Modeling run around loop in openstudio

answered 2015-11-17 10:32:56 -0600

updated 2015-11-17 10:51:31 -0600

I've been doing this lately using an HeatExchanger:AirToAir:SensibleAndLatent (with zero latent effectiveness) on a 100% outside air loop that I can manipulate to have the relief side of the outdoor air mixer stand in for my exhaust air path. This facilitates workflow entirely in the application or with OpenStudio measures, without needing EMS (yet). The previous external calculation/EMS/plant loop method was based on the flawed assumption of constant thermal effectiveness at various air flow rates. The air-air hx allows for effectiveness to vary linearly based on airflow, which I've confirmed with some manufacturers is valid for run around loops. However, typical constant fluid flow loops will see a reduction of thermal effectiveness at turned down airflow. If your system has variable fluid flow, it's possible to see an effectiveness increase at lower flow rates. Another advantage of the Air to Air HX is it can support the effect of adding adiabatic cooling of the exhaust air with an evaporative cooler to increase cooling effectiveness.

The original work was done in OpenStudio, but using EnergyPlus measures to insert the EMS code. Neither of those two tasks are exceedingly complicated in themselves: reference the 'EnergyPlus measures' section of the measure writing guide, the Application Guide for EMS, and with some digging you can find handy examples of energyplus measures that use EMS.

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Thank you. I am probably trying to learn how to do too many complex things at once. For starters I am trying to model a single-pass 100% outside air VAV unit with DX cooling and HW reheat to serve a laboratory. I should probably have deferred my heat recovery question until after I learn how to do the basics of setting up my air system in OS with supply fan and exhaust fan. Right now I have my system hard-set for the full design flow, but since it is VAV I need to first understand how to input some kind of schedule to vary my fume exhaust fan flow rate. Any advice?

SVVitale ( 2015-11-19 13:59:02 -0600 )edit

There is definitely a lot to learn! Regarding exhaust fans: if you model your lab exhaust as a relief fan on the air loop (i.e. the fan is located on an outlet node of the OutdoorAirSystem object), then it will automatically run to balance the flow in the loop. If you model the exhaust with Fan:Zone:Exhaust objects as zone equipment, then I suggest you review the 'System Availability Manager Coupling Mode' input field description.

ericringold ( 2015-11-19 14:51:48 -0600 )edit

Thanks. So I have a variable flow fan placed on the return duct node of my vav dx air handler. I made a schedule with varying usage percentages by time of day (like an occupancy schedule) and associated that schedule to the variable flow fan. I then hard-specified maximum flow cfm and minimum flow cfm for the fan where the minimum flow cfm is the lowest value that my usage schedule would be. Is that correct?

SVVitale ( 2015-11-20 14:12:40 -0600 )edit

Possibly. See here for the placement of the relief fan. The availability schedule for that fan might be overridden based on the loop operation, which is governed by the loop control.

ericringold ( 2015-11-20 14:35:16 -0600 )edit

I placed the fan as shown in the first screenshot of the two. You suggested the relief position so I will change it to the second position. My simulation is converging. One difficulty I'm having is detecting if I'm getting reasonable results. Par for the course, I'm sure.😀

SVVitale ( 2015-11-20 18:55:24 -0600 )edit

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