Need help with capillary tube questions

ChrisJ

I'm hoping someone on the forum may be very familiar with capillary tubes and designing systems using them.

I need to talk to someone regarding what actually takes place when the system first starts up, and when it shuts down. I've read a lot on it and many people seem to disagree about what specifically takes place.

I'm working with a small 1/8HP system that has a flooded evaporator, used to be a highside float system but is now converted to a capillary tube and I'm trying to get the charge correct.

pecmsg

See if this helps

hvacfreak2

So it used to be the most forgiving as far as refrigerant amount but is now converted to a " critical charge " metering device. If this machine does something important I would investigate an expansion valve ( thermostatic or automatic depending on some specifics ).

If the system capacity is known and since it is relatively small it may be possible to add a heat source at the evaporator and charge to a target superheat and staying just under the FLA rating on the compressor motor ( assuming that the evap. is at capacity ). Maybe a small electric heater with a known wattage for the evap. load would work.

ChrisJ

hvacfreak2 said:

So it used to be the most forgiving as far as refrigerant amount but is now converted to a " critical charge " metering device. If this machine does something important I would investigate an expansion valve ( thermostatic or automatic depending on some specifics ).

If the system capacity is known and since it is relatively small it may be possible to add a heat source at the evaporator and charge to a target superheat and staying just under the FLA rating on the compressor motor ( assuming that the evap. is at capacity ). Maybe a small electric heater with a known wattage for the evap. load would work.

I doubt anyone will ever find a TXV or similar for it as it uses methyl formate as a refrigerant. This is for my monitor top refrigerators.


I don't think superheat will work, but maybe I'm wrong?
The evaporator is a flooded type in the shape of a U with a large header on each side. The two sides are connected with what they call an equalizer tube that carries primarily vapor and the suction tube is between two baffles with a skimmer setup to remove oil off of the top of the refrigerant in one header. Essentially, the suction tube never sees anything but vapor even when badly overcharged.

It appears both an undercharge and an overcharge have very similar results, shortened off periods. But I don't understand why undercharged would cause shortened off periods?

Thermostat goes by evaporator temperature, typically 10F - 20F range. When the evaporator hits 20F it turns the compressor on and when it hits 10F it shuts down.

Techman

Hi ChrisJ. You and your projects!!! Cool! Well anyway, here goes.
#1- "Commercial Refrigeration for the AC Technician" by **** Wirz, Second Edition, pg#116-122.
#2-RSES- SAM(ServiceApplicationManual # 620-98) "Capillary Tubes: Theory and Practice"
#3-RSES-SAM # ?, "Servicing Cap-Tube Systems"
#4- SUPCO(SealedUnitPartsCo.)-"Theory and Field Problems of the Capillary Tube" by Henry Ehrens
#5- SUPCO, "Theory of the Cap Tube as a Refrigerant Control" by Henry Ehrens
#6- SUPCO- "Practical Education Information Based on Forty Years of Experience" by SUPCO
#7-JB Industries Co.-"Copper Capillary Tubing"- Application and Engineering Data for R&AC.
#8- the NEWS(publication magazine) -"Ice Breaker: How to Size Replacement Cap Tubes"
"Ice Breaker: Diagnosing a Restricted Capillary Tube"
"Charging Capillary Tube Systems" all by Joe Marchese
#9- Service Reporter Magazine-"Cap tube systems present a charging challenge"
#10- Tecumseh Co. "Engineering Recommendation on: Cap Tube Sizing"
#11- Have a Good Read my Friend. LOL with you!

KC_Jones

Chris you better start reading I expect a full report next week!

hvacfreak2

Andy Schoen ( spelling , guess ) at Parker / Sporlan is my hero regarding refrigeration topics. He used to post over at htalk but I have not noticed any posts for awhile.

Alot more information has been presented since my first reply , I did not know if it was a beer cooler or an in row server rack unit that was the topic. So I'm understanding that the cap tube is working but the off cycle times get weird. If the original high side float closed off tight that could have prevented refrigerant migration during the off cycle. A check valve or liquid line solenoid may be the ticket for the current cap tube configuration.

unclejohn

I wonder if a electronic TEV would work. They shut off 100% just like you old float.

ChrisJ

unclejohn said:

I wonder if a electronic TEV would work. They shut off 100% just like you old float.

Could one be used to regulate a strange refrigerant that was only used in 1933-1934 though?

unclejohn

I don't see why not. They open and close with a sensor not a temp. bulb. The unknown would be if the refrig. would eat up the insides of the valve.

ChrisJ

unclejohn said:

I don't see why not. They open and close with a sensor not a temp. bulb. The unknown would be if the refrig. would eat up the insides of the valve.

What kind of sensor?
For some reason, the sulfur dioxide machines used machined steel float valve components. These methyl formate ones used bronze.

What I can say for sure is anything plastic or rubber doesn't seem to like it including lexan and refrigerant hoses.

unclejohn

The electronic TEV uses voltage to open and close. Not suction line temp. Well the temp of the line determined the voltage and the valve opened or closed. There was a system called Becon I think Bohn made them. They aren't in production anymore mainly because most refig. mechanics refuse to read the manual. You can contact Bohn service and talk to one of there tech guys and they might know, or rather should know what the valves are made of, I think they were stainless steel on the outside anyway.