low suction pressure

[fridgy09]

small 404a bitzer unit running 3 deli cases, sucton pressure comes down to fast so unit short cycles. sight glass doesnt flash. changed dryer.. has eprs so have them set rigt but to keep temp have to run the unit to very low pressure with big diff on pressure control... any ideas would be appreciated...as unit still shirt cycles just not as bad.. thanks

[desA]

Short of gas?
Evap fan/s operating?
Evap blocked/dirty - airside?
TXV dirty/blockage?

[Gary]

Check the evaporators for airflow problems, fans, defrost, etc.

[fridgy09]

all fan good coils clear, tx valve strainers clean. it does give signs of short of gas.. but liquid line sight glass stays full....

[desA]

... it does give signs of short of gas.. but liquid line sight glass stays full....

Is there any reference to the original refrigerant charge for this system?

Are you anywhere near this?

[Gary]

Any unloaders?

[Gary]

Outdoor condenser? Condenser fan control? High side pressure set too low?

[fridgy09]

im going back there tomorow. charge not stated. no unloader. high side pressure is about 240 fan comes on at 260psi doesnt come on very often and cuts out about 245... i think from memory..

[fridgy09]

seems almost like the unit could be over size and it needs an unloader?

[fridgy09]

yes sorry out side cond as well

[Gary]

seems almost like the unit could be over size and it needs an unloader?

Does it maintain temps in the deli cases?

Does the pressure drop into vacuum?

Has the system functioned differently in the past?

Why do you believe this is a problem?

[ramranga]

hi
im ram i want the know how can i charge the oil in screw comp. and also i wanna know wt is system of oil heat pump

[fridgy09]

well yes the case temps are ok. no it doesnt drop in to a vacume. im not sure it may have operated like this for years... i just thinnk the compressor is cycling to much....

[Gary]

It sounds to me like the system is operating properly, but the pressure control should be set lower to avoid short cycling.

If the pressure were dropping into vacuum I would recommend an unloader.

[desA]

What if the refrigeration system were over-sized for the application? Would this not perhaps cause a fair deal of short-cycling?

[Gary]

What if the refrigeration system were over-sized for the application? Would this not perhaps cause a fair deal of short-cycling?

The refrigeration system must be sized for heavy load on all 3 deli cases.

As the load reduces, the EPR's restrict flow in order to maintain high humidity conditions in the cases. This drops the downstream pressure, causing the suction pressure to be low, reflecting the actual collective heat load.

If the heat load in the cases drops enough, the compressor cycles.

As long as the pressure does not drop into vacuum and the compressor is not short cycling, then I would not be concerned.

If the pressure drops into vacuum, then it should have some form of unloading or false loading.

[desA]

Thanks, Gary. Nice explanation.

[fridgy09]

thanks gary

[Steve Wright]

What is case number and bitzer number and how many times per hour is it cycling?

[paddyaircon]

gary knows his onions- i had similar situation on 404a display chiller- i too was looking for a fault that wasnt there. one thing i have learned is that gauges are not the only way of seein whats happening in a system, but temps, loading and gut instinct all come in

[cadwaladr]

are the cases fitted with there own controllers/mag valves?what is the ambient at condensor/temperature of the cases enviroment?

[Peter_1]

....
As long as the pressure does not drop into vacuum and the compressor is not short cycling, then I would not be concerned.
Just stumbled over this thread Gary..only thing to be concerned of will be serious increase in SH and discharge temperatures.