and there is no further change.
>Consider the following scenario:
>You have a refrigerant tank that is half full (or half empty if you
>prefer). At the top of the tank, you have a heat source. At the bottom
>of the tank, you have a cooling source. Let's imagine that they are
>equal so that the liquid level doesn't change.
>The vapor at the top of the tank will be superheated!!!
>The liquid at the bottom of the tank will be subcooled!!!
You're injecting another variable, external energy input, into the
equation. The vapor will be in the process of expanding in reaction to the
heat source. The heat energy will not have traveled down to the liquid
yet, so it's temperature has not changed, and therefor it is not sub-cooled.
As the pressure increases in the container, the balance of liquid and gas
will be attempting to adjust to the new conditions.
>Saturation (and therefore the pressure/temperature relationship)'
>exists only where the vapor and the liquid are in direct contact.
ASHRAE Fundamentals Chapter 1 states " If a substance exists as
vapor at the saturation temperature, it is called saturated vapor." It
makes no reference to a border phenomena.
>This explains why the liquid in a liquid line can be subcooled even
>while vapor is still present. Saturation exists only where the vapor
>is contacting the liquid.
How so ? That doesn't make sense to me.
>It also explains why the vapor in a suction line can be superheated
>even while liquid is still present. Saturation exists only where the
>liquid is contacting the vapor.
I think that is better explained by the fact that there is an
external energy input going on, and the working fluid has not yet reached
equilibrium.
The same could be said of the freon leaving the TEV - the changes
in pressure and temperature it undergoes are not neccesarily in exact
balance at every moment and every point, leaving the potential for
sub-cooled liquid, or superheated saturated gas. The rules of stasis can
not apply to a system in motion.
Paul
_________________________________________________________
No Disclaimer needed - This is my own PPP account, and my
employer doesn't care what I think anyway ! :~)
_________________________________________________________
http://www.elitesoft.com/sci.hvac/ is the home page of
sci.engr.heat-vent-ac
Paul Milligan
tme...@gatecom.com (Gary Lloyd) wrote:
~~>Here is the bottom line (under standard operating conditions):
~~>
~~>Solid liquid occurs between 10-15F subcooling.
~~>
~~>Solid vapor occurs between 5-10F superheat.
~~>
~~>Note: This assumes the system is running <:-)>
Well - this is what I was taught ( as general rules )- it probably
contains some truth and some old wive's tales, but it seems to work, and
I've never heard anyone object strenuously to it ( yet :~) )
All superheats are measured at the evaporator exit, at the position
of the TXV sensing bulb ( or equivalent ), never at the compressor. SH at
the compressor will be considerably higher.
All superheats must be taken when the unit ( IE controlled space )is
within it's normal design operating conditions - you can't superheat an
AC unit when the indoor space temp is 95, nor a freezer when the space temp
is 50.
High temp ( AC ): 10 -> 15 degrees superheat
Medium temp ( 34 -> 38 degree space temp )5 -> 7 degrees SH
Low temp ( -10 -> + 10 degree space temp )3 -> 5 degrees SH
Ultra-low ( -70, etc ) ???? 3 -> 5 also ????
All of these are of course approximations that may be useful to
ballpark a unit, especially if no superheat chart is available for the unit.
I've also seen factory charging charts that specified 1 degree
superheat for a particular R22 air conditioner under certain conditions.
In many real-life situations, no unit-specific chart is available.
Also, much time is wasted trying to obtain 'the perfect superheat'. Techs
have been known to spend hours trying to adjust from 12 up to 13 SH, which
is silly. There is an inherent error in any normal set of field gauges
and temperature meters, plus other variables like the perfection of the
insulation around the temperature sensing probe, etc, that make such
precision A) impossible and B) irrelevent.
I suggest that if you were to take ten of any unit at the factory,
set them up under factory controlled conditions, and have two factory
specialists measure and adjust superheat, you would still come up with
a range of at least 2 degrees difference between them.
