Air Cooler with Warm Air Recirculation Control
Air Cooler with Warm Air Recirculation Control
(OP)
Good day, forum
We struggle with air cooler control loop design. We have a new air cooler:
- air cooler with warm air recirculation
- ambient air min/max -35/+35°C
- process fluid freezing temperature +43°C
- louvers control - automated with pneumoactuators from DCS
- fans control - first fan on/off from DCS, second fan VFD controlled from DCS
- process fluid control - thermocouple in air cooler outlet, signal to DCS
- cooler internal air temperature control - 2 internal thermocouples, signal to DCS
Vendor has provided us with manual where it described that there 6 cases exist for air cooler autocontrol:
- 3 for overheating protection (modes "cold summer", "moderate summer", "hot summer")
- 3 for antifreezing protection (mode "cold winter", "moderate winter", "warm winter")
And 1 more mode for manual start-up during winter using steam coils.
All seems OK but we can't realize how overall control loop works and finish process and instrument diagram. We can't imagine and explain to a programmer how algorithm works. We have 3 input signals and 5 output signals in DCS:
1. How to choose air cooler internal temperature that is controlled during each mode?
2. When do we have to switch one mode to another?
If anybody have detailed P&ID or algorithm how air cooler with warm air recirculation works please share it.
We struggle with air cooler control loop design. We have a new air cooler:
- air cooler with warm air recirculation
- ambient air min/max -35/+35°C
- process fluid freezing temperature +43°C
- louvers control - automated with pneumoactuators from DCS
- fans control - first fan on/off from DCS, second fan VFD controlled from DCS
- process fluid control - thermocouple in air cooler outlet, signal to DCS
- cooler internal air temperature control - 2 internal thermocouples, signal to DCS
Vendor has provided us with manual where it described that there 6 cases exist for air cooler autocontrol:
- 3 for overheating protection (modes "cold summer", "moderate summer", "hot summer")
- 3 for antifreezing protection (mode "cold winter", "moderate winter", "warm winter")
And 1 more mode for manual start-up during winter using steam coils.
All seems OK but we can't realize how overall control loop works and finish process and instrument diagram. We can't imagine and explain to a programmer how algorithm works. We have 3 input signals and 5 output signals in DCS:
1. How to choose air cooler internal temperature that is controlled during each mode?
2. When do we have to switch one mode to another?
If anybody have detailed P&ID or algorithm how air cooler with warm air recirculation works please share it.





RE: Air Cooler with Warm Air Recirculation Control
RE: Air Cooler with Warm Air Recirculation Control
Without seeing the detail, the vendor seems to have provided you with a number of different "cases", however unless there is some sort of step change between them in terms of control output, the control function remains the same.
The only thing that will change is the speed of the fan and the amount of air being re-circulated.
Without reading the process and control description presumably supplied by the vendor I can't be certain, but I would imagine that the system tries to maintain a fixed air cooler internal temperature by use fot he louvres and a fixed fluid oultelt temperature by use of the fan speed.
There may be times when if the fans are max speed then the control point fot he internal air temp is reduced.
Might be a bit of a process map that needs to be followed for all the combinations, but that's what control systems are for.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Air Cooler with Warm Air Recirculation Control
RE: Air Cooler with Warm Air Recirculation Control
We are just a detailed design Contractor and do what Principal and Licensor had prescribed. If we shall drive screws with hammer - ok, we do our best as better as possible.
Thank you. I thought I should use "program" language as my question is concerned only DCS.
No. See vendor's manual.
https://drive.google.com/open?id=0By-iev9g_oHZZU8y...
I did not understand what you are talking about.
RE: Air Cooler with Warm Air Recirculation Control
Looks pretty straightforward to me.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Air Cooler with Warm Air Recirculation Control
RE: Air Cooler with Warm Air Recirculation Control
As with all such complex schemes, process control loop configurations evolve with time before you settle down with something that is perceived to be as simple as it can be, stable and cover all operating cases/ modes ( and you have carry an increasingly irate project manager to boot during this evolution). So take your time on this loop / scheme before you declare it to the project team. Be prepared for field changes when / if things dont turn out the way you intended.
