List of "Dumb Questions"...

[R12rules]

What is a "drop leg" in a market system?

Just wanted a brief answer. nothing extensive.

Thanks

[R12rules]

when you got a system with a headmaster valve controling hi side pressure... do you cycle the condensor fans?

[mccool]

Drop leg is just the return line from condenser. Many market systems will have both a drop leg and discharge sensor.

Cycling fans with headmaster? Yes if a rack and fans are controlled by pressure ( setpoint for fans 10- 15 psi. above holdback), but not always neccessary, lots of rack condensers use ambient control only and work fine, depends on the rack, system variances and climate.

[engineerdave]

The only "Dumb Question" is the one that gets asked after the fact. I'd wager that every one of us has run into situations beyond what we knew how to fix. We get past that by asking someone else who no doubt thinks it's a "Dumb Question". I have no fear of the telephone at 3:00 AM. "What was I supposed to have done instead?" is probably the DUMBEST question. Call me while we still stand a chance, not AFTER the ice cream is soup.

[SupaHeat]

I read the title of the thread and decided this would be a good place to ask these questions that i have, without starting a thread for each one.

is there only one drop leg coming off the condenser?

What is a discharge sensor? Is it a pipe? If so how and were is it piped?

How do you cycle fans? I mean i know why you cycle them, but what is the device(s) that actually control the cycling?

sorry if these seem real basic, just very curious

[MikeySq]

How do you cycle fans? I mean i know why you cycle them, but what is the device(s) that actually control the cycling?



if the condenser is for one refrigeration circuit you should use pressure controls to cycle fans, pressure rises fans come on, ideal setup would have the first fan allways on(systems equiped with headpressure control, headmaster ect.) and supsequent fans come on when needed(pressure rise)

if you have a rooftop condenser with multiple refrigeration circuits(systems) piped to it you would have to use temperature controls to cycle the fans

[engineerdave]

Drop Legs
I've only ever seen or heard of one Drop Leg per circuit. There are Condensers that have multiple circuits.

Example: One Evaporative Condenser. Four Racks. Each Rack has it's own Drop Leg (Liquid travelling from Condenser Outlet to Receiver Inlet).

Fan Cycling
See the previous posts by McCool & MikeySq.

Discharge Sensor:
Pressure and / or Temperature Tranducer sensing Compressor Discharge Gas.

[SupaHeat]

right on thanks guys

[markettech]

Why does a guy who's been doing this work for as long as I have forget to put a quick connect on my high side hose when checking out an outdoor condensing unit on a -15F day?:(

Why does this same guy also forget to put on a pair of gloves and blister the sh!t out of a couple fingers?:o

[engineerdave]

Humility, man. We all need it from time to time.;)

[yotaman16]

not to steal the thread but i had a question that seemed to fit.. If you didnt know im 6 months out of tech school and trying to learn the MANY things that i still dont no so here it goes.. WHat is the procedure for checking superheat.. Not how to do it but is the reading going to be 100% accurate if the ambient is 0 to -10 making the load on the unit way less then 100 degree summer day.. So basicly i was wonderin if i take the super heat 6in form the comp on the suction line when its really cold out if this is going to be accurate or if i should just get it close and fine tune when its under a load..

Thanks in advance

[markettech]

not to steal the thread but i had a question that seemed to fit.. If you didnt know im 6 months out of tech school and trying to learn the MANY things that i still dont no so here it goes.. WHat is the procedure for checking superheat.. Not how to do it but is the reading going to be 100% accurate if the ambient is 0 to -10 making the load on the unit way less then 100 degree summer day.. So basicly i was wonderin if i take the super heat 6in form the comp on the suction line when its really cold out if this is going to be accurate or if i should just get it close and fine tune when its under a load..

Thanks in advance

You really should be checking superheat at the outlet of the coil. SH at the compressor doesn't always mean much......if you want accurate information - check it at the evap.

Dialing in a txv should be done when refrigerated space is near its desired temperature as well

[yotaman16]

if i check it at the outlet of the coil can i still use my suction gage thats on the compressor and account for aproximate pressure loss? What would you guys recomend

[markettech]

You can..............it's always best to check pressure at the coil if at all possible. In order to get the most accurate measurements a person should try to eliminate as much "guessing" as possible.

Do I ALWAYS check superheat by reading psig at the coil? No, not always - but a very good percentage of the time.

If it's a problem call, I'll pump the circuit down and braze on a tap at the coil if I have to.

[passintyme]

Yes in most systems under 100 ft your pressure drop is going to be negligable your temp from the evap to the condenser however will vary greatly with surrounding ambient,insulation and piping conditons. 6 inches from evap coil is generally where you want to take your reading.

[refrige-nate]

i was taught by accounting for line loss, 1psi for every approx 12 1/2 feet.

[frostmonkey]

i was taught by accounting for line loss, 1psi for every approx 12 1/2 feet.

I Can't see that method being at all accurate.Refrigeration PIping is sized for an equivalent pressure drop 2 deg F/ 100 Feet. So on a R-507 system running at -24 Deg F SST( 15 PSIG) you would expect to see the pressure back at the rack to be 13.7 psi. ( Assumiing the line run is100 feet) By your method youd expect to see it at 7 PSIg.( 100'/12.5=8, 8x1psi/12.5'=8 PSIg delta P)I bet you have a hard time getting superheat setting accurate. Rules of thumb are great but it must be an accurate rule of thumb. If I'm missing something here feel free to correct me.:D

[markettech]

i was taught by accounting for line loss, 1psi for every approx 12 1/2 feet.