Like anything else, there is an acceptable range ( the span of which
is probably debatable )around any 'ideal target' number, in anything. This
range is known as LRW, or 'living in the real world'.
Paul
Dennis
> In the above example, shut off the heat source and leave the cooling
> source running. The entire mass of vapor will be at saturation
> temperature, while the liquid will be subcooled except where it
> contacts the vapor. This gives us a dynamic model of the subcooling
> process in the condenser (and the liquid line) with its external
> cooling source. As more and more liquid is formed, there must be a
> point where the subcooling effect exceeds the saturation effect, and
> that point must precede the disappearance of the vapor. Two competing
> effects, with one overcoming the other. Think about it. It cannot
> possibly be otherwise.This is quite intriguing. Maybe I don't understand what you say about one effect exceeding
another, but I must assume you are referring to disappearance of vapor only in the evap. However,
keep in mind that the head pressure will lower along with suction, affecting the subcooling. And
does your theory assume a hypothetical situation of no moisture to freeze and insulate the evap.
but an atmosphere that will continue to cool till the extreme condition exists? It's a given that
the refrigerant will migrate to the evap. leaving little in the condenser to create any
subcooling, (but of course we'll lose the compressor before that happens).
>
> Conversely if, in the example, we shut off the cooling source and keep
> the heat source running, the liquid will be at saturation temperature
> and the vapor will be superheated, except where it contacts the
> liquid. This provides us with a dynamic model of the superheating
> process in the evaporator (and the suction line) with its external
> heat source. As more and more vapor is formed, there must be a point
> where the superheating effect exceeds the saturation effect, and that
> point must precede the disappearance of the liquid. Again, it cannot
> be otherwise.
Here again, keep in mind that the suction pressure will raise along with the head, affecting the
superheat. And does your theory assume a hypothetical situation of an atmosphere that will
continue to heat till the extreme condition exists? It's a given that the refrigerant will
continue to vaporize to critical temp. till superheat is not measurable. Sound right?
-Dennis-
Dennis
-Dennis-
BTW, did you know one of the most used texts teaches that a sight glass is the best way to charge
a system, even a critical charge system? Wild, huh?
paul milligan
~~>source, the relatively cool walls of the condenser tubing. It quickly
~~>cools down to saturation temperature (aka condensing temperature).
Or below, depending on that wall temperature, and remembering that
the system is in motion, and has not ( will not ! )come to equilibrium.
~~>condenser walls. These are the two competing effects. The vapor tries
~~>to keep the liquid at saturation temperature
'At a superheated condition', actually, which is what the vapor is.
~~>As the liquid/vapor mixture continues through the condenser, the
~~>amount of liquid increases and the amount of vapor decreases.
~~>Eventually, the liquid is in contact with the cooling source more than
~~>it is with the saturated vapor, at which point the average temperature
~~>of the mixture is lower than saturation temperature.
Kinda depends if you measure it by mass or volume :~) But still,
you're talking about an 'average', which does not apply here, because there
is not equilibrium of mixture conditions. It is in a state of flux. To
compare - talking about the 'average' wind speed of the eye of a hurricane
averaged with the wall doesn't tell much.
Another important factor to remember here is that heat does not
transfer instantaneously. Put an ice cube into a pot of boiling water,
you'll have liquid, solid, _and_ gaseous states existing in that container
for a while, until they equalize.
{Watcha think, Gary ? :~) }
Hey, Andy... If we make that pot a pressure cooker, and put the lid
on, does the process change from isobaric to isochoric ? :~)
In other words,
~~>the mixture is subcooled before all of the vapor has condensed.
Nope, only if you look at the whole thing as being a homogeneous
mass, which it is not at this point - it is severely stratified, and the
various regions are in different conditions.
Ever see those cute 'lil smoke stick things they use in wind tunnels
to illuminate, so to speak, the aerodynamics going on ? Would it be valid
to describe the average smoke content of the entire airstream through the
tunnel ? Nope ! You could easily come up with a number, but that number
would not help describe the process occurring.
