VAV system with cfm flow charge out
The clients wanted to do charge out rate to the tenant for air condition used.
The problem arises when One unit AHU is supplying to 4 different Tenants per floor.
I have been told to use the VAV air flow to do CFM totalizer eventually to do charge out rate.
However, I don't agree with the concept using VAV to do charge out rate since the VAV does not act independent as fcu or vrv system.
I know there is kw/hour, btu/hour but i never heard of cfm/hour.
Would appreciate anyone can share some thoughts on this issue or can direct me to the explanation website/page since i am in controls system.
The VAV boxes proportionally take air from the air handler that was cooled either by DX or chilled water. The only way to measure the amount of air delivered to a given zone is via the flow rings in each VAV box (converting FPM to CFM).
i think your greater challenge is determining what an equivalent rate for CFM/hour might be as compared to a chilled water meter (gallons consumed). The air handler blower requires energy, along with the VFD, and whatever means employed to chill the AHU coil. And you're dividing that among four tenants, each of whom may have more than one VAV box per tenant space. CFM totalization seems your only route...figuring out a rate to charge...maybe your client just wants you to set up the CFM/hour totalization on the front end and let them worry about what to charge. If so, I would do as they ask and let them worry about the money angle.
Building Physics Rule #1: Hot flows to cold.
Building Physics Rule #2: Higher air pressure moves toward lower air pressure
Building Physics Rule #3: Higher moisture concentration moves toward lower moisture concentration.
If I understand you correctly, they want to charge the tennant $ for CFM delivered.
This can be calculated (CFM turned into BTU's or KW x $), but I can see some serious problems down the road, if you have VAV problems (and you will).
We have a controls system where the building areas, Chillers, AHU etc. have power meters on them. The client is charged for the %of energy used according to his floor space. Because of lease contracts this is in the implecation process only, but in time this will be the standard way of billing.
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If they are trying to proportionally split the costs among tenants that utilize the same AHU, then the VAV flow is the only way to split it up. You still ahve to have some metering on the AHU to meter how much energy IT is using, then you take that total energy use, and split it by the same proportions of the total CF delivered to each zone over the time period.
If I were the tenant I would ask that my min cfm is set to zero. Do these vav's have any form of reheat? Just kidding about the zero cfm min. I think a supply airflow station should be utilized to check the accuracy of vav cfms. Most systems now have trending capabilities so trending of each vav in groups per tenant would be how I approach it. Trending of suplly air cfm would indicate totals to check tenant totals against.
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