Floating rack pressure

[Fridgy Dave]

I hear about this all the time as far as saving energy. Can someone explain to me exactly what it is and the advantages and disadvantages of floating the rack pressure.
Thankyou

[HVACRsaurus]

The philosophy is that on certain conditions (digital input from time clock, outside air temperature, or whatever) the suction pressure setpoint can be offset (raised), resulting in a reduced compression ratio and reduced demand on compressors. Obviously there are limitations as to how much, and how long, the suction pressure can be offset. I'm not convinced that it is the silver bullet for energy management, and I'm also not certain that it makes a particularly big difference to energy consumption. No doubt system float may be a part of system optimisation, but by itself probably yeilds little gain.

[joe magee]

Actually floating suction does save energy. Lets say you have a +15 suction group with numerous circuits. The meat being the coldest. If you have epr valves at the rack with all different pressure settings you could manually open them have the E-2 controller look at the meat case temp. If that temp is satisfied the suction will start to rise(shut off compressors) All the other temps should be fine. Basically the suction header is wide open and you have more mass flow. You are not driving the suction down to satisfy temps. One large company I used to service would drive the suction down to zero and just set epr valves. What a waste. I'm simplifying this.

As far as head pressure goes you can float the head down with the ambient. We use vfds. Flood the condenser so we are using all of it then use a 10 degree split. Drop it down to a min. Low head equals $$$$ saved

[750 Valve]

Yep big dollars to be saved by floating heads and suctions, electronic TX valves make it really easy to float head pressure down provided they are sized adequately for the minimum pressure drop, mechanical valves are a little bit harder to get right - even balanced port valves have a significant operational change when comparing design max and a typical floating scenario, as long as they don't starve too much or hunt excessively causing floodback most of the time the fluctuations in operation go un-noticed.

Suction pressure strategy as described by Joe is spot on and works a treat, you need a decent controller like a Danfoss 55, 255 or CPC to do it, stuff like Carel will struggle when it comes to this function.

[Y L Lai]

Floating pressure only works if you have very stable communication. From where I come from, communication always fails due to what soever reason. Everytime it happens, front end will be confused, and will remain in it's state, and will not resume normal even though load is required. Be very carefulwhen you activate it, and make sure you set maximum float value.
On Danfoss decentralized system (using stanalone controllers), make sure it's properly configured as instead of floating upwards, it will dive lower than your setpoint when load is required, wasting energy i stead of saving. Just take note on this.

[georgedvf]

Suction pressure strategy as described by Joe is spot on and works a treat, you need a decent controller like a Danfoss 55, 255 or CPC to do it, stuff like Carel will struggle when it comes to this function.

Have you used Danfoss SC255 to set the floating suction??? Could you explain me how does it manage to raise the pressure if only temperatures of each evaporator is measured???

[THE DUDE]

Hello everyone. 80% of the supermarkets I take care of ( in Minnesota) are set to float the suction and head pressure. Walking into the compressor room in winter is crazy....nothing is running = Big savings. Some racks float the suction up 14 psi.

[750 Valve]

Have you used Danfoss SC255 to set the floating suction??? Could you explain me how does it manage to raise the pressure if only temperatures of each evaporator is measured???

You need to setup suction group for dynamic suction float, then when programming the circuits you decide which circuits to include in the float (usually ones that operate on the lowest sst on the rack), when the circuits are at or below setpoint (it works a bit differently depending on whether circuits are standard, case control with injection only or case control with cut in/cut out control) the controller will allow the rack to float up (or down if you enable it in programming) to slowly add to the suction setpoint (to a designated maximum value). Ypu can exclude cases while on defrost and pulldown.