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Ventilation controls for diesel generator room 2

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Norm01

Mechanical
Apr 18, 2008
27
Hello, I'm relatively new to the HVAC and HVAC controls industry and one of the projects I have been involved with is upgrading the ventilation system controls for an existing diesel generator room that is not working properly. The existing room is equipped with an O/A damper, return damper and relief damper all operated with pneumatic actuators and pneumatic controls (see attached sketch). The building is equipped with a BMS. The client wants to upgrade the ventilation control system to electric controls, but is wondering if the existing pneumatic actuators can be reused. Most of the actuators have been replaced in the last 4 years, but a few are probably more than 10 years old. My questions are as follows,
Is it worth using the existing pneumatic actuators or should they just be replaced with new electric actuators?
If the pneumatic actuators are used how are they connected to the electric control system?
What will the control system consist of? New electric temperature sensor(s), actuators and a new DDC control panel?
Any information that can be provided to these questions or to the control system in general would be of great help. Thanks.
 
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Norm01,

I have done enough projects to tell you that trying to modify a control system into a hybrid electrical/pneumatic system to save the cost of a few actuators is biting off your nose to spite your face. The interface work and fights you will get into over warrantee, finger pointing, "not in my scope" whining, "that's not in our service contract" excusing, nonsense is definitely NOT worth the cost. Your client will pay tenfold in the long run. Design a complete, single point of responsibility control system that you absolutely know will work.

You may need temperature sensors to operate the system normally to provide ventilation when the generators are not running. (You also did not state if this is a prime power/cogeneration type system or a Standby/emergency backup type system-I have assumed the latter). But the system must run anytime the generators are running, period.
You show the exhaust piping but did not state if the silencers are within the space or outdoors. I assume there is no switchgear in the generator room? That changes the air and temperature parameters.

You did not elaborate on what "...not working properly" means, but if I am looking at your sketch correctly, I would hazard a guess that the room is getting hot and the generators are not getting sufficient air to run at the temperatures at which they are designed to run. This is especially probable if the partitions in your sketch are full height partitions.

Your sketch shows that the air from the generator room is exhausted out by fans. It also shows that a relief air louver and damper is present to allow outside air to relieve the exhausted air. But what I also see is a "return" air damper between the relief air louver and the exhaust fans. The relief air is being short cycled and exhausted through the "return" louver. Unless the relief air is ducted and delivered by a relief air fan to the front side of the generator sets to allow that air to wash over the alternator and the engines, there is no cooling air available to the gensets.

It doesn't matter what the controls are. Unless you get sufficient air to flow from the front to the rear of the gensets, they are going to run hot and reduce their performance and life expectancy. I would cut very large louvers into the wall where you indicate the doors. You can filter them to minimize dirt intrusion, but you need airflow.

I know this is a little off topic, but we've done many of these plants and this is a common problem.

Good luck,
EEJaime
 
EE Jamie,
thanks for your comments. I have included an updated sketch, showing the locations of the switch gear and transfer switch, as well as the silencers. The walls are full height and the generators are for emergency backup. Also the three fans blow air into the room as shown.
As it stands right now, one of the fans is constantly running to provide air circulation in the room. As shown I the sketch there is a transfer damper that has been closed because air was not getting to Gen 3. From talking to the maintenance staff, the sequence has never worked properly from the start. From what I’ve been told, when each generator runs, the corresponding fans start and then the sequencing of the dampers begins.
You are correct in that the room does get warm and that is why they have they run the fan and sometimes leave one of the doors slightly open.
From your comments the damper should be set up to modulate to maintain a setpoint temperature even when the generators do not run (which currently does not happen). What would a typical setpoint temperature be in a generator room 25 deg C (77 deg F)?

 
 http://files.engineering.com/getfile.aspx?folder=481da250-9e29-4b9b-b49a-5e5bbbd9b2ab&file=Genset_Room.pdf
Hello Norm01,
I had a look at your sketches and I do recognize something in the building design and dampers, I have seen a similar design in an exchange building once
It looks to me that gen 1 & 2 were first installed and that there were doors to the area where now gen 3 is and the smaller room next to it.
The air from the fans was intended to flow though the room with the two generators and pressurize the room so you would get a flow back to the room before the fans thought the open transfer damper.
In this small room, which I will cal return air room, the return air was either complete or partially exhausted or returned back to the generator area. The return air amount is controlled by the return air damper. The relief damper, or a back draft damper, would let the excess air flow out of the building. In the room before the fans the return air is mixed with outside air, the quatities of return air and outside air would depending on the condition (temperature) in the room.
The room where now gen 3 is was not used.
In a later stage gen 3 was installed and the doors to generator room and to the return air room have been removed and the transfer damper got closed in order to give gen 3 some cooling/ air flow.
This is a thing you could maybe check with one people that has been working there for a longer period.
I would first check if the airflow through the building is sufficient to cool the area when all three generators are running. The design might have been for two generators only.
The airflow pattern would be one to look at as well, since I have the feeling that generator 3 gets his air heated first by generator 1 & 2 and looking at your sketch it bypasses most of the generator 3 when it is going back to the former return air room.
The control system change to, as you said to New electric temperature sensor(s), actuators and a new DDC control panel? The actuators have to modulate the dampers (RA & OA dampers to control the temperature). See site for the actuators.
For the temperature in the room, I am looking at a deign from Lindenberg for north Africa and the temperatures mentioned here are Tmin is 10 ?C and Tmax 49 ?C.
I hope this will help a little.
 
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