~~>If you remove refrigerant slowly until the sight glass begins to
~~>bubble, the subcooling will be about 10-15F. Adding refrigerant until
~~>the sight glass clears yields the same results.
Fill a blender with water. Turn the beasty on, and watch where the
bubbles _start_. Before any air can be sucked in, there are bubbles down
there at the impellor. Why ? Can this be described as a boiling process ?
I don't think so. BTW, I have no idea why this phenomena occurs, and I
really hope someone can answer it here. Comments on the nature of 'Frappe'
will not be considered elucitory ! :~)
~~>The process in the evaporator is similar, with the competing effects
~~>being the saturation temperature of the liquid versus the relatively
~~>warm walls of the evaporator tubing. Eventually, the mixture is
~~>superheated before all of the liquid is evaporated. Solid vapor occurs
~~>between 5-10F superheat.
Again, I would have to suggest that the mixture can not be averaged
as a whole, and the result then applied to it's parts, as per the smoke
example.
Comments ?
George Goble
[SNIP]
> Fill a blender with water. Turn the beasty on, and watch where the
>bubbles _start_. Before any air can be sucked in, there are bubbles down
>there at the impellor. Why ? Can this be described as a boiling process ?
>I don't think so. BTW, I have no idea why this phenomena occurs, and I
>really hope someone can answer it here. Comments on the nature of 'Frappe'
>will not be considered elucitory ! :~)
Cavitation of the impeller. The pressure around parts of the impeller
has dropped below the boiling point of water at the current temperature,
so some water flashes into vapor (the bubbles). There also may be dissolved
air in the water which comes out.. Try this with freshly boiled or distilled
water and see if you still get bubbles? If you shut off the blender, the
bubbles should "vanish" instead of rising very far to the top.
Liquid pumps for propane (and refrigerants) are very easy to cavitate, since
the vapor/liquid are at or close to equalibrium, and the slightest pressure
drop (a pump trying to "suck" liquid) will create huge amounts of flash
gas and and pump cavitation. Any thing like undersized pump input lines,
or trying to suck liquid uphill or thru fine screens, etc, leads to
extreme cavitation and quickly destroys a pump. I once had to build a
3 ton refrigeration unit to subcool liquid 40-50 degrees due to someone's
dumbass attack of running liquid refrigerant lines uphill into the pump
inlet and it was easier to build the subcooler than to get the piping
changed. Now they listen. THe subcooling reduced the pump cavitation.
--ghg, Inventor R-406A.
http://worldserver.com/R-406A
http://ghg.ecn.purdue.edu
George Goble
> I understand the concepts of what you're saying, George, but in
>ten years of field work, I've never seen nor heard of 'booster pumps'
>being applied to refrigerant lines. I don't doubt that it's done, but
>in what type of applications, and why ?
>
>Paul
>
A few years back, there was a company called "HY-SAVE", located near
Seattle, who patented the concept of a small (1/4 hp or less)
magnetic coupled "booster pump" for liquid lines in refrig systems.
Allowed running head lower as pump would get rid of flash gas.
They had tons of thermodynamic calcs also... If you are interested,
I can try to find them again.
I just pinged an addr "www.hysave.com" and it responded (I dont have
web capability at home).. but I bet that is their homepage..
Applic was mostly supermarkets, long liquid line runs, etc.
paul milligan
g...@cidmac.ecn.purdue.edu (George Goble) wrote:
~~>Cavitation of the impeller. The pressure around parts of the impeller
~~>has dropped below the boiling point of water at the current temperature,
~~>so some water flashes into vapor (the bubbles).
Yup, I realized this after my post - thanks, George !
~~>Liquid pumps for propane (and refrigerants) are very easy to cavitate,
I understand the concepts of what you're saying, George, but in
ten years of field work, I've never seen nor heard of 'booster pumps'
being applied to refrigerant lines. I don't doubt that it's done, but
in what type of applications, and why ?
