RE: [EnergyPlus_Support] how can I change the HOT WATER PLANT HW SUPPLY PUMP power in a HVAC Template System VAV

Dear Richard,

I’m missing something…. Where does “%NomPowerUse” come from?

To make sure we have the same understanding:

Pump power = Flow * DeltaP * ConversionFactor / Mechanical Efficiency / Motor Efficiency

You could also say:

Pump power = Flow * DeltaP * ConversionFactor / TotalEffic

Combining these efficiencies is essentially what you show:

PumpEffic = TotalEffic / MotorEffic

Reformulating the 2^nd equation above gives:

TotalEffic = Flow * DeltaP * ConversionFactor / Pump power

This basically matches your formula of:

TotalEffic = FlowRate * PumpHead / NomPowerUse

Finally the question! EnergyPlus does not know the pump power use (when autosizing for example), so how can this equation be calculated without assuming something for pump efficiency?

I hope I’m not missing something obvious! (but I appear to be missing something!)

The Building Performance Team
James V. Dirkes II, P.E., BEMP , LEED AP
1631 Acacia Drive NW
Grand Rapids, MI 49504
616 450 8653

From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of Richard Raustad
Sent: Thursday, June 16, 2011 11:31 AM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: Re: [EnergyPlus_Support] how can I change the HOT WATER PLANT HW SUPPLY PUMP power in a HVAC Template System VAV

No. In the pump, motor efficiency is motor efficiency. Total eff is calculated as I showed. And in the fan, the fan efficiency input = (motor eff x fan eff) and motor eff is also an input. The fan and pump code was written by two different individuals long, long ago.

Your confusion may lie in the fact that the fan input field named Fan Efficiency might be better named Total Efficiency (power out divided by electrical power in). The phase "The fan efficiency is power..." in the description actually refers to the fan mechanical efficiency (power out divided by shaft power in).

Field: Fan Efficiency

The ratio of the power delivered to the fluid to the electrical input power. It is the product of the motor efficiency and the fan efficiency. The motor efficiency is the power delivered to the shaft divided by the electrical power input to the motor. The fan efficiency is power delivered to the fluid (air) divided by the shaft power. The power delivered to the fluid is the mass flow rate of the air multiplied by the pressure rise divided by the air density. Must be between 0 and 1.

On 6/16/2011 9:12 AM, Jim Dirkes wrote:

Well……

In order to calculate pump power, you must know flow, head, motor efficiency AND pump (mechanical) efficiency. Since there is no input field for pump efficiency in E+, I looked into this a while back and the documentation indicated that the pump efficiency was a constant 78%. Since you are indicating otherwise, I checked again and see that things appear to have changed!

So, is it the case that pumps now follow the E+ fan convention and use a combined (motor efficiency * pump mechanical efficiency) as the “Motor” efficiency input field?

p.s., I always thought that this was confusing, since it’s not a normal engineering convention and the field label clearly says “Motor efficiency”. It’s workable, just confusing J.

The Building Performance Team
James V. Dirkes II, P.E., BEMP , LEED AP
1631 Acacia Drive NW
Grand Rapids, MI 49504
616 450 8653

From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of Richard Raustad
Sent: Thursday, June 16, 2011 8:56 AM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: Re: [EnergyPlus_Support] how can I change the HOT WATER PLANT HW SUPPLY PUMP power in a HVAC Template System VAV

The pump efficiency is not actually used in the calculation but it is calculated internally to provide warnings for the user. The Engineering Reference shows the pertinent calculations for the pump (search for "Variable Speed Pump" or "Constant Speed Pump").

The calculation for pump efficiency is:

TotalEffic = PumpEquip(PumpNum)%NomVolFlowRate * PumpEquip(PumpNum)%NomPumpHead / PumpEquip(PumpNum)%NomPowerUse

PumpEquip(PumpNum)%PumpEffic = TotalEffic / PumpEquip(PumpNum)%MotorEffic

On 6/15/2011 7:31 PM, Jim Dirkes wrote:

.. and E+ always assumes pump efficiency to be a constant value (78% I think.)

The Building Performance Team
James V. Dirkes II, P.E., BEMP , LEED AP
1631 Acacia Drive NW
Grand Rapids, MI 49504
616 450 8653

From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of Jason Kirkpatrick
Sent: Wednesday, June 15, 2011 6:57 PM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: Re: [EnergyPlus_Support] how can I change the HOT WATER PLANT HW SUPPLY PUMP power in a HVAC Template System VAV

Jose,

You can modify the pump power by changing the pump rated head in the HVACTemplate:Plant objects that you need to include to create the chilled water, hot water, and condenser water loops.

Pump Electric Power = Pump Volume Flow Rate x Pump Head / Total Pump Efficiency

Refer to the EngineeringReference.pdf documentation that came with your install of energyplus for more detail.

Alternatively, once you have an expanded input data file that works, you could directly input the rated power consumption into the pump object and autosize the pump head.

In order to accomplish this you will need to rename the file that end with .expidf to a file with name .idf and then you can edit and run the expanded idf like any other input file.

Hope this is of help.

Jason
2011/6/14 José Ernesto Castillo Peralta <joercape@xxxxxxxxx>

Hi,
how can I change the power of the HOT WATER PLANT HW SUPPLY PUMP, CHILLED WATER PLANT CHW SUPPLY PUMP and CHILLED WATER PLANT CNDW SUPPLY PUMP in a HVAC Template System VAV

--

Jose Ernesto Castillo Peralta

--

Richard A. Raustad

Senior Research Engineer

Florida Solar Energy Center

University of Central Florida

1679 Clearlake Road

Cocoa, FL  32922-5703

Phone:   (321) 638-1454

Fax:     (321) 638-1439 or 1010

Visit our web site at: http://www.fsec.ucf.edu

UCF - From Promise to Prominence: Celebrating 40 Years

--

Richard A. Raustad

Senior Research Engineer

Florida Solar Energy Center

University of Central Florida

1679 Clearlake Road

Cocoa, FL  32922-5703

Phone:   (321) 638-1454

Fax:     (321) 638-1439 or 1010

Visit our web site at: http://www.fsec.ucf.edu

UCF - From Promise to Prominence: Celebrating 40 Years

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