I Can't see that method being at all accurate.Refrigeration PIping is sized for an equivalent pressure drop 2 deg F/ 100 Feet. So on a R-507 system running at -24 Deg F SST( 15 PSIG) you would expect to see the pressure back at the rack to be 13.7 psi. ( Assumiing the line run is100 feet) By your method youd expect to see it at 7 PSIg.( 100'/12.5=8, 8x1psi/12.5'=8 PSIg delta P)I bet you have a hard time getting superheat setting accurate. Rules of thumb are great but it must be an accurate rule of thumb. If I'm missing something here feel free to correct me.:D

Are you better able to see why it's best to just check it at the coil if you can?

[frostmonkey]

Are you better able to see why it's best to just check it at the coil if you can?

Checking the pressure at the coil outlet is of course the best way however sometimes it is not always possible. I was more puzzled by his rule of thumb of 1 psi delta p per 12.5 feet of pipe.

[markettech]

Checking the pressure at the coil outlet is of course the best way however sometimes it is not always possible. I was more puzzled by his rule of thumb of 1 psi delta p per 12.5 feet of pipe.

I'm with ya, that does seem like a lot of drop, but I was just trying to drive the point home to Yota that once you get away from checking SH at the coil there are SOOOO many variables - seems like everyone has their own "rule of thumb".

[frostmonkey]

Thats one ofthe reasons why I like working on markets so much. 99% of the time theres a Service port at the coil,and if there isn't the store manager doesn't complain about the extra time to put one in.

[refrige-nate]

im a student and dont have much real field time so im just telling you what my teacher told me. of course he often tells us a little bit when we really need to know alot, not his fault we can only learn small peices at a time. we work on small walk in units with 134a right now, which has about ten feet of suction line and im able to take temp reading right at the end of the evaporator. im sure this rule of thumb must have just applied to the units we are working on right now sorry for the confusion. thanks for the info! there must be something he is planning on teaching later like a better way to account for line loss so now i geuss ill be ahead of the game.

[frostmonkey]

You Should learn the the 2Deg/100f theroy when you are taught how to size pipe in the mean time I think I have a link to a great site with lots of info on piping practices let me see if I can find it.

[frostmonkey]

Sorry can't seem to find it off hand right now. I'll do some more digging later. Gotta go rescue my apprentice. NEVER A DULL MOMENT :rolleyes:

[markettech]

............OOPs - wrong thread

[jappell]

How does a suction line trap stop oil slugging?

[MikeySq]

how do you mean stop oil slugging?

are you saying if there was no trap you would have Oil slugging back to compressor?

Oil traps aid in oil Return to compressor, i'm not sure what you're sayin:confused:

-Mike

[markettech]

How does a suction line trap stop oil slugging?

A trap doesn't stop oil from slugging a compressor - it insures oil return to the crankcase. A trap is only needed if the evaporator coil sits at a lower level than the controlling compressor and/or there is a suction riser 3' or higher.

[jappell]

A trap doesn't stop oil from slugging a compressor - it insures oil return to the crankcase. .

O K thanks, now, expand please, how does it insure oil return?

[MikeySq]

oil pools up in the trap and reduses the ID of the pipe causing higher refrigerant velosity which will move/carry the oil upwards easier

[markettech]

O K thanks, now, expand please, how does it insure oil return?

Read through this (http://www.heatcraftrpd.com/res/pdf/support/Oil_Pressure_Problems.pdf)

[skotup]

I read the title of the thread and decided this would be a good place to ask these questions that i have, without starting a thread for each one.

is there only one drop leg coming off the condenser?

What is a discharge sensor? Is it a pipe? If so how and were is it piped?

How do you cycle fans? I mean i know why you cycle them, but what is the device(s) that actually control the cycling?

sorry if these seem real basic, just very curious

Im a sparky specialising in refrigeration, i'll try to answer you...
Yes only one "drop leg" ,although ive never heard that term, from the condensor unless the discharge is used for heating elsewhere then it may return, condensed, to the reciever from other applications such as water heating or air con.

discharge sensor in my experience is a resistor that varies its resistance based on temperature (discharge temp probe). Its just cable ties onto the discharge line. You also have suction probes aswell on the the suct header.

Fan cycle in my work is controlled a computer program and also the discharge pressure which is obtained from pressure sensors or 'pressure transducers' on the discharge and suction lines. The pressure determines how many fans are on and at what speed they should run at.

[R12rules]

superheat measured at the compressor is useful to tell you if your coming back with a slug of liquid and gonna eventually kill the unit from flood back. Or is the return gas so overheated that the compressor is NOT getting any cooling coming back which is critical to its long life.

Superheat measured at the evap coil outlet is the superheat of the coil itself. Whereas as system superheat is measured at the compressor.

A balanced system, in other words, one where no engineers were involved... a properly made system will have the right amount of return gas temp to the unit as well as the right superheat coming off the evep coil(s).

But you cannot let engineers be involved otherwise it is all fouled up. Especially Husman "Engineers"!!!


(pun intended)

(using the word "engineer" in the same sentance with the word Husman is an oxymoron)

[300 Win Mag]

If you have a split system a/c with pipes running thru an attic and you checked the superheat at the coil, by the time it gets to the compressor you might have 50* super or better. The comperssor MFR's recommend checking near compressor. I can probably dig up a warranty card from copeland that states this, but correct my if I am wrong. If the system is right, you will probably have 10-12*F at coil and 18-22*F at comp. Most MFR's want around 20*F super at comp to protect from slugging and overheat. This is for A/C not ref. Ref specs are sometimes different.