Paul
Gary Lloyd
>In article <4i0a8r$9...@Twain.MO.NET>, asc...@mo.net wrote:
>~~>In <4hkj56$1...@castle.nando.net>, Paul Milligan <p...@nando.net> writes:
>~~>>
>>> All superheats are measured at the evaporator exit, at the position
>>>of the TXV sensing bulb ( or equivalent ), never at the compressor. SH at
>>>the compressor will be considerably higher.
>>>
>~~>Paul, too many servicemen out there trying to adjust TEVs to keep compressors
>~~>cool, and not bothering to figure out what the superheat is at the valve's
>~~>sensing bulb. Dumb, dumb, dumb....
> Begging for floodback or starving ! :~( See it all the time !
> For the record here - if you adjust superheat for compressor
>cooling, you are adjusting _ONLY_FOR_THAT_PARTICULAR_MOMENT !!! The instant
>the load conditions change ( controlled space or ambient ), you kiss your
>'compressor cooling adjustment' goodbye ! Not good ! When you come back
>on your call-back ( which you will have ), you'll either have an overloaded,
>over-heated compressor, or a severely flooded compressor, or, quite likely,
>a broken compressor ! :~(
> Just to point out here for new folks to the trade - compressor
>cooling is a valid concern, but it needs to be addressed in other ways,
>such as load limiting, de-superheating, liquid injection, etc. :~)
That's compressor cooling_ as indicated by compressor inlet superheat_
is a valid concern.
> I've left the above point about the location of superheat
>measurement in this follow-up, because it _really_ does matter !
It certainly does matter!
I agree that TXVs should be ADJUSTED for coil outlet superheat.
Compressor inlet superheat is a TROUBLE SHOOTING TOOL.
If the service technician is familiar with the variables involved, it
can be an accurate indication of, and in fact, more informative than,
coil outlet superheat. TXV adjustment is (or should be) a rare
occurence, whereas trouble shooting is every day on every job. A
blanket statement of "never measure at the compressor inlet" robs the
service tech of an extremely valuable tool.
>~~>I like to limit minimum superheat to 4F for all low temp DX systems.
>~~>Many servicemen cannot measure superheat accurately enough to set TEVs
>~~>lower than 4F superheat.
> If I were working on a freezer or lower temp unit, and had 4 degrees
>SH, I would certainly not try to lower it - there's no benefit in capacity
>to be gained ( which is what you'd be looking for ), and much risk to the
>compressor.
>~~>I can remember listening to a service explain to me that he was measuring
>~~>a steady 2F superheat on a low temp unit, but he couldn't figure out
>~~>why the compressor was running so cold.
> Andy, I would _truly_ *hate* to be a serviceman trying to explain
>my actions to you on a service call ! :~) ( VBG ) I mean _truly _ ! :-)
> I've seen worse - on one call ( second compressor ), someone tried
>to tell me that 'superheat is fine', but the accumulator was full of
>liquid, and I mean _full_, to where it was feeding pure liquid out !!!!!
Probably thought that 1 degree of superheat meant solid vapor.
>~~>> I've also seen factory charging charts that specified 1 degree
>~~>>superheat for a particular R22 air conditioner under certain conditions.
>~~>
>~~>Probably a cap tube system, and the low superheat specification was for a
>~~>very high ambient temperature.
> Yup :~) And low load condition - pre-cooling a movie theater before
>showtime influx load. I seem to remember target space temp was like 60
>or something, on an EMS timer that coincided with show times.
>Paul
If this was a specification for compressor inlet superheat (which most
cap tube charging charts are), it was in floodback. Under those
conditions, I would throw away the chart and take it up to 5F.
Magitouch
someone who is just getting a start in the biz and til now only relying on
very dry technical text / lab conditions / theory.......interesting stuff
guys....thanx!
paul milligan
tme...@gatecom.com (Gary Lloyd) wrote:
~~>I agree that TXVs should be ADJUSTED for coil outlet superheat.
~~>Compressor inlet superheat is a TROUBLE SHOOTING TOOL.
