Air Cooled Heat Exchanger -Fan Speed Control 1

[npf]

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

 

[nouanda]

since condensation will create vacuum, pressure will significantly drop.
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

 

[thermmech]

1. Confirm whether there are any louvers on the top.

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.

 

[Limitedone]

I guess the pressure control is cascade with level controller of the dowstream receiver. Frankly speaking I do not know the reason, but I think with this configuration the level of the receiver can be controlled smoothly.
I hope my explanation raise some new idea.

 

[dcasto]

there is absolutely no reason to have speed control on a propane condensor, UNLESS its a reflux condensor.

 

[EmmanuelTop]

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,

 

[maddocks]

I've only speed controlled a propane condenser to reduce nighttime fan tip noise. I set the pressure at 1200 kPa and let the speed vary to try and reach this. It also kept the propane warm for the required subcooling and process fluid warming.

 

[MJCronin]

A couple of questions...

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

 

[maddocks]

We ALWAYS use finned tube condensers in air cooled propane condensing service. Vacuum is not necessary - it's impossible to get a vacuum in propane service unless you're colder than -45F. There's a number of ways to control the condenser:

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.

 

[RotaryG]

It is not clear why speed control of the air cooler fans is required in this service. Once money is invested in the air cooler surface area, does it not make sense in using the exchanger as efficently as possible? Better is the cooling, lower is the condensing pressure. Lower is the condensing pressure, lower is the power consumed by the compressor..

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!

 

[797989]

Guys

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

 

[maddocks]

The only way you can reach vacuum is is the propane actually cools to below -40F. Since this is an extremely remote possibility, there is no need to design for vac. As far as induced vs forced draft, ultimately it's the designer's choice, but I've used nothing but forced draft in the 50 or so that I've installed in propane condensing service. Variable speed connected to pressure control will allow some op cost savings with reduced power draw, but you need some pressure to move propane through the system. Presumably this is a refrigeration system. Unfortunately, rotaryguru, the propane moves around the loop based on differential. You need to maintain the accumulator at a sufficient pressure to get flow of liquid refrigerant.

 

[RotaryG]

Maddocks is right. We need DP to push the stuff round and round. However, in a well engineered system, a situation that warrants artificial creation of pressure in the condenser shall not arise. If that is the case, then the condenser is bigger than needed. Ie, we engineers have wasted money and contributed to the global financial crisis! Abnormal events such as storms and rain can create abnormal situations as well as system instabilities. Hence, induced draft fan systems are preferred for condensers - particularly large ones! This does not mean forced draft systems will not work. Both will work as long as they are well engineered and their strengths and limitations are understood.

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

 

[maddocks]

Anytime the ambient temperature is below the design, the condenser is essentially oversized. This has nothing to do with wasting money. Even a 5 Deg F drop in ambient will make double the cooler LMTD and it will be oversized and the temperature/pressure will drop.

 

[dcasto]

I worked on little 100 to 500 HP propane refrigeration systems for about 10 years in the natural gas industry, seen it all. I went to a huge ethylene cracker that had a 30,000 HP propylene refrigeration system. What a site.

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.