Cooling Tower Staging
Cooling Tower Staging
(OP)
Is it normal to have chilled water plants that do not stage up a 2nd cooling tower before a 2nd chiller?
I would expect that having two cooling towers (2 towers 1 pump = 1/2 flow to each tower) at the same flow rate would increase the latent heat rejection to atmosphere and prolong the stage up of a 2nd chiller.
I would expect that having two cooling towers (2 towers 1 pump = 1/2 flow to each tower) at the same flow rate would increase the latent heat rejection to atmosphere and prolong the stage up of a 2nd chiller.





RE: Cooling Tower Staging
The setpoint itself could be reset by outdoor wet bulb, as is the case in certain plant optimization strategies. I like to see towers operated that keep centrifugal chillers comfortably away from their surge envelopes, personally.
RE: Cooling Tower Staging
Why do you try to prevent the chiller from surging? I'm still in the learning process and wish to know more about chiller operations.
RE: Cooling Tower Staging
Chiller surge is bad. A centrifugal compressor is a non-positive displacement pump. If the pressure difference becomes too high between the condenser and the cooler (evaporator), gas flow reverses across the impeller, at the same time attempting to stall the impeller. This causes a huge jump in compressor motor amps and also strains the drivetrain of the motor to impeller power transfer. On the higher efficiency chillers, tolerances between the impeller and its surrounding enclosure is very small; unchecked surging can lead to increased bearing play and gear lash, which in turn can lead to the impeller striking a stationary surface. Not good!
Many chillers of more recent vintage have surge prevention algorithms. I've set mine a bit conservative so it will go into surge prevention mode quicker when operating conditions deteriorate to that point. Conditions favorable for surge to occur is high entering condenser water temperature, not to mention clogged tower and condenser water pump strainers, fouled tubes due to scaling and/or debris, or a fouled tower. Good water chemical treatment and annual punching of condenser tubes are vital to keep a chiller healthy and away from surge territory, along with good tower management.
RE: Cooling Tower Staging
RE: Cooling Tower Staging
Either way, sounds like it's time for me to speak with my Trane rep/engineers for some more information on both parts of the system.
RE: Cooling Tower Staging
This chiller also has a way to detect how many times the compressor surges, in case the prevention algorithm can't react strongly enough to prevent a surge. Every time a surge occurs there is also a concurrent surge of amp draw, which the computer monitors via current sensors on the main power feed to the compressor. When a preset count is exceeded, the chiller trips and the compressor stops.
Additionally, one can select a "demand limit", which limits how high the chiller can be loaded. This is measured in percentage of FLA.
I'm curious as to how what I've described above compares to how different brands of chillers, such as Trane in your case, control to prevent surging from happening.
RE: Cooling Tower Staging
My own towers are upflow forced draft and don't have that problem, but do have an overflow issue if the tower staging and floats are not set correctly. This is because an equalizer line separate from the pump suction was not installed when the towers were new. We plan to change that sad fact at some point.