Gotta differ with that - compressor _temperature_ is, and yes, it's
related to superheat among other things, but still, compressor inlet
superheat is only a side-efect of evap exit SH, which is what must be
measured and controlled first, and then possibly some of the other
remedies I mentioned might be needed for compressor temperature, although
these are in 99 out of 100 cases going to have been factory designed
in if needed, and are not 'field' remedies. Adding liquid injection is a
design decision, not a field repair.
~~>If the service technician is familiar with the variables involved, it
~~>can be an accurate indication of, and in fact, more informative than,
~~>coil outlet superheat.
How ? Assuming no liquid injection, etc. ?
~~>> I've also seen factory charging charts that specified 1 degree
~~>>superheat for a particular R22 air conditioner under certain conditions.
~~>If this was a specification for compressor inlet superheat (which most
~~>cap tube charging charts are), it was in floodback. Under those
~~>conditions, I would throw away the chart and take it up to 5F.
A) No chart I've ever seen has said 'measure AT the compressor,
and B) No chart I've ever seen was designed to allow floodback conditions.
I can not accept the concept of 'throw away the factory chart, it's
wrong'. Making a service decision to do something unusual, because of a
known and understood unusual circumstance, OK, but not 'The factory
charging chart is wrong'. Maybe 'The chart doesn't cover this condition',
but not 'it's wrong, it's designed for floodback'.
If you know of one that is, I'd like to see a copy of it.
Gary Lloyd
>In article <4hv6ot$a...@www.gatecom.com>,
> tme...@gatecom.com (Gary Lloyd) wrote:
>~~>It's tempting at this point to launch into a discussion of dynamic
>~~>equilibrium vs static equilibrium, but nah...
> Hey, go for it, Gary ! What is 'dynamic equilibrium' ? I
>understand the words, but how can a system in motion, and under outside
>influence ( compressor, fans, etc ) acheive equilibrium ?
Equilibrium being the balance of opposing forces, we think of it as
static. But forces that are continuously changing, as long as they
continue to be in balance, are in equilibrium at any given point in
time. Hence, dynamic equilibrium as I have been given to understand
(or misunderstand) it.
I claim no expertise in this area.
>~~>> Another important factor to remember here is that heat does not
>~~>>transfer instantaneously. Put an ice cube into a pot of boiling water,
>~~>>you'll have liquid, solid, _and_ gaseous states existing in that container
>~~>>for a while, until they equalize.
> You can't skip this one, Gary - isn't it on point ?
I agree with this statement, Paul, but fail to see your point.
>~~>I'm not certain from what perspective you consider average temperature
>~~>to be unimportant. From a trouble shooting point of view (that which
>~~>matters most to me), the all important measurements are the surface
>~~>temperatures on the liquid and suction lines.
> Yes, it's certainly important, and is my perspective also, as a
>service tech. The surface temps, the pressures _at_ the ports, is all I
>have to work with. What I'm suggesting is that they only give a snapshot
>of certain points, and only averages of those points at that. Then add in
>instrument error and field conditions, and you come up with the need to
>talk about 10-15 degrees superheat, instead of one, which I think was your
>original thesis.
> I can't ( and have no desire or need to ) get into a pipe and
>measure the different temperatures in the laminar stratification that is
>likely present, or the various pressure zones at that cross-section that
>are being pushed around by the pump. I must take the 'snap-shot', and
>work with it, and thus the theoretical 'perfections' can not apply.
>Paul
I'm not sure, but I think you're saying you agree with me, right?
Gary Lloyd
>In article <4i4srm$s...@www.gatecom.com>,
> tme...@gatecom.com (Gary Lloyd) wrote:
>~~>I agree that TXVs should be ADJUSTED for coil outlet superheat.
>~~>Compressor inlet superheat is a TROUBLE SHOOTING TOOL.
