Home » Posts » Chiller Plant Efficiency Series Chiller Efficiency – Part 2

Chiller Plant Efficiency Series Chiller Efficiency – Part 2

We move to the variable that dominates centrifugal chiller electrical efficiency: Compressor lift. If there is one concept every operator should understand about chiller performance, it is this: Lift drives compressor kW/ton. But as we will see, reducing lift is powerful — and dangerous if misunderstood.

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Lift, Mass Flow, and Why Lower Isn’t Always Better

In Article 5, we discussed evaporator heat transfer and the relative importance of ΔT, flow, and fouling.

Now we move to the variable that dominates centrifugal chiller electrical efficiency:

Compressor lift.

If there is one concept every operator should understand about chiller performance, it is this:

Lift drives compressor kW/ton.

But as we will see, reducing lift is powerful — and dangerous if misunderstood.

What Is Lift?

In a centrifugal chiller, lift is the pressure difference between:

Evaporator refrigerant saturation pressure
Condenser refrigerant saturation pressure

Or more simply:

Lift = Condenser Pressure – Evaporator Pressure

In temperature terms:

Evaporator saturation temperature is set by chilled water temperature.
Condenser saturation temperature is set by condenser water temperature.

Lower condenser water temperature → lower condenser pressure → lower lift. Higher chilled water temperature → higher evaporator pressure → lower lift.

Lower lift means the compressor does less work to move refrigerant vapor from suction to discharge.

That is why lift is so important.

Why Does Lower Lift Improve kW/Ton So Much?

Centrifugal compressors are highly sensitive to compression ratio.

As lift decreases:

Compression ratio decreases (condenser pressure – evaporator pressure)
Compressor can move more refrigerant for any compressor speed
Required work (Power) per pound of refrigerant decreases

Compressor efficiency improves dramatically.

This is why even small reductions in condenser water temperature often produce noticeable improvements in kW/ton.

But to understand this fully, we must look at capacity.

Capacity = Refrigerant Mass Flow × Δh

Evaporator capacity is:

Q = ṁ × Δh

Where:

ṁ = refrigerant mass flow rate
Δh = enthalpy change across the evaporator

Now here is where things get interesting.

When lift decreases:

Compressor resistance to refrigerant flow decreases
Refrigerant mass flow rate increases
Compressor moves more vapor per unit time

But what happens to Δh?

Why Δh Does Not Change as Much as You Might Expect

In a flooded evaporator:

Refrigerant boils at nearly constant saturation temperature.
Evaporator Δh is mostly defined by the latent heat of vaporization.

When lift changes slightly:

Evaporator saturation temperature typically remains controlled by chilled water requirements.
Latent heat does not change dramatically over small temperature ranges.

So:

Mass flow increases significantly with lower lift.
Δh changes only modestly.

That is why:

Capacity is far more sensitive to changes in mass flow than to changes in Δh.

However — and this is important — The relationship between capacity and lift is not perfectly linear.

Why?

Because:

Δh shifts slightly with evaporating temperature
Compressor volumetric efficiency shifts
Refrigerant density shifts

So capacity and lift are strongly related — but not strictly proportional.

*Conceptual Relationship Between Refrigerant Mass Flow Rate and Lift*

An AHA Moment About Lift and Capacity

Here is something that surprises almost every operator:

We have seen 1,000-ton centrifugal chillers producing 1,300 to 1,400 tons under low-lift conditions.

If you ask most operators whether a 1,000-ton chiller can produce 1,400 tons, the intuitive answer is:

“No. A 1,000-ton machine is a 1,000-ton machine.”

But centrifugal chillers are not fixed-displacement machines. Their capacity is not mechanically limited to a single number.

When lift is reduced:

Compression ratio drops
Refrigerant mass flow rate increases significantly
Compressor work per pound decreases

Since capacity equals refrigerant mass flow × Δh, and Δh does not change dramatically, the large increase in mass flow can temporarily push output well above nameplate rating.

Of course