Fan outlet pressure vs temperature
Fan outlet pressure vs temperature
(OP)
Our glass container manufacturing plant has several 350HP VFD-controlled fans for bottle cooling. They normally run close to full speed (60Hz), since the outlet pressure setpoint is normally set to 60"H2O, the fan´s rated pressure at rated airflow. However, since we would like to have the highest possible pressure WITH the lowest possible temperature, we need to know if there are temperature v.s. airflow v.s. pressure fan curves, or formulas or rules of thumb, which could tell us how much the outlet air temperature rises with respect to pressure and airflow in centrifugal fans.
We want to see if under some cases the overall cooling effect could be increased by LOWERING the fan´s speed, and thus the temperature, right now we only control outlet pressure and disregard temperature.
Related comment: sometimes when we stop the bottle machine and airflow goes to a minimum, the outlet pressure actually decreases rather than increase, an effect which has been puzzling us a little, since normally you expect the pressure to rise in a centrifugal fan when you demand less airflow under constant speed. Anyone out there can point to some literature on this subject? Thanks a lot for your help.... Normally the manufacturer supplies pressure vs airflow vs power vs system curves, but no curves or formulas take into account outlet temperature rise. Thanks again for any comments on these matter.
We want to see if under some cases the overall cooling effect could be increased by LOWERING the fan´s speed, and thus the temperature, right now we only control outlet pressure and disregard temperature.
Related comment: sometimes when we stop the bottle machine and airflow goes to a minimum, the outlet pressure actually decreases rather than increase, an effect which has been puzzling us a little, since normally you expect the pressure to rise in a centrifugal fan when you demand less airflow under constant speed. Anyone out there can point to some literature on this subject? Thanks a lot for your help.... Normally the manufacturer supplies pressure vs airflow vs power vs system curves, but no curves or formulas take into account outlet temperature rise. Thanks again for any comments on these matter.





RE: Fan outlet pressure vs temperature
Temperature rise = Total Pressure Rise /(2.7 * Fan Efficiency)
Temperature rise in degrees F
Total pressure rise in inches of water.
RE: Fan outlet pressure vs temperature
RE: Fan outlet pressure vs temperature
i am wandering why you are so certain that pressure should be maximized? it is air flow rate that brings you cooling effect - assuming that we talk about heat rejection; pressure only serves to transport that flow.
you can google "fan curve", "ventilation system curve", "ventilation system working point" to get basic idea of it.
RE: Fan outlet pressure vs temperature
RE: Fan outlet pressure vs temperature
RE: Fan outlet pressure vs temperature
For example yesterday, the machine worked with only one or two molds for 8 hours, airflow obviously was minimal, and pressure dropped from 60"wg to 58"wg, causing the automatic pressure control to increase fan speed from 56Hz to 60Hz, and as a result temperature rose from 26 degC to 46 degC!!!
Obviously the formula [Temperature rise = Total Pressure Rise /(2.7 * Fan Efficiency)] didn't hold in this case, because pressure remained the same but air still heated up, with airflow reduced to a minimum. Is this because this formula applies only under constant speed or under non-extreme conditions like the one I'm mentioning? Is there another formula that approximates fan behaviour under these extreme circumstances? Thanks for your comments.
RE: Fan outlet pressure vs temperature
Do you measure the pressure upstream of the manifold (in the plenum)?
RE: Fan outlet pressure vs temperature
RE: Fan outlet pressure vs temperature
You've checked that the pt is piped properly with no blockages?
RE: Fan outlet pressure vs temperature
RE: Fan outlet pressure vs temperature
from what you described i would firstly assume that there is flaw in cooling-blowing concept IF the mold dampers do not posses pressure-independent flow control ability. if those are ordinary on-off dampers, system curve "jumps" over fan curve and maintaining constant differential pressure by changing fan rotation gives different fan-system interaction for each and every on-off combination.
that means fan should be "ordered" different pressure for every combinations of damper open (all open, all but one open..., etc.), but i am not sure can you set it up at fan speed controller.
in that case primary solution could be disconnecting all mold dampers leaving them open all the time.
i am not sure how would it affect energy consumption. if in you current practice energy consumption would be significantly increased, than setting up control solution as mentioned could be long-term cure.
RE: Fan outlet pressure vs temperature
I still do not understand your process completely, but if there are other machines that work there is no reason for pressure vs temperature to change.
RE: Fan outlet pressure vs temperature
In our plant, operations personnel blame the air temperature swings for some of the bottle quality problems, so that is why they want us to maintain a stable air temperature, which obviously is not feasible cost-wise. And that is why we have been looking closely at those temperature increases related to airflow, trying to figure them out. I'll keep you posted, while I try to understand a bit more those temperature effects, since I am just the control/instrumentation guy resonsible for the control systems, with very limited HVAC knowledge.
RE: Fan outlet pressure vs temperature
I made a test today on one of the fans, at fixed speed (56 Hz), checking the plenum pressure under various conditions:
The whole machine (10 molds) running: 58"H2O
machine stopped (0 molds): 54"H2O
1 mold running: 56"H2O
2 molds running: 58"H2O
3 molds running: 59"H2O
4 or more molds: 58" H2O
so as HerrKaLeun said, it is definitely a surge condition, right?
I had seen surging in compressors, but not on centrifugal fans. I suppose it only happens with higher outlet pressure fans, as the link posted by HerrKaLeun shows fan curve examples with a surge region, but the pressure never actually decreases at low airflows, it remains more or les stable, but outlet pressure is only 10"H2O.
RE: Fan outlet pressure vs temperature
Only axials and FC centrifugals operate have surge regions in the fan curve.
RE: Fan outlet pressure vs temperature
RE: Fan outlet pressure vs temperature
RE: Fan outlet pressure vs temperature
RE: Fan outlet pressure vs temperature