> Gotta differ with that - compressor _temperature_ is, and yes, it's
>related to superheat among other things, but still, compressor inlet
>superheat is only a side-efect of evap exit SH, which is what must be
>measured and controlled first, and then possibly some of the other
>remedies I mentioned might be needed for compressor temperature, although
If the compressor inlet superheat is within range, then the compressor
temperature will be within range, and the coil outlet superheat will
be within range, and checking the (much more difficult to get to) coil
outlet superheat and/or compressor temperature is unneccessary. If the
compressor inlet superheat is not within range, but the coil outlet
superheat is, then there is a problem in between. It's a process of
elimination.
>these are in 99 out of 100 cases going to have been factory designed
>in if needed, and are not 'field' remedies. Adding liquid injection is a
>design decision, not a field repair.
Factory designs are generally pretty good. After all, they test them,
right? On the other hand, as you pointed out in an earlier post, 10
units of the same design will all work differently. I would consider
it to be a really bad day if I couldn't improve the performance of
most of them.
Adding liquid injection is a drastic step, and as you say, not usually
a field repair.
>~~>If the service technician is familiar with the variables involved, it
>~~>can be an accurate indication of, and in fact, more informative than,
>~~>coil outlet superheat.
> How ? Assuming no liquid injection, etc. ?
Before going any further, it should be noted that almost anything that
affects any part of a system will affect the superheat. This is
especially true for cap tube systems. Therefore, on the list of tests
that I perform, it is very near the last thing that I check.
Most systems (with notable exceptions) are designed with both coil
outlet superheat and compressor temperature in mind. It should be
obvious that the compressor inlet superheat will be within a
predictable range for these systems if everything is working properly.
>~~>> I've also seen factory charging charts that specified 1 degree
>~~>>superheat for a particular R22 air conditioner under certain conditions.
>~~>If this was a specification for compressor inlet superheat (which most
>~~>cap tube charging charts are), it was in floodback. Under those
>~~>conditions, I would throw away the chart and take it up to 5F.
> A) No chart I've ever seen has said 'measure AT the compressor,
>and B) No chart I've ever seen was designed to allow floodback conditions.
Well, you got me on this one. Over the years, I have developed my own
charging methods, and I never use the "one size sorta fits all"
charging charts. I guess I just assumed that cap tube charts would use
compressor inlet superheat, since it would make more sense to do so.
What do you do when the chart is missing?
> I can not accept the concept of 'throw away the factory chart, it's
>wrong'. Making a service decision to do something unusual, because of a
>known and understood unusual circumstance, OK, but not 'The factory
>charging chart is wrong'. Maybe 'The chart doesn't cover this condition',
>but not 'it's wrong, it's designed for floodback'.
> If you know of one that is, I'd like to see a copy of it.
What I said was that IF the charging chart called for floodback, I
would throw it away.
This may shock you, but I have slightly less faith in the "experts"
than you do. But then, sometimes I don't even believe myself :-)
Russ French
Superheat serves only one function, to protect the compressor. All other
reasons to measure it, adjust it, or whatever serve no real purpose. Its just
a 'general' evaluation of one part of the performance of the entire system.
And what are you trying to protect? The mechanical moving parts of the
compressor. (and this is generally refering to re-cips) you would ideally
like to measure it, just as it enters the suction valve, which you can't do
so you pick some place else. Yes, those charts are actual measured
conditions, and they sometimes indicated 1-2 deg. F superheat at extreme
operating points, might even indicate floodback, but by the time the vapor
passes by the motor, and enters the low pressure area at the suction valve,
its no longer a saturated vapor.
Of course now with the rotary pumps we have, liquid in the compressor ain't
no problem, also increase efficiency, since superheat was just a waste of
entropy anyway...
-rp
asc...@mo.net
>In article <4i2jle$n58...@grail1617.nando.net> p...@nando.net (paul milligan) writes:
>>ten years of field work, I've never seen nor heard of 'booster pumps'
>>being applied to refrigerant lines. I don't doubt that it's done, but
>>in what type of applications, and why ?
>>
>>Paul
>>
>
>Seattle, who patented the concept of a small (1/4 hp or less)
>magnetic coupled "booster pump" for liquid lines in refrig systems.
