Staging Chillers on Return Water Temperature - It Doesn't Work

I've had this idea in my my head for a few years now.
That you can not stage variable primary pumping chillers on return water temperature.
Well, you can stage on return water temperature if energy efficiency is not a priority.
Whether you believe it or not, there are few interesting points to consider here.
Chiller Minimum Flow Control: • Chiller Minimum Flow C...
Chiller Bypass Control: • Chiller Bypass Control
Chiller Evaporator Pressure Sensor Selection: • Chiller Evaporator Pre...
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[@rodkington7996]

Great content again, yes I reckon you are correct, in a variable flow chilled water loop the temperature of the return water is not an effective way to control chiller staging.

[@xhajjar]

In a variable primary system, you have two ways to stage the chillers. 1- building load (BTU meter) 2- chillers load (FLA% or actual ampere)

[@janbijkersma1484]

True. Neither return water temperature alone, nor excess deficit water flow is sufficient. Since the 'thermal capacity rate' = 'mass flowrate' * 'specific heat capacity' * 'temperature'. Then the other problem is that flowrate and temperature can vary faster than the chiller / plant can tolerate.

[@janbijkersma1484]

To steady the thermal variation in time, the water is first to be buffered in a vertical stratification tank. With water entering at the bottom, and leaving at the top. The tank size should be sufficient to steady the temperature & thermal capacity. The temperature sensor to be situated in the stratified zone, or top of the tank.

[@matthewbeatty7798]

Love this

[@bryceanderson17]

Thanks Matthew.

[@tonydavison9821]

As a BMS/HVAC tech. I've been arguing this very issue with design & software engineers for 20 years. Most of them just don't get it.

[@bryceanderson17]

Yeah, it's a bit left field. I was pretty worried when I posted this video as it really goes against what everyone knows. And as you say, it's not exactly simple to understand. Thanks for sharing, it's good to get some confirmation that other people have a similar opinion :-)

[@123banop321]

Been trying to convince people at work to ditch this from the standard spec for years. Hard to find a good alternative as everyone is set in there ways.

[@masterchef6694]

How about Delta T as a source of current/real time load estimate, lets's say design delta T is 5 (i.e. 7 & 12) Celsius therefore 1 degree (.i.e 7 & 11) celsius drop in delta T is a 20% reduction in plant load....2 degree (.i.e 7 & 10) is a 40% drop in plant load, therefore at this point certainly a chiller & its corresponding primary pump needs to be turned off to match plant load. Would do think about this method?

[@osamamashal2602]

What if I control the staging based on the differential pressure transmitter of the load that I'm using to control pumps speed? Just an idea came up to my mind

[@bryceanderson17]

We don't do this, but I guess it may work in some situations, as a secondary condition. In Australia we reset the system pressure set point depending on the load. So, in high load conditions we reset the pressure set point up to design, say 250kPa. in low load, we reset the pressure set point down to say 125kPa. This squeezes out a little more energy. However, relevant to what you are saying, this means that the system pressure may not be a good indication of the load as we are continually changing it.