ReplyExcept its not really a secret. Some folks just dont readOriginally Posted by Poodle Head Mikey
Regular Guest
This is what I do. Seemed like common sense at the time. First charge as much liquid as you can through the liquid side, and then throttle it in with the gauges at 5-10 psi over the system suction pressure to top it off.
Always worked like a charm.
Oh, and BTW - I never use a digital gauge. Just seems to make me lose touch with whats goin' on. I dont even use a digital vacuum gauge - but instead I collect (yes, I have 4) of the Thermal Engineering vacuum gauges. They are ridiculously cheap if you can find them.
Maybe I am just too much of an ole codger
BTW - looks like ALL of those Sporland links are ded.
Regular Guest
Except its not really a secret. Some folks just dont read
Professional Member
Yeah, I've realized that.
http://sporlanonline.com/literature-...lating-valves/
Look there for more Sporlan info
Regular Guest
I guess the bottom line is if there is a head pressure control the charge must be calculated? You cant just charge based on subcooling like any other system?
If i came to a system and was unsure i it was properly charged, i would remove recover refrigerant until i saw bubble and then topped it off following sporlans procedure?
P.S. i have only charged two systems with vapor (the first two systems i ever charged) the rest? Always liquid to the suction. I cant imagine doing it any other way with exception of through the liquid service valve.
Professional Member
Ding! Ding! Ding!
You are exactly correct, Sir.
I'd laminate a 90-30-1 or save it to your phone or tablet for easy access.
*edit*
To be completely clear, if the system has a receiver, you cannot accurately charge by subcooling.
Regular Guest
When you say if it has a receiver you cant charge by SC, does that also include systems with receivers but DO NOT have a head pressure control?
Professional Member*
Receivers
R I P Icemeister
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Regular Guest
How do you charge with a receiver (but without a HPCV)?
Professional Member*
Sight Glass / Weight
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How do you determine the charging-to weight for the system?
PHM
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PHM
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When faced with the choice between changing one's mind, and proving that there is no need to do so, most tend to get busy on the proof.
Membership Chair/ARP Committee / Professional Member*
Let's say that we can't charge by subcooling because the system has a liquid receiver. OK; but then we need to set the TXV / suction superheat and look up the maker's instructions. Let's pretend that these say that we need 8-10º of subcooling at the inlet to the TXV in order to ensure that our suction superheat numbers and the valve's operation is accurate. OK; so then after we do that - can we assume that the system is charged properly?
PHM
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PHM
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When faced with the choice between changing one's mind, and proving that there is no need to do so, most tend to get busy on the proof.
Regular Guest
If it has a receiver, charge per flooded evaporator calculated procedure. Then adjust txv. Then check sight glass assuming cooler is under normal load conditions and adjust charge appropriately?
Professional Member
There are a couple of different things going on here.
A small refrigeration system with a receiver (not outdoors) will not require a flooding charge at all. Clear the glass, MAYBE a little extra to allow for ambient temp variations and move on. You're done.
An outdoor system with a low ambient control device is where the flooding charge calculation comes into play.
PHM, the only thing about your process that I'll dispute is the need for 8-10 degrees SC. What we actually need is a solid column of liquid.
Now, if a manufacturer says that they expect 8-10 SC at the TEV, then that's what it is, but it can't really be taken as a general assumption or as a general rule of charging.
Regular Guest
Wait till load is normal before clearing the glass?
Membership Chair/ARP Committee / Professional Member*
Well; that is stating it a little too briefly I think. I'm pretty sure that: "8-10º degrees of liquid subcooling at the TXV inlet" should have the words: "under all conditions" after it. So yes; 1-2º of SC may allow the TEV to control properly at that moment - but there would be no allowance for changing conditions; cooler condenser, higher evap loading, and so forth. Which is what the 'in excess of momentary need' subcooling (along with the 'stores the surplus' nature of the receiver) does for the system.
For my own systems I do as you suggest because I am willing to tolerate the SSH drifting higher than setpoint during warm pulldown in return for having a nearly empty condenser and the resulting greater condenser surface area to to trend head pressure lower during the stabilized operation - which is the vast majority of the time. But I wouldn't readily do that for a customer.
I would want to think about this more - but isn't a certain degree of actual SC designed into the functioning of the TXV? Without sufficient SC doesn't the refrigerant 'flash too soon'? That is: flash excessively at the orifice and so force the distributor tubes to act more like capillary tubes? Metering more and distributing less? I would expect it to be difficult to stabilize TXV operation with only 'a solid column of liquid' if the SC was only a degree or two.
Your thoughts?
PHM
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PHM
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When faced with the choice between changing one's mind, and proving that there is no need to do so, most tend to get busy on the proof.
Professional Member
Some of this is a moot point because these systems, refrigeration systems with receivers, are designed with an adequate degree of subcooling built in.
Now, I'm not really an engineer type. Although I try to be analytical in my thinking, I don't have the qualifications nor the education to be an engineer.
We typically don't know what that subcooling measurement is, unless we are in contact with the guy who did engineer it OR we took detailed readings on the equipment on startup and have them recorded in a log, and the second is still something of an assumption because there could still have been a latent problem that we are (or were) unaware of which lead to inaccurate readings.
I tend to think that the conditions that you're keying in on, most specifically warm pull down, aren't really operational conditions that we need to be overly concerned with. They are transient. How long does a warm pull down last? How much longer would it last with a slightly lower charge? Are we talking a minute, two minutes difference between one charging method and the other?
Once the system is operating and at a fairly normal operating temperature, then our load is generally fairly stable and our operating conditions will also be fairly stable. This is the operating condition that we need to key in on and make run at its optimum.
Yes, as the TEV is sized, a degree of subcooling is considered in that sizing. I've actually found undersized TEVs because mechanical subcooling was considered as a sizing factor when it actually was not present. It amounted to an additional valve size on a Sporlan Balanced Port valve that was required to make required BTUs. In those cases, it was R-22 and we're talking about upwards of 50 degrees of subcooling that was not there but which was designed for (105 entering liquid vs 55 entering liquid)
Subcooling certainly affects the sizing of a TEV, but I think that before we have a significant sizing issue, we need to deviate more than a few degrees of subcooling. Even the difference between 1-2 which is theoretical and 8-10 which is what you're calling optimal, and I don't really disagree by the way, but the difference in capacity that we'd see in that range is so small as to be almost insignificant.
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