FMEA for HVAC System

[offshorehvac]

Hi Friends

I have been asked to check how the system will perform if 25% of the cooling capacity is lost.

The system is in design stage and the system sizing considering all units operating is as follows

Plant Configuration : 4 x 125 KW
Ambient : 44 deg C/90% RH
Installed Total Load : 500000 Watts
Coil SHR : 0.631
Coil Sensible Load : 315500 Watts
Coil Latent Load : 184500 Watts
Air Flow over the Coil: 23172 l/s
Fresh Air : 1158 l/s
Return Air: 22014 l/s

Off Coil DB : 14 deg C
RH : 90%

Supply Temp DB 15.5 deg C ( 1.5 deg C heat gain ssumed)
RH: 82.1 %

Room Actual Cond : 24 deg C
RH : 50%
RSHR : 0.92

Is there any way to check what would the room conditions if 25% capacit is lost (all others like air flow, room loads etc remain same). I have attached the Psy Chart.
Thanks

 

[IRstuff]

How are you sizing the system right now? Wouldn't you use the same process with a 25% smaller capacity?

TTFN

FAQ731-376

 

[offshorehvac]

Hi IRstuff

In the system with full load available, I know the inside conditions and can calculate the mix temperature. But when system capacity is reduced by 25%, I am not sure where the process will stop when it follows the RSHR line.
Because I can not calculate the mix temperature, I am not able to find the off coil temperature.

If I fix the room temperature at 30 deg C with 50% RH and design the system as a new system then I need about 460KW of cooling capacity. But what I am looking for is room conditions for a known cooling capacity.

 

[SAK9]

That is an interesting question!

From a fundamental perspective you can have a steady temperature in the room only when steady state is achieved ie Heat generated = Heat removed.

With 125x3 Kw your heat removal rate is 375 kw in a second.But generation rate still remains at 500Kw .So the short fall will steadily heat the room up.Temperature rise can be calculated as follows:

Enthalpy change across coil at 375 kw capacity=375x1000/1.184x23172 =13.67KJ/Kg

Enthalpy change in the room =500x1000/1.184x23172=18.22KJ/Kg

Shortfall=18.22-13.67=4.5 KJ/Kg

From the psychro chart,this equates to an increase in Dry bulb temperature of 1.5C per second.

Eventually this situation will lead to tripping of the other plants on high oil temperature.

 

[quark]

I would suggest you to validate the design conditions against the actuals if your system is already existing. There may be a great redundancy in the system design. For example, 44C and 90%RH is something like negative Utopia. If the chiller capacity is designed for peak loads then you can be safe.

 

[offshorehvac]

The system does not exist. It is in the design stage. I understand that 44C/90% RH are unrealistically high but this is what we see now a days in HVAC specifications for offshore applications. Recently I got a design basis asking us to design the system for 35C/100% RH ambient??

Anyways, For this issue I understand that the way to go is to use the psychart of the system when running at 100% . Only know factors when we have reduced capacities are:

1. Coil SHR
2. Room SHR
3. Ambient

I tried to plot few lines but there are far too many unknowns.

Other option is to recommend closing the shutoff damper at the fresh air inlet and run the plant temporarily with full recirculation. But this might not be acceptable to classification/regulatory society…

Are there any software available for HVAC which can deal with “what if scenario”

 

[AbbyNormal]

may find those ambients on Venus but not outside on the earth

44C and 90% check out that dewpoint, I do not know if the fact that there would be know way your body could shed heat that would kill you or if you would drown from all the condensation.

Lop 25% off of that ambient load and you are all set to take on the worst in the Persian Gulf.





Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[marcoh]

We use a software package called IES which can simulate scenarios like this.

http://www.iesve.com

Alternatively redo the heat load calcs and increasing the space temperature by a degree at time until the system cooling demand is 375kW. The psychrometrics will be totally different to the design condition psychrometrics