Dyamic calculations / Heating // Cooling ramps

[vinhermes]

Good morning to you all,

For a series of ships (tugs) actually being built, I am facing an HVAC engineering issue. To start with, I should say that we only do steady-state calculations using ISO 7547, SNAME T&R Bulletin 4-16 or equivalent. In other words, for technical rooms, we calculate ventilation flowrates and/or FCU’s powers so that the Delta T ensures that we remain below the maximum temperature knowing all the heat gains (+ verification of R/C required of course).

But I have a specific room (thruster room) in which is an equipment with a relatively high dissipation (15 kW) but only used over a limited period of time (10 minutes per hour maximum).

The first option, ventilation only, has been discarded as we could not integrate the ducts and goosenecks associated with such high flowrates. So we decided to go for an FCU in the room. Knowing all the heat gains, etc., when doing steady-state calculations, we reach a cooling load requirement of 17 kW. This will be very difficult to integrate and this small room with a piece of equipment used once in a while becomes the biggest consumer for the chilled water unit…

So here is my question, how would you do a dynamic calculation so that, knowing by iteration the power of the FCU :

- We know the initial temperature at equilibrium with the equipment off
- We calculate the heating ramp based on the dissipation over 10 minutes
- We calculate the cooling ramp when the equipment is stopped
- We get back to the initial equilibrium temperature with the equipment off in less than 50 minutes so that we are ready to start the equipment again

Here is a sketch to clarify if needed:

Main data are :
- Maximum temperature indoor temperature: 45°C
- Outdoor conditions : 37°C, 110 kJ/kg, 70% RH
- Volume of room: 16,5m3
- Fresh air: 20m3/h supply
- Heat dissipations from equipment: 15kW over a period of 10 minutes continuous per hour
- Other gains: 200 watts

When looking at the HVAC books and manuals I have, there is no dynamic calculation available (or I am not looking at the right place…). Any guidance would be highly appreciated on how to demonstrate that. Thanks, Vincent

 

[nuuvox000]

I know this doesn't answer your question so feel free to disregard but I wouldn't mess around with undersizing the cooling for this load and tiny room at all. See below calcs with explanation, let me know if I made any mistakes.

I will go by Imperial units but you can convert them. In a room that small you have 583 cubic feet which is 44 pounds of air. Starting at 70 degrees F and 50% R.H., you would only need 300 BTU's to heat the space up to 100 degrees F! 15 kW is about 51,000 Btu/hr so it would only take a minute to reach high temperatures in the room. I know this disregards the walls and flooring and other things in the room that can absorb heat but that is still frightening to me. Just imagine 15 space heaters in a room the size of a bathroom all turned on at the same time. The fresh air supply is almost nothing so you can't count on that for much.

If it were me, I'd probably do an exhaust fan that completely changes out the air every 15 or 30 seconds. You could exhaust the air outside or you could exhaust it to a large area of the building (I'm not sure how your building is set up). You can put filters in the intake grilles if needed. Interlock your exhaust fan with the heat generating machine to make sure they start at the same time.

 

[vinhermes]

Thanks nuuvox000. I'll need to convert tomorrow.znd will try your approach. Anyway, my curious mind would have loved knowing loads in and loads out how to calcu the temper on a time scale...

 

[nuuvox000]

No problem, you can also use a remote fan coil that you can duct into and out of the room if they really want a fan coil.

 

[MintJulep]

Take a look at these two threads as a starting point.

thread403-343325

thread403-232882

Your Qx will be negative.

I suspect that you are being lied to about 10 minutes per hour. What bad things happen when they need one more minute and the thruster stops working because it got too hot?

Are you cooling the actual thruster motor, or the power electronics that drive it?

Have you considered DX?

 

[IRstuff]

Since the room's temperature is dependent on the Joule Heating, couldn't you start with the room at 15, or even 10, degC? That would buy you some headroom, and allow for some sloppiness in timing and cooling capacity.

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[vinhermes]

MintJule, I went through these posts before posting, especially the first one. I am still working on trying to adapt this. I might share my results today. Can you please explicit Qx and Dx?

IRStuff, this is one of the options. It would be acceptable to start the sequence between 15°C and 18°C for example and buy some inertia this way.

I'll keep you updated.

 

[MintJulep]

Qx was defined in the earlier threads. It is the difference between heat load and cooling capacity at any instant.

DX is Direct eXpansion refrigeration. It might be a better option for the application.

You should also remember that the equipment temperature will be higher than the room air temperature. What is the operating limit of the equipment?