>Allowed running head lower as pump would get rid of flash gas.
>They had tons of thermodynamic calcs also... If you are interested,
>I can try to find them again.
>
>I just pinged an addr "www.hysave.com" and it responded (I dont have
>web capability at home).. but I bet that is their homepage..
George, you got that right. :-)
>Applic was mostly supermarkets, long liquid line runs, etc.
Many liquid pumps have been installed in supermarket systems to
prevent flashing in the liquid line at low ambient temperatures.
Basic problem is large supermarkets will often have 200 ft
liquid line runs. To reduce energy, many of these systems
float head pressures down during low ambient conditions. In the
northern climates, allowing condensing temperatures to fall
to 70F and low is fairly common. If liquid lines run in heated
areas, however, flashing can occur. A liquid pump is one method
of addressing the problem. Heat exchangers and mechanical
subcoolers can also be used to eliminate flash gas.
Andy Schoen
asc...@mo.net
paul milligan
>~~>> Another important factor to remember here is that heat does not
>~~>>transfer instantaneously. Put an ice cube into a pot of boiling water,
>~~>>you'll have liquid, solid, _and_ gaseous states existing in that container
>~~>>for a while, until they equalize.
> You can't skip this one, Gary - isn't it on point ?
I agree with this statement, Paul, but fail to see your point.>
Only that a measurement on the outside of the container will reflect
only an average, and only a 'spot' one at that, while different things
are actually occuring inside, and yet no natural laws are violated.
>I can't ( and have no desire or need to ) get into a pipe and
>measure the different temperatures in the laminar stratification that is
>likely present, or the various pressure zones at that cross-section that
>are being pushed around by the pump. I must take the 'snap-shot', and
>work with it, and thus the theoretical 'perfections' can not apply.
>Paul
>I'm not sure, but I think you're saying you agree with me, right?If the
compressor inlet superheat is within range, then the compressor
temperature will be within range,
Hopefully, although there are other factors also.
> and the coil outlet superheat will
be within range,
Probably
and checking the (much more difficult to get to) coil
outlet superheat and/or compressor temperature is unneccessary.
Nope - can't go for it. The spec that was given was for the coil
outlet, not the compressor. There are several factors that can affect a
change inbetween - what about cold weather, or for that matter hot weather
in the other direction, the unknowns of insulation efficacy or lack thereof,
etc.
Checking the coil outlet may be more difficult on many systems,
but it is the <only> valid place to check. The only device controlling
( not just 'influencing' ) superheat is the TXV, and it does so at the
bulb location at the coil outlet, not at the compressor.
> If the
compressor inlet superheat is not within range, but the coil outlet
superheat is, then there is a problem in between. It's a process of
elimination.
Agreed, and that is why you must measure at the coil. What if
coil SH is too low, and there's a lot of gain in the piping, that happens
to net out to close to right at the compressor ? You're still gonna have
problems, at the very least inefficiency
>Before going any further, it should be noted that almost anything that
affects any part of a system will affect the superheat. This is
especially true for cap tube systems. Therefore, on the list of tests
that I perform, it is very near the last thing that I check.
Assuming that you means SH is the last, that makes good sense.
>Most systems (with notable exceptions) are designed with both coil
outlet superheat and compressor temperature in mind. It should be
obvious that the compressor inlet superheat will be within a
predictable range for these systems if everything is working properly.
Yes, but what is that number ? It's not on the unit's chart !
> A) No chart I've ever seen has said 'measure AT the compressor,
>and B) No chart I've ever seen was designed to allow floodback conditions.
>Well, you got me on this one. Over the years, I have developed my own
charging methods, and I never use the "one size sorta fits all"
charging charts.
I'm not aware of any such charts ????
> I guess I just assumed that cap tube charts would use
compressor inlet superheat, since it would make more sense to do so.
Not that I've ever heard of ( which doesn't rule out a whole
lot ) :~)
>What do you do when the chart is missing?
