Cooling Tower Fan & Pump Speed Calculation

[pop29684 14]

I've been handed a project that includes writing code to control the fan and pump speeds in a process water cooling tower. I will know 1) the outside air temperature (reference), 2) the temperature of the incoming return process water at the tower, and 3) the temperature of the outgoing process water supply at the tower. The customer has specified a linear relationship between TWST and velocity of the tower fan and pump speeds. They also want both the tower fan and tower pump speeds to match each other. I have two questions: 1) what other data might be necessary to consider, and 2) what would the algorithm look like? The VFDs will be driven by a 0 - 10V analog signal. This seems to me that a percent of speed would be easier to implement. I'm guessing that the setpoint of TWST needs to be subtracted from the reference OAT (if OAT is higher than TWST-setpoint), and this difference should be manipulated into a percent of the 10V analog signal to drive the VFD. Is there an easier direction that can be recommended? Thanks!

[pturmel 160]

12 minutes ago, pop29684 said:

customer has specified a linear relationship between TWST and velocity of the tower fan and pump speeds

Seems odd.  Fans and centrifugal pumps have higher-order relationships between rotor speed and product velocity.  If they want the latter to be linear, then use a square root or cube root function to drive the motor speed. (I can never remember which.)

[chelton 66]

Not sure why you would want the fan and pump speeds to match? It's not uncommon for cooling tower fans to be just idling over moving enough air to maintain temp. A centrifugal pump running this slow wouldn't be operating within its operating range.