joesinger12
commented
1 year ago - Milestone: ComDevIssue --> CBECC2022DevIssues
Original comment by: joesinger12
Open joesinger12 opened 5 years ago
joesinger12
commented
1 year ago Original comment by: joesinger12
Based on [Issue 2929] (https://sourceforge.net/p/cbecc-com/tickets/2929/)
Here is the brief summary of what I found in 2929
The problem encountered by the CBECC user • Two identical models other than that one uses FPFC and one uses HP. The fan power in the FPFC model was 50% higher than the HP model
The cause of the difference • The supply air in the FPFC model is significantly higher than the HP, that the required airflow is more to meet the same load.
Why the FPFC supply air temperature higher than expected despite having a large cooling coil? • We found the air and water delta T correlate to the flow rate in FPFC. The higher flow rate results in lower delta T. • The delta T is also governed by the coil UA. The higher UA gives higher delta T. • UA is determined by coil capacity (which is calculated by air flow rate, inlet and outlet air temperature) and water flow rate, per E+ engineering reference. • The higher air flow rate and air delta T increase UA, but a higher water flow rate decrease UA. • For an unknown reason, E+ doesn't use user input air flow rate to calculate coil capacity. • E+ auto sizes coil capacity with an unknown air flow rate and 20F delta T. ( I tried hard input design inlet and outlet air temperatures in E+) • If the water flow rate is too high (as we found in this case), E+ under sizes coil UA. • The undersized coil UA yields very small delta Ts, and hence high supply air temperature.
What can the CBECC user do? • Reduce the capacity of the system to better match the zone loads. As mentioned above, E+ does not respect the user input air flow and dT when calculating UA. Therefore, if the user air flow and water flow rate is closer to the required load. The model air flow and capacity reflecting close to 1 cfm/ft2 and 400 cfm/ton, which is too large of a system for the given zone loads using a lower value, like 0.5 cfm/ft2 and 400 cfm/ton is more reasonable.
What can we do? • We did find that the defaulting rules for cooling coils needs some attention, mainly a circularity in the rule for capacity, and that if CaptTotNetRtd is input for DX coil, but then switched a chilled water coil to input gross capacity, the defaulting of the water flow rate does not reflect the change. • We should inquire with E+ team or check code to see what the assumed air flow rate is for coil UA, and we may be able to adjust our rules or translation to compensate for this.
The Result of reducing capacity • After reduced the size according to "Auto-Hardsize Parameters", 0.5 cfm/ft2 and 400 cfm/ton, the minimum supply air temperature drops to 62F, and the delta T increases 4 F to 16F. • The water temperature difference is also increased. • The interior fans energy drops from 2907 kBtu/yr to 2067 kBtu/yr. Still higher than 1719 kBtu/yr from the HP model, but closer. • The difference is due to the different coil characteristics of FPFC and HP
Reported by: joesinger12
Original Ticket: cbecc-com/tickets/2967