is it possible to calculate the return air temperature in the system if no heat gains and losses are taken into account (except for temperature rise due to the fan) knowing the supply air temperature, temperatures of different ventilation zones, airflow, and outdoor temperature?
HVAC ducts are not designed for heat transfer. They may even be insulated to minimize heat loss/gain. Compared to an exchanger specifically designed to transfer heat, ducts have a lot of conveyance area and little transfer area by proportion; yet, the same principles hold. You can still calculate the heat transfer from Q=ΣUA⌂T. Area is easy enough to calculate. You can estimate U (use low end of typical range values). The biggest challenge will be the relatively large length scales over which the temperatures vary. Heat transfer (in or out) will impact the temperature near the walls, not in the core of the flow. Assuming temperatures are evenly distributed across the flow would be quite inaccurate. Assuming a single temperature for air flowing in a 2 cm diameter tube is a reasonable assumption, but not if it's flowing in a 20 cm duct. Unlike a shell-and-tube heat exchanger, where there are some entrance and exit effects but mostly developed flow, you shouldn't consider anywhere to be "fully developed" in a typical HVAC system such as I think you're suggesting. I can't image this would be worth an elaborate CFD model. I also doubt there is a general solution which you might apply to any system, making this a configuration-specific problem.
Dudley J Benton Thanks for your detailed answer. I am working with the standard EN 16798-5 (Calculation methods for energy requirements of ventilation and air conditioning systems) where it is suggested to calculate heat losses and gains per section of duct using air leakage, temperature difference, and properties of air. However, as it is not possible for me to assume a number of duct sections, I can not take it into account. Moreover, I need delta T to calculate everything per standard but a method of calculation is not given in the standard and I can't seem to find a way that doesn't involve on-site measurements and hence my question here asking if it is possible to use some kind of a formula/ assumption to get the delta T between the supply and return air temperature. Thanks again for your answer.
Dear Nadeen,
A high return temperature means that the temperature difference across the network ( TFLOW – TRETURN ) will decrease. The biggest difference between supply and return vents is the direction in which the air flows. In a supply vent, the air flows out of the ductwork. In a return vent, the air flows into the ductwork.
Mounting a thermostat near a return won't cause any issues because the air returning to the furnace is at room temperature. The return duct should be positioned near the floor or ceiling, which will provide several feet between the thermostat and the moving air.
CFM outside air divided by CFM of supply air; subtract the percent of outside air from each return air grille airflow in the system to find the required adjusted return airflow.
Ashish
Without site-specific dimensions and conditions, anything you come up with will necessarily be a guess and no more accurate than your assumptions. This would be true for any problem, not just HVAC. In cases like this (no meaningful information available), there are tables of yearly average temperature by region and expected air conditioning/heating loads by building type. There's a chapter in the ASHRAE Handbook of Fundamentals containing such information, though I've never used any of it.
Dear Dudley J Benton,
Thank you very much for the very clear and seemingly logical answer and approach to the issue that is being discussed. It is not my subject but I have managed to learn from the different views given by the various researchers in this field.
Best Regards
Desalegn Abraha
Dear Nadeem Hassam,
How to calculate the return air temperature in a system? is a very interesting question. However, as it is not in the field of my qualification I can not make any contribution in the dicsscussion. However, I will follow the discussion in order to be able to understand what it is and how it functions.
Thank you very much for the very interesting and timely issue that you have taken up for discussion.
Best Regards
Desalegn Abraha
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