7 replies to this topic

[Propacket]

Dear all,
I have a question related to a hot oil system. The hot oil system is designed to supply hot oil to two reboilers at 518°F. Hot oil return temperature is 400°F. Hot oil is heated in a waste heater being heated by flue gases coming from combustion turbines at 997°F. I was astonished to see a trim cooler on hot oil line just before entering the waste heater. What is the significance of this cooler? Heat duty of cooler is not known.

[ankur2061]

Dear all,
I have a question related to a hot oil system. The hot oil system is designed to supply hot oil to two reboilers at 518°F. Hot oil return temperature is 400°F. Hot oil is heated in a waste heater being heated by flue gases coming from combustion turbines at 997°F. I was astonished to see a trim cooler on hot oil line just before entering the waste heater. What is the significance of this cooler? Heat duty of cooler is not known.

P. Engr,

It is not such a mystery to have a hot oil trim cooler when you are using a Waste Heat Recovery Unit (WHRU) using exhaust gases from gas turbines. In fact for all systems that I have seen for hot oil with heating by WHRU this is the correct configuration. And the reason is simple. Accurate control of hot oil temperature from the WHRU is extremely difficult due to the varying characteristics of the exhaust gases from the GT's. The varying characteristics or more precisely the varying heating value &/or flow rate of the exhaust gases could be for a number of reasons such as GT turndown to minor variations in inlet gas composition. In fact the variation in the characteristic of the exhaust gases from GT is most commonly found in areas where there are extremes of ambient temperature during summer & winter & in such a case the trim cooler becomes an absolute necessity to have accurate hot oil temperature control.

As the description says 'trim cooler', a side stream of the pumped hot oil from the hot oil expansion tank enters a trim cooler (air cooled heat exchanger) & exits at a controlled lower temperature which then is mixed with the hot oil exiting from the WHRU using a 3-way temperature control valve (TCV) to obtain accurate temperature control of the mixed hot oil stream. I have seen this configuration many number of times and it works perfectly fine.

Hope this makes things clear.

Regards,
Ankur.

Edited by ankur2061, 26 February 2010 - 11:25 PM.

[Padmakar Katre]

Dear,
Could you just provide the terminal conditions across the trim cooler.

[Padmakar Katre]

Dear all,
I have a question related to a hot oil system. The hot oil system is designed to supply hot oil to two reboilers at 518°F. Hot oil return temperature is 400°F. Hot oil is heated in a waste heater being heated by flue gases coming from combustion turbines at 997°F. I was astonished to see a trim cooler on hot oil line just before entering the waste heater. What is the significance of this cooler? Heat duty of cooler is not known.

P. Engr,

It is not such a mystery to have a hot oil trim cooler when you are using a Waste Heat Recovery Unit (WHRU) using exhaust gases from gas turbines. In fact for all systems that I have seen for hot oil with heating by WHRU this is the correct configuration. And the reason is simple. Accurate control of hot oil temperature from the WHRU is extremely difficult due to the varying characteristics of the exhaust gases from the GT's. The varying characteristics or more precisely the varying heating value &/or flow rate of the exhaust gases could be for a number of reasons such as GT turndown to minor variations in inlet gas composition. In fact the variation in the characteristic of the exhaust gases from GT is most commonly found in areas where there are extremes of ambient temperature during summer & winter & in such a case the trim cooler becomes an absolute necessity to have accurate hot oil temperature control.

As the description says 'trim cooler', a side stream of the pumped hot oil from the hot oil expansion tank enters a trim cooler (air cooled heat exchanger) & exits at a controlled lower temperature which then is mixed with the hot oil exiting from the WHRU using a 3-way temperature control valve (TCV) to obtain accurate temperature control of the mixed hot oil stream. I have seen this configuration many number of times and it works perfectly fine.

Hope this makes things clear.

Regards,
Ankur.

Dear Ankur,
Nice piece of information indeed. Just got a little confusion over it. As you said to take care of extreme temperatures (winter/summer)it's needed to route the hot oil through the air cooler. Is there any recircualation of hot air to maintain the air cooler inlet air temperature as I apprehend about the viscous hot oil localized stagnation in air cooler tubes in case of extreme winter.

[Propacket]

Dear all,
I have a question related to a hot oil system. The hot oil system is designed to supply hot oil to two reboilers at 518°F. Hot oil return temperature is 400°F. Hot oil is heated in a waste heater being heated by flue gases coming from combustion turbines at 997°F. I was astonished to see a trim cooler on hot oil line just before entering the waste heater. What is the significance of this cooler? Heat duty of cooler is not known.

P. Engr,

It is not such a mystery to have a hot oil trim cooler when you are using a Waste Heat Recovery Unit (WHRU) using exhaust gases from gas turbines. In fact for all systems that I have seen for hot oil with heating by WHRU this is the correct configuration. And the reason is simple. Accurate control of hot oil temperature from the WHRU is extremely difficult due to the varying characteristics of the exhaust gases from the GT's. The varying characteristics or more precisely the varying heating value &/or flow rate of the exhaust gases could be for a number of reasons such as GT turndown to minor variations in inlet gas composition. In fact the variation in the characteristic of the exhaust gases from GT is most commonly found in areas where there are extremes of ambient temperature during summer & winter & in such a case the trim cooler becomes an absolute necessity to have accurate hot oil temperature control.

As the description says 'trim cooler', a side stream of the pumped hot oil from the hot oil expansion tank enters a trim cooler (air cooled heat exchanger) & exits at a controlled lower temperature which then is mixed with the hot oil exiting from the WHRU using a 3-way temperature control valve (TCV) to obtain accurate temperature control of the mixed hot oil stream. I have seen this configuration many number of times and it works perfectly fine.

