[Equest-users] Changing Head Setpoint for Condenser Water Pump Head

[Doebber, Ian]

Paul

 

Thanks for your response.  Yes, the Condenser Loop Total Pressure Drop is 57' which seems very high.

 

Typical design has been :

 

1.	Adding up the Negative Factors (Vapor Pressure =1' + Friction Losses = 20' + Chemical Treatment = 6') = 27'

 

2.	Adding up the Positive Factors (Static Suction Head=1' + Atmospheric Pressure=34' + Positive Pressure = 0') = 35'

 

3.	Calculating the Net Positive Suction Head Available = Positive Factors - Negative Factors = 35' - 27' = 8'

 

Good design for a typical plant should limit the Frication Losses to 20' to 25'.

 

As long as the Net Positive Suction Head margin of safety meets minimum NPSH margin ratios.  ANSI/HI Standard 9.6.1-1998 recommends min NPSH margin ratios. For Cooling Towers the NPSHA/HPSHR = 1.3-1.5 with 2.0 being very high.

 

That said, the Baseline for ASHRAE 90.1-2004 Appendix G specified Condenser Water Pumps to meet 19 W/gpm with Single Speed Operation.  For a typical Pump Efficiency of 77%, the Pump Head needs to be 68 ft.  If you look at your pumping energy, the Condenser Water Pumps can easily consume more Energy than the Secondary Chilled Water Pump.  

 

For example on a ASHRAE Baseline New Orleans 200,000 ft2 High School the Condenser Water Pumps consumed 50% of the total pumping energy.  Note that in such a warm climate the HHW pumping requirements are negligible.  Obviously in a colder climate the HHW Pumps will significantly lower the 50%.  Yet this exercise emphasizes the importance of the Condenser Water Pumps which can contribute an annualized 0.14 kW/ton to the Total Chilled Water Plant Efficiency.  This is Significant.

 

Has anyone made an argument for Variable Speed on the Condenser Pumps?     How would the Flow Rate be controlled without fighting the control of the Cooling Tower Fans?

 

Regards

Ian

 

________________________________

From: Paul Riemer [mailto:Paul.Riemer at dunhameng.com] 
Sent: Tuesday, September 16, 2008 8:08 AM
To: 'Xavier García Casals'; Doebber, Ian
Cc: equest-users at lists.onebuilding.org
Subject: RE: [Equest-users] Changing Head Setpoint for Condenser Water Pump Head

 

Look at the head on the tower, condenser barrel of the chiller, and your loop, etc.  These make up the head your pump has to overcome.

	 

	
________________________________


	From: equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Xavier García Casals
	Sent: Tuesday, September 16, 2008 8:58 AM
	To: Doebber, Ian
	Cc: equest-users at lists.onebuilding.org
	Subject: Re: [Equest-users] Changing Head Setpoint for Condenser Water Pump Head

	Did you try the right click/restore to default, or the right click/User default options to free this input?
	
	En/na Doebber, Ian ha escrit: 

	The default Head Setpoint for the Condenser Water Pump on an Open Cooling Tower is 70 feet which is way too high!  I want to set it to 40 feet but EQuest automatically increases it to 70 ft.  

	How can I reduce the Head to 40 ft without Equest resizing it?

	
	Regards

	Ian

	
	
	
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