RE: Air Cooler with Warm Air Recirculation Control
Imagine a shutdown situation. The air cooler is filled with a material which solidifies at a temperature above ambient temperature, and you lose flow. What's your plan?
Yeah, I know it's not water- we're not going to rupture the tubes- but we still have a problem, don't we?
What do you do when power goes down? What do you do when the material has solidified in the tubes as a result? You're going to try to "cook" it out so you can get it flowing?
You're presumably going to try to close louvers on the fan box and then attempt to HEAT the material in the tubes- the top of which are still probably exposed to the environment. OK- let's say that the whole thing is boxed in, so you can avoid the problem I've just described. Seems to me that you've just impaired the NORMAL functioning of the unit, just to handle a start-up case!
Personally I consider running a material which solidifies above ambient temperature through an air cooler to be bad design practice. It might be a necessary compromise of bother for capital savings if the unit is huge, though, especially if the unit rarely shuts down.
The licensor likely has experience doing this and thinks what has been recommended is just fine. Frankly though I'd speak with a couple USERS of this design first and see if they have complimentary things to say about this air cooler arrangement.
RE: Air Cooler with Warm Air Recirculation Control
The operating modes are a set of options ranging from convection cooling controlled by the inlet and outlet louvres, fan driven with re-circulation control to start up ( total re-circ with a steam coil).
Not the simplest of cooling systems but equally not too hard either once you read the operating manual and get it clear in your head. Lots of combinations of temperatures and changing of control functions but quite doable.
LI
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Air Cooler with Warm Air Recirculation Control
Yes, we have.
Do you mean electricity or all kind of power simultaneously? If only electricity was shut down:
Winter - switch control loop to mode "winter 2" (see vendor's manual). If it can't help then lower process fluid flowrate to control fluid outlet temperature no less 43°C. If it can't help then switch control loop to manual mode, close intake louvers, close exhaust louvers, open recirculation louvers, open valves on steam coil inlet and outlet.
Summer - switch control loop to mode "summer 1" (see vendor's manual). If it can't help then lower process fluid flowrate to control desired fluid outlet temperature.
Switch control loop to mode "winter 1" (see vendor's manual) and manually open valves on steam coil inlet and outlet.
Call to storage control room and warn them about emergency, open cooler bypass, switch control loop to mode "winter 1" (see vendor's manual).
It seems to me you misunderstood. Sorry for my obscure posts. We don't intend to automate start-up or shut down. Emergency, start-up and shut down are to be done in manual mode. My question is concerned only normal operation - how air cooler has to be controlled using 4 input signals (ambient air temperature, first internal temperature, second internal temperature, process outlet temperature) and 5 output signals (intake louvers, exhaust louvers, recirculation louvers, first fan start/stop, second fan VFD)?
It is a straight run vacuum gasoil hydrocracking facility (~180 barg) revamp, capacity is to be increased from 1.00 to 1.25 Mt per year. Process fluid is main fractionator bottom product 360+°C. Overhaul period is 3 years.
Yes, it has a long reference list.
We have no these USERS. We have nobody to ask. It is a first air cooler with air recirculation that is fully automated.
I have done many times and this manual starts to be dreamed. I can't catch how:
1. to choose set point of internal temperature. Yes, I understand that vendor has provided equations in manual, but they give us a wide range of possible set points.
2. to define boundary conditions when DCS has to switch modes from "summer 1" to "winter 2" and back.
3. to control all 3 louvers (intake, exhaust and recirculation) simultaneously. Should it be linear and equal or stepwise or something else?
4. ambient temperature should be included in algorithm.
RE: Air Cooler with Warm Air Recirculation Control
RE: Air Cooler with Warm Air Recirculation Control
- intake louvers
- exhaust louvers
- recirculation louvers
- first fan start/stop
- second fan VFD
See vendor's manual, page 2, modes 3 and 4 (link above).
RE: Air Cooler with Warm Air Recirculation Control