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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.
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 ------ Originally Posted by jpsmith1cm 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.
Wait till load is normal before clearing the glass?
Originally Posted by wdshea 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? 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.
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?
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 ------ Originally Posted by wdshea 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?
How do you determine the charging-to weight for the system? PHM ------ Originally Posted by pecmsg Sight Glass / Weight
Originally Posted by wdshea How do you charge with a receiver (but without a HPCV)? Sight Glass / Weight
How do you charge with a receiver (but without a HPCV)?
Receivers
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?
Originally Posted by wdshea 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. 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.
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.
Originally Posted by lexscripta BTW - looks like ALL of those Sporland links are ded. Yeah, I've realized that. http://sporlanonline.com/literature-...lating-valves/ Look there for more Sporlan info
Originally Posted by Poodle Head Mikey One of the best kept secrets in the HVAC/R business must be that all refrigerant gets charged as a liquid. <g> Except its not really a secret. Some folks just dont read
Oh yeah. 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. Originally Posted by Poodle Head Mikey One of the best kept secrets in the HVAC/R business must be that all refrigerant gets charged as a liquid. <g> Who the hell has the time to warm the cylinder repeatedly and then wait for pounds and pounds of refrigerant to boil off? Don't your service managers scream over all the time you are taking? There are little vaporizing widgets you can charge through but why not just crack the suction side gauge handle and let it boil/vaporize in through the crack? If you are very curious (as I was <g>) use two 1/4" flare MxF sight glasses. Put one before the suction hose - screw it onto the left bottom side of your gauge set - and put the other one onto the suction access flare on the unit. Then crack the suction gauge manifold handle a little and see what the refrigerant looks like in the sight glasses. With the manifold gauge cracked - tell me what you see. And also note the suction pressure increase on your gauge (valve open versus valve cracked) initially 'set' it for 5-10 lbs over running suction pressure. Use an analog gauge and watch the needle flutter. If the needle is steady the gauge valve is either open too far or closed too much. <g> PHM -------
Oh yeah.
Originally Posted by Poodle Head Mikey No it's not. I just posted so it's a current thread. Perhaps you meant to say that this thread had it's genesis almost four years ago? <g> PHM ------ Ahhhh. No I'm sticking with the fact that this thread is almost four years old. Now the POSTING are recently current but the thread at it's genesis is approaching four years of age.
Originally Posted by cthegreat1 the link http://sporlan.jandrewschoen.com has expired.... http://sporlanonline.com/90-30-1_072012.pdf Try that one.
. No it's not. I just posted so it's a current thread. Perhaps you meant to say that this thread had it's genesis almost four years ago? <g> PHM ------ Originally Posted by VTP99 Plus this thread is almost four years old.
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. You could measure all the tubing in the system piping and both coils and calculate the internal volume. Then work out where there would be various vapor volumes at the various pressures. And so forth. Or you could look up the total receiver volume for the system refrigerant and then use 75% of that as the charge. If you have to bid the job - use 100%. <g> I have worked on systems which could not be pumped down - but not very many. PHM ------- Originally Posted by cthegreat1 I have a Bailey walk-in freezer.... How would I calculate the amount of refrigerant needed for this system?
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