refrigerant velocity and t/stat bulb placment

[theapprentice]

hey everyone I hope this thread is in the right spot to post these questions but im new and no sure where to post certain questions. What Im no sure is how to measure the velocity of the refrigerant inside the pipes to make sure we maintain correct velocity in the run to provde good oil return.
Im also abit curious why exactly when installing thermostats that the bulb goes into the coil rather then just return air.

thanks for everyones help I am learning little bits and pieces from this site,
cheers,
chris.

[US Iceman]

Hi Chris,

The refrigerant velocity is found by this:

cubic feet of vapor (or liquid) per minute divided by the pipe cross section area : CFM/sq.ft. = feet per minute

or

cubic meters of vapor (or liquid) per second divided by the pipe cross section area : CMS/sq. meter = meters per second

If you can find some refrigerant booklets, these usually have charts that list the line velocity based on refrigeration capacity flowing through the pipe. The charts account for the little bit of math shown above.

As to the bulb placement; I normally recommend the bulb be placed in the return air path to control room temperature.

Hope that helps....

[wkd]

Are the thermostats you are installing for temperature control or defrost.US Iceman is correct for temperature control but you may put a bulb in the coil to initiate a defrost when a unit is a reverse cycle heat pump.

[750 Valve]

Smaller commercial cap tube systems (mainly bottle cabinets) quite often have the t'stat bulb buried in the coil. This is because a cap tube requires longer off cycles to allow the pressure to equalize for starting of the compressor, usually these stats hava quite a large differential on them (5 to 8K) to allow this long off cycle and also allow a good off cycle defrost to clear the coil, the coil may run down to -2C and cut in again at say 5C - hence the large diff and the stat placement.
Can't beat cycling on return air for an upright cabinet or room on txv though.

The velocity in the lines is something that is done when designing a system but is a good thing to keep in mind when starting up new systems, and even maintenance and repair, always checking for correct piping and that the installers actually did what the engineer wanted and didn't throw any old pipe in.

[theapprentice]

yeah my tafe teacher was saying velocity is usually caluclated by system designeers therefore just follow the plans and yeah I understand now about bulb placement in return air aposed to coil placement. thanks guys im learning little bits and pieces from this forums,
chris.