Hope this makes things clear.

Regards,
Ankur.

Ankur,
Very nice explanation.
1-Will the trim cooler be still required if flow of exhaust gasses is controlled by the temperature of hot oil exiting from the waste heat recovery unit? If temperature of hot oil increases than the required, flow of entering gasses will be reduced and extra gasses will be routed to stack.
2- If we need to design the cooler, for what conditions it should be designed? I mean to say that I do not know the maximum temperature of hot oil entering the cooler. I have following information in this regard:
The hot oil system is designed to supply hot oil to two reboilers at 518°F. Hot oil return temperature is 400°F. Hot oil is heated in a waste heater being heated by flue gases coming from combustion turbines at 997°F.

[ankur2061]

[/quote]
Ankur,
Very nice explanation.
1-Will the trim cooler be still required if flow of exhaust gasses is controlled by the temperature of hot oil exiting from the waste heat recovery unit? If temperature of hot oil increases than the required, flow of entering gasses will be reduced and extra gasses will be routed to stack.
2- If we need to design the cooler, for what conditions it should be designed? I mean to say that I do not know the maximum temperature of hot oil entering the cooler. I have following information in this regard:
The hot oil system is designed to supply hot oil to two reboilers at 518°F. Hot oil return temperature is 400°F. Hot oil is heated in a waste heater being heated by flue gases coming from combustion turbines at 997°F.
[/quote]

P. Engr,

The option 1 which you have suggested should definitely lead to the elimination of the trim cooler. However, the accuracy of controlling exhaust gas flow using dampers is suspect in my personal opinion. Maybe these days with new control techniques it is possible to obtain precise flow control of exhaust gases. However, you need to check this out with the WHRU supplier.

The WHRU supplier should be able to give you the maximum outlet temperature achievable for a supply temperature of 400°F to the WHRU. Alternatively for a new WHRU you could give this as an input to restrict the maximum outlet temperature or skin temperature from the WHRU to a given value. This maximum outlet temperature with or without a margin could then be utilized as design temperature for the trim cooler. I am suggesting this because in one of the scenarios that I can envisage is that the hot supply to the reboilers is by-passed or no heat exchange of heat is taking place in the hot oil reboilers & the temperature of the return stream is same as the supply stream to the reboilers with the WHRU still in operation. However, this scenario can be discounted if you have appropriate instrument controls to by-pass the WHRU with the exhaust gases directly going to the stack in case the reboilers are not in operation or by-passed.

Hope this helps.

Regards,
Ankur.


























































































7

[JoeWong]

P. Engr,

Besides managing momentary temperature variation cause by imperfect control and physical construction, a trim cooler may be required for other reasons :

i) Heat removal to facilitate initial start-up
ii) heat balance and plant operability during turndown
iii) heat balance and plant operability during operating mode change e.g. production with NGL extraction or without NGL extraction
iv) heat balance and plant operability during operating maintenance of particular set of equipment, units, or trains.
v) Accelerate heat removal during maintenance and total shutdown of hot oil system

There may be more subject to process system.

You have to address each of the scenarios and derive the trim cooler duty accordingly.

[JMW]

Hot and cold flow stream mixing is a way in which to obtain very fast response to rapid temperature transients in the incoming fluid stream and is necessary due to the problem that heat exchangers cannot respond fast enough.

However, if there is a trend in temperature (or an underlying trend) then it can also become important not just to regulate the proportion of hot and cold streams being blended together but to also regulate the flow of heat exchange medium (coolant). This is because maximum responsiveness will be obtained when the control valve is operating at around its optimum postion. But if there is a significant trend in the temperature of the input flow stream, then the valve will progressively move away from its optimum control position, either trying to increase the cold flow more and more or less and less as appropriate. Of course, the faster the rate of flow of the flow through the heat exchanger, the less the estent to which the flow is cooled.... this isn't a problem when handling transients but it can become a problem when responding to an underlying trend. So, as the flow through the exchanger increases, more coolant flow should be factored in. As this takes effect then the cold flwo temperature will reeduce and the flow control vlave will modulate back toward its optimum control position - this necessarily is a slower control response than the mixing valve control and it can be done by monitoring the temperature of the flow stream emerging from the heat exchanger.
This technique is employed in some analyser sample loops where precise temperature regulation is required.


Incidentally, it is usually necessary to control heating in order to regulate viscosity, the viscosity of the fuel has to be optimum for correct atomisation.
In which case, if viscosity can vary due to quality, then precise temperature control becomes irrelevant unless the set point temperature can be varied in response to quality change. Hence, rather than keep sampling the fuel and calculating new EVT (equi-viscous temperature) values, it is easier and more cost effective to use inline viscosity measurement to regulate the flow mixing either directly or by adjusting the temperature set-point. Incidentally, viscosity sensors are often faster responding than temperature sensors which adds to their effectiveness for control.
In burner applications reliance on temperature as a control parameter means frequent fuel sample collection and analysis, flame inspection and excess oxygen flow. All of which adds to costs. Large diesel engines have used viscosity for control for many decades and the earlier capillary style viscometers have largely given way to modern vibrating element sensors which are more accurate and faster responding (and virtually maintenance free, even in burner applications where fuel cleanliness leaves a great deal to be desired).

(For info on viscometers see www.viscoanalyser.com or search for fuel oil heatre control viscometers which will bring up a range of manufacturers. http://viscoanalyser.../wp9dbce0e8.png, shows an analyser loop using a heat exchanger with a mixing valve - in this case the incoming flow stream is split and part by-passes the heat exchanger, and it shows the heat exchange medium being regulated.)