How to model the pumps according to ASHRAE 90.1/2010?

asked 2021-01-22 08:38:05 -0500

Nathalia gravatar image

updated 2021-01-22 09:33:03 -0500


I'm simulating a 10-story office for LEED ID+C certification ID+C. I'm usind System 8 for the Baseline and according to ASHRAE 90.1/2010, the building have less than 300 tons. So the number and type of chiller will be "1 water-coolled screw chiller. In addition, according to ASHRAE: "Chilled-water pumps in systems serving less than 300 tons cooling capacity shall be modeled as a primary/secondary systems with secondary pump riding the pump curve".

My question is how to create in OpenStudio a "1 water-coolled screw chiller with a primary / secondary pump systems with secondary pump riding the pump curve. I am unable to find an explanation of what this "secondary pump riding the pump curve and screw chiller" would be. Can anybody help me.


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As far as I know, riding the pump curve simply means the pump operates at constant speed. If you know what pump make and model you have, you can plot its head rise against volume flow rate and its mechanical efficiency against flow rate. For example, head rise may vary non-linearly from 80 ft at 0 gpm to 0 ft at 500 gpm and mechanical efficiency should vary non-linearly from 0 % at 0 gpm to (also) 0 % at 500 gpm, with a peak somewhere in between, say 60% at 300 gpm. The design (rating) point should be near those 60%, where the head rise then may be 60 ft.

mattkoch gravatar image mattkoch  ( 2021-01-24 16:36:21 -0500 )edit

The "riding" is dictated by the simulation. At any given hour, the pump will need to provide a certain flow, usually different from the above 300 gpm. For example, if the required flow is 250 gpm, the head rise will be more than the above 60 ft (riding up the pump curve to the left), if it is 350 gpm, the head rise will be less than those 60 ft (riding down the curve to the right), and mechanical efficiency is typically less than at 300 gpm also. At any point, the required mechanical power is then simply the product of flow, head rise and inverse of mechanical efficiency - in suitable units.

mattkoch gravatar image mattkoch  ( 2021-01-24 16:42:01 -0500 )edit

And mechanical power multiplied by the inverse of electrical efficiency (motor efficiency) - in suitable units - then is the electrical power recorded for the pump by EnergyPlus for energy modeling purposes.I believe in the HVAC plant loop, if you simply add a constant speed pump, it will assume a "typical" pump curve and do what is needed. However, it may also be that you need to use a variable speed pump and force it somehow to operate at 100% speed, because for that, you can specify actual pump curve parameters.

mattkoch gravatar image mattkoch  ( 2021-01-24 16:46:04 -0500 )edit

This here may also help.

mattkoch gravatar image mattkoch  ( 2021-01-24 20:16:27 -0500 )edit

To be certain, the pump curve used for a variable pump in OpenStudio is not head rise or efficiency vs. flow rate, but rather mechanical power to rated mechanical power vs. flow rate to rated flow rate. So it is head rise and mechanical efficiency already combined into mechanical power. That is all EnergyPlus cares about, unless you do a complete simulation (not usually done for ASHRAE 90.1).

mattkoch gravatar image mattkoch  ( 2021-01-24 20:29:05 -0500 )edit