Air Cooled Heat Exchanger -Fan Speed Control
Air Cooled Heat Exchanger -Fan Speed Control
(OP)
All:
Currently I am working on Air Cooler P&ID. Here are the details of the air cooler-
Its a forced draft, single bay, 2 fan/bay cooler. The propane vapor enters at 210F and 300psig and leaves as liquid at 120F. The deltaP across the cooler is 5 psig.
The motors are of VFD type.
The speed of the fan is controlled via pressure controller on the inlet header that feeds the air cooler
Could anyone explain to me this control scheme? Also the use of temperature controller as oppossed to pressure controller?
Any help would be appreciated.
Regards,
npf
Currently I am working on Air Cooler P&ID. Here are the details of the air cooler-
Its a forced draft, single bay, 2 fan/bay cooler. The propane vapor enters at 210F and 300psig and leaves as liquid at 120F. The deltaP across the cooler is 5 psig.
The motors are of VFD type.
The speed of the fan is controlled via pressure controller on the inlet header that feeds the air cooler
Could anyone explain to me this control scheme? Also the use of temperature controller as oppossed to pressure controller?
Any help would be appreciated.
Regards,
npf





RE: Air Cooled Heat Exchanger -Fan Speed Control
if the pressure dicreases too much, this means you condensate too much, then you need to decrease the speed of the fans. and vice-versa. position of pressure indicator is not important, as long as it is between the head of your column and your reflux pump (the set point may vary, due to friction head loss, hydrostatic head loss, etc...)
the temperature control may be usefull if you need to eliminate some compounds that will condensate at lower temperature than propane. Controlling the temperature will assure you to condensate only propane, and other compounds will be separated in remaining vapor phase.
I hope my answer is useful
RE: Air Cooled Heat Exchanger -Fan Speed Control
2. Refer yourself to API 661 C.3.1 Airflow and air temperature control systems, and
Figure C.1 — System A — Airflow control with automatically controlled variable-pitch fans
Figure C.2 — System A — Airflow control with automatically controlled outlet louvres and
fixed-pitch fans
... substitute variable-pitch fans with VFD driven fans ...
Cheers
S.
RE: Air Cooled Heat Exchanger -Fan Speed Control
I hope my explanation raise some new idea.
RE: Air Cooled Heat Exchanger -Fan Speed Control
RE: Air Cooled Heat Exchanger -Fan Speed Control
This is a poor control scheme - controlling the tower (overhead) pressure by manipulating fan speed. It is proven to be very inefficient, mostly because of extended dead time between the controller input and VFD speed. As the result, tower pressure will fluctuate.
If this is total condensation service, what you can do is to modify this scheme with hot vapor bypass or to try how ACHE will work as flooded condenser. Hot vapor bypass is simple and straightforward, and it works in 100% of cases.
Best regards,
RE: Air Cooled Heat Exchanger -Fan Speed Control
RE: Air Cooled Heat Exchanger -Fan Speed Control
Is it normal and customary to use a finned-tube air cooler in condensing service ???
Or is this a case where equipment was "left over" and put into this service bt management.
The entire cooler should be rated for a vacuum...is it ?
If we compare this configuration to a steam surface condenser (in power plant service), I note that the condenser is really controlled by the level of condensate in a "hotwell".
Does your plant have a collection hotwell ?
-MJC
RE: Air Cooled Heat Exchanger -Fan Speed Control
1.) Do nothing - surprisingly enough, many plants still do this.
2.) Head pressure control - maintain a controlled backpressure downstream of the condenser - this backs up the condenser until the pressure of the condensing fluid is at the setpoint.
3.) Backpressure and condenser bypass - this holds a backpressure upstream of the condenser. The bypass around the condenser is a hot gas push to make sure that the liquid can get pushed around the system.
4.) Put a pressure controller on the propane and control the louvers or fan speed - not as effective but still rudimentary control.
RE: Air Cooled Heat Exchanger -Fan Speed Control
Secondly and perhaps more importnatly, condensers of this type are usulaly, INDUCED draft type rather than the FORCED draft. I think that it is beacuse, the induced draft fan provides a more uniform cooling over the entire exchanger area and it also avoids sudden creation of vacum if it rains on the exchanger tubes....
Regards
Evolution rather than Revolution!
RE: Air Cooled Heat Exchanger -Fan Speed Control
Could you tell me why should'nt we rate the air cooler for vaccum,since because of the condensation nad sudden cooling during rain storm ,there is a possibility of vacuum ?.
Please help out.
Vinod
RE: Air Cooled Heat Exchanger -Fan Speed Control
RE: Air Cooled Heat Exchanger -Fan Speed Control
Vinod: It is highly unlikely that the propane condenser will ever experience vacuum where most of the forum members live and build plants. However, it is more likely that under some situations, the compressor suction could drop below atmospheric pressure. It is preferred to avoid such a situation for the danger of sucking in air and moisture in to the propane loop through flange joints. If there is a chance of the compressor suction pressure becoming sub-atmospheric, its seal system shall be designed to create artificially higher sealing pressure.
Regards
RE: Air Cooled Heat Exchanger -Fan Speed Control
RE: Air Cooled Heat Exchanger -Fan Speed Control
It was a cool winter day in LA (USA). The operators were complaing that could not keep the pressure up in the accumulator, it was below 180 psia!!! I asked what's wrong with that? Whats the steam load to the turbine. It was lower than normal. I asked was that good? They said yes. so what's the issue? The issue was that the first stage flash economizer valve was wide open and the system could not pass enough C3+ from the accumulator if the pressure was below 160 psia. I laughed until I cried!!!!
I told the Ops manager that he needed to buy a 2" class 300 control valve to put is series with the 8" one. He told me that would not do any good. Oh well, those big chem plant guys know so much more than us little gas processing guys.