Use the numbers I've previously posted, or call the manufacturer
if that doesn't provide satisfactory performance.
In article <314900...@nwlink.com>,
Russ French <rpfr...@nwlink.com> wrote:
~~>Gary Lloyd wrote:
~~>>
~~>> p...@nando.net (paul milligan) wrote:
>Superheat serves only one function, to protect the compressor. All other
reasons to measure it, adjust it, or whatever serve no real purpose. Its just
a 'general' evaluation of one part of the performance of the entire system.
Whooo boy......Let's address that one at a time - The purpose of
superheat is to ensure a combination of capacity and efficiency, while
also protecting the compressor. That's a bit different than the statement
above.
The reasons to adjust, measure, etc, are as I just stated.
It is not a 'general evaluation', but rather a specific indicator,
which is quite different.
The need for superheat is based primarily on the fact that more
energy is moved in the changing of state of a substance than in the warming
or cooling of a gas or liquid. Thus, the least amount of working fluid
( freon ) to be moved ( and therfore the least energy spent to move it ),
will be obtained by utilizing the change of state. If this didn't matter,
then compressors would be designed as liquid pumps ( quite easy to do),
and a fluid would be moved around, instead of a liquid -> gas -> liquid
change.
Given that the above primary function of the machine ( best energy
movement effect, with least energy cost to move it )demands that the moving
device ( compressor ) be a gas-handler rather than a liquid-handler, it
then becomes a matter of mechanics that a device that is designed to move
gas well will not move liquid well. Thus, it follows that the device
( compressor )must not be presented with liquid to move, thus the liquid
must all have been turned to gas, thus superheat.
>And what are you trying to protect? The mechanical moving parts of the
compressor. (and this is generally refering to re-cips) you would ideally
like to measure it, just as it enters the suction valve, which you can't do
so you pick some place else.
No, I'd ideally like to measure it _where_ I wanted the change
of state to be happening, which is where the work I was paying to accomplish
is supposed to take place. That means the coil outlet.
>Of course now with the rotary pumps we have, liquid in the compressor ain't
no problem, also increase efficiency, since superheat was just a waste of
entropy anyway...
But it _is_ a problem ! Just because the compressor won't cough
up a hairball doesn't mean the work ( heat transfer ) is taking place
where you want it to - in the conditioned space. Pumping liquid around
without it having changed state and picked up the maximum amount of heat
it can is a waste of energy, and therefor money.
And of course there's always oil washout to be considered....
Paul
-rp
paul milligan
tme...@gatecom.com (Gary Lloyd) wrote:
~~>Never let it be said that I violated a natural law :-)
Good thing you said that - I was about to report you to Hillary's
NLP ( Natural Law Police ) :~)
~~>> Agreed, and that is why you must measure at the coil. What if
~~>>coil SH is too low, and there's a lot of gain in the piping, that happens
~~>>to net out to close to right at the compressor ? You're still gonna have
~~>>problems, at the very least inefficiency
~~>
~~>What you're "gonna have" is hunting.
Maybe or maybe not. It could be a bad TXV, which might not hunt.
Or simply set too low.
~~>> Yes, but what is that number ? It's not on the unit's chart !
~~>
~~>It's in my books !
Ah, but are your books there on site with the unit,like the
chart ? :~) I know, we should all have copies, but......:~)
I'm not aware of any such charts ????
~~>The numbers you've previously posted are for TXV systems. The numbers
~~>for cap tube systems must be adjusted for conditions. Try again.
I don't have a separate set of numbers for cap systems. Superheat
is a result of the application. The main difference is
that a cap tube system _must_ be in it's design 'satisfied' range to measure
superheat, while a TXV has a bit ( but not infinitely so )more room to
breath, so to speak.
~~>>and a fluid would be moved around, instead of a liquid -> gas -> liquid
~~>>change.
~~>
~~>Both liquids and gases are fluids, but otherwise I agree.
Yes, and I meant to use the word 'liquid' in that first instance,
of course.
Paul