Changing Head Setpoint for Condenser Water Pump Head

5 posts / 0 new
Last post

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

Doebber, Ian's picture
Offline
Joined: 2011-09-30
Reputation: 0

Did you try the right click/restore to default, or the right click/User
default options to free this input?

*Xavier Garc?a Casals*

=?ISO-8859-1?Q?Xavier_Garc=EDa_Casals?='s picture
Joined: 2011-09-30
Reputation: 0

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.

Paul Riemer's picture
Offline
Joined: 2011-09-30
Reputation: 0

Xavier, thanks. You were correct.

One odd item was then when I specify the User default option to 40', then it changes all the Defaults of the Pumps. Only when I closed EQuest and reopened it did the 40' only apply to CW pumps. Very Strange.

Doebber, Ian's picture
Offline
Joined: 2011-09-30
Reputation: 0

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

Doebber, Ian's picture
Offline
Joined: 2011-09-30
Reputation: 0