5 replies to this topic

[Lyne]

Hello everyone,

I'm designing a new hot oil loop and I got confused about the trim cooler. Should it be designed for the highest consumer duty or should it be as per the fired heater turndown duty ?
The duty percentage for each consumer are as follow:
-Heat exchanger 1 represents 74% of the total duty.
-Heat exchanger 2 represents 10% of the total duty.
-Heat exchanger 3 represents 12% of the total duty.
-Heat exchanger 4 represents 4% of the total duty.

The exchangers 1 and 2 doesn't work all the time, one can be working and the second not working or both not working.
The exchangers 3 and 4 work always at the same time.

The lowest duty will be the duty of exchanger 2. So I've got confused how should I design my trim cooler as per the highest duty, the lowest duty or the fired heater turndown ?

Your advice and guidance are much appreciated

Edited by Lyne, 30 March 2024 - 08:11 AM.

[Pilesar]

The trim cooler is to help control the hot oil temperature. When you are running exchangers 1 and 2, the fired heater is adding heat to the oil at 84%. When you turn off heat exchanger 1, there is a huge drop in heating demand. The trim cooler would fill the gap in demand to smooth out the oil temperature for the transition to lower firing. You will need to understand the control system response for best design choice. At first glance, I would think the trim cooler design should approach the duty of heat exchanger 1.

Edited by Pilesar, 30 March 2024 - 08:58 AM.

[breizh]

Hi,

You may find pointers in the link attached:

https://read.nxtbook..._bechtel_i.html

Breizh

[Pilesar]

The article breizh supplied gives a better answer than I did! It seems there is no simple answer for design trim cooler duty without knowing the rest of the system and its controls.

[Lyne]

Thank you for your response, this was really helpful 

[deft clay]

Hi,

You may find pointers in the link attached:

https://read.nxtbook..._bechtel_i.html

Breizh

 

Solid article.

 

Here's a few key question on hot oil systems:

 

1. What is the max turndown rate of the fired heater?
2. Do exchangers 3+4 remove all of that produced duty at turndown? If so, the trim cooler could be fairly small.
3. If not, at that turndown rate, what are the inlet and outlet conditions?

 

Your trim cooler needs to be designed to enable the fired heater to run at turndown without shutting off due to high temps. Remember, there a few bottlenecks in turndown:

• burner turndown - The burner manufacturer will tell you how much the burner can turn down
• hydraulic turndown - You must pump some minimum of fluid through the system. Your pumps may dictate this, or if the heater is multi-pass, you need to hit a certain dP across the coils in order to guarantee balanced flow (10 psi is a rule of thumb)
• heat transfer turndown - If the heater has multiple burners, it's possible that max turndown is achieved by turning some of them off. You still have to ensure that film temperatures and flux rates are acceptable in that case, and with that if the heater is multi-pass all passes are getting the same heat transfer.

How did this end up getting resolved?