14 Replies Latest reply on May 12, 2015 6:49 PM by Kamil Zyskowski

    Co2 concentration

    Pradip Aryal

      As we know, most HVAC system recirculates indoor conditioned air to save energy which may lead to higher Co2 concentration. In a steady-state analysis, I want to define volume fraction of Co2 at supply inlet as a function on indoor Co2 concentration, which increases over time due to occupants breathing.

      If anyone could help me up with defining dependency boundary condition, I would be very thankful.

      Regards,

        • Re: Co2 concentration
          Bill McEachern

          why do you need a dependency? Why not just put a source emitting CO2 at the emission rate . You need a place for the pressure to equalize but that must exist in the real system (bldg. leaks or a device).

          • Re: Co2 concentration
            Jared Conway

            won't your method create an ever increasing level of C02 if they are dependent on each other?

             

            or are you thinking that your system should cause it to level out at something? in that case, why not use bill's suggestion of just having the source?

              • Re: Co2 concentration
                Pradip Aryal

                Well, the concept is supply inlet of conditioned air will be recirculating with same concentration of Co2 as room, but to neutralize the ever increasing level of Co2 I have different inlet for fresh ventilating air and of-course pressure outlet to equalize the pressure. So I am assuming at some point the concentration at the room and at the supply should level out.

                  • Re: Co2 concentration
                    Jared Conway

                    I think a diagram here might be helpful.

                     

                    But again, if this is a steady state solution, I don't see how the inlet can be dependent.

                      • Re: Co2 concentration
                        Pradip Aryal

                        I have tried to come up with a simple model but with the same idea. There is ventilating fan that sends in outdoor fresh air (Co2 concentration:350 ppm). There is Co2 generating source (Sleeping human). So this 4 way-cassette air conditioner sucks in warm air at the middle and cools it off and discharge it back to the room. In reality, the discharged conditioned air has same Co2 concentration as the air that gets sucked in. Now the problem is how do I define Co2 concentration of cool conditioning air at discharge inlet?

                        Regards

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                          • Re: Co2 concentration
                            Bill McEachern

                            I am probably missing something here but I wold put an inlet on the human that discharges the CO2 input and just run the fans and what not and sine they have a certain circulation some steady state value of CO2 will remain n the room. Further you could define multiple species of air and CO2 at all the inlets and pressure BC's so that they reflect the 350ppm and turn it on and see what it settles out at for the room average concentration. That would likely be the a global goal on CO2. But given the simplicity of doing that I must not understand something I suspect.

                              • Re: Co2 concentration
                                Pradip Aryal

                                Greetings,

                                Yes, I have defined multiple species of air and Co2. The incoming outdoor fresh air (concentration by volume fraction: Co2: 0.00035, air: 0.99965), inlet at human (concentration: Co2:1 , air: 0). But when it comes to defining concentration at the air conditioning inlet, I am afraid I can't define Co2 concentration of 0.00035 (350 PPM), I believe it is higher than 350 PPM because in reality discharged air from air conditioning inlet is not outdoor fresh air, it is just the re-circulation of indoor air having higher co2 concentration. Concentration at the pressure outlet isn't important because the mixture of air is flowing outside of the room.

                                well, I know in simulation, we can't make air to re-circulate like in reality. we can only define the boundary condition at the desired inlet. So, If in some way I could define the concentration of co2 at the air conditioning inlet equal to the one at the pressure outlet where the mixture of air is exiting the room, I believe that would give me the desired result.

                                Thanks Bill,

                                Have a nice day!!!

                              • Re: Co2 concentration
                                Jared Conway

                                sounds like a fit for an internal fan instead of trying to get fancy with the dependency

                                  • Re: Co2 concentration
                                    Kamil Zyskowski

                                    Hi,

                                     

                                    first of all boundary conditions looks not well - I mean Enviroment pressure on HVAC unit shouldn't be there - there must be some "underpressure" and in the same room some extractor should be used to remove "used" air - where? it's a task for cfd :-)

                                     

                                    Jared, you suggest to use internal fan - it's ok but please consider that HVAC system changes parameters of air (T and Hr) and, as I know and try, it's not so easy with Fan feature.

                                     

                                    Ok, we can put there simple f(t) chart but its just rough estimation... or maybe I'm wrong.

                                     

                                    I'd like to use internal fan but for T @ Thermodynamic Parameters I'd like to have it in function of cooling capacity and inlet air parameters - like in real life.

                                     

                                    Ciao!

                                      • Re: Co2 concentration
                                        Jared Conway

                                        sounds like a great enhancement

                                         

                                        but today you're limited to what is in the software

                                         

                                        if it is critical to create a transient simulation which can adapt to the inlet temp, you're going to have to build the whole air handling unit in flow to behave the same rather than "virtualize" it

                                        • Re: Co2 concentration
                                          Pradip Aryal

                                          Well, Thanks!

                                          Flow simulation need at least one pressure opening to equalize pressure and to run the simulation. Ideally, there should be an extract grilles with low pressure sucking in the "used" air. But in flow simulation, volumetric flow rate has to be conserved. So whatever the volume flow rate is going into the room, same volumetric flow has to exit the room from some opening. So the boundary condition Environment is basically a pressure opening to equalize the pressure and conserve the volume flow rate.

                                           

                                          And yes, about using Fan. At first. I was also concerned about changing the air parameters of conditioned air (T and RH). I am not familiar with internal fan feature. I will follow your suggestion and see if I can set temperature and RH as a function of inlet air parameters.

                                          Regards,

                                            • Re: Co2 concentration
                                              Kamil Zyskowski

                                              Ok, I agree but in this case env pressure opening shouldn't be in HVAC unit, for sure. Fresh air should be delivered untreated or delivered by HVAC unit. I'd add to your model a hole with Enviromental pressure (as I wrote - place should be found by simulation)

                                               

                                              Wish you good luck!