Controlling a compressor off suction line temp, or pressure

[desA]

Scenario:
1. RHVAC system operates in a hot country, say at T,ambient = 30'C;
2. Trip compressor on suction line temp of 25'C;
3. Want system to stay off for preset time;
4. After time, system to re-start, run & pull suction temp down to below trip temp.

Problems:
a. If normal direct action controller is used, it will not re-start, since final temp above trip temp;

How to manage this situation as inexpensively as possible?

[NoNickName]

How to manage this situation as inexpensively as possible?

Push the compressor contactor with your index finger, until suction temp drops below trip temperature, then leave.
Cost = 0 krugerrand.

[desA]

Mmhhh... LOL.

I meant as a permanent control strategy - perhaps using a Carel-type controller. Has me a bit flumoxed.

[Gingerair]

Wire start/start delay & off delay timer in parralell with suction trip device so that it energises on suction trip.

When suction trip device returns to normal off delay timer energised to avoid momentary loss of control circuit power..

[NoNickName]

Yes, normally closed contact with delay on opening, just like gingerair said.

[sedgy]

yes this situation was common < in the old days< pre-1960s most fridge units did not have stats they were controled off the L-p switch so when the evap went below the desiered temp off on L-p setting up was time consuming but in them days for a pound you could get 8 pints of beer land sea + air < fish chips + peas <
and a taxi home, the good old days.

[monkey spanners]

yes this situation was common < in the old days< pre-1960s most fridge units did not have stats they were controled off the L-p switch so when the evap went below the desiered temp off on L-p setting up was time consuming but in them days for a pound you could get 8 pints of beer land sea + air < fish chips + peas <
and a taxi home, the good old days.

Have seen a few old milk tanks (alfa laval mostly) that regulated the ice bank on the lp switch, worked surprisingly well.

[desA]

Wire start/start delay & off delay timer in parralell with suction trip device so that it energises on suction trip.

When suction trip device returns to normal off delay timer energised to avoid momentary loss of control circuit power..

Thanks so much for your input, Gingeair. Very much appreciated. :)

[desA]

Yes, normally closed contact with delay on opening, just like gingerair said.

Thanks very much 'NoNickName'.

[powell]

Scenario:
3. Want system to stay off for preset time;
4. After time, system to re-start, run & pull suction temp down to below trip temp.

3. How long?

4. If the units are running high SST, how would restarting them pull the SST down? Will the load change?

[desA]

@ powell:

Some very valid questions. The idea is to guard against a certain combination of load & ambient conditions which pushes suction line temp (!not SST!) above a certain threshold.

The system should re-activate after an allowed off-time. If combined conditions again push the suction line temp over the limit, then stop again.

Practically, this strategy could be improved with a number of additional features eg. lockout after so many re-starts, manual re-set etc.

For the systems I work with, the load can most definitely change in the off-time, so again - it is the combination of load & ambient acting on affecting suction line temp.

[powell]

desA,

Actually a 30C (86F) ambient is not that hot. We see 43C (110F) plus here in Nevada all the time. Most refrigeration system units work well.

Are you dealing with a system thats undersized, extra low temp (maybe R22), has possible control problems etc.?

What type of system do you have?

I would address the high superheat / overheating problem and cure the cause.

[desA]

desA,
Actually a 30C (86F) ambient is not that hot. We see 43C (110F) plus here in Nevada all the time. Most refrigeration system units work well.
Are you dealing with a system thats undersized, extra low temp (maybe R22), has possible control problems etc.?
What type of system do you have?
I would address the high superheat / overheating problem and cure the cause.

Let's say the system is designed to run with high condensing temperatures. As such, this tends to pull up Te,sat & suction temp in sympathy - this is a byproduct of the particular technology. The system runs just fine, as designed. I'm working through a particular control strategy - trying to keep it as simple as possible.

Actually, in my region, we often go over 42'C, at RH 72-75%. The 30'C was a typical value found in let's say, more moderate climates. :D

[sedgy]

hi all is this thread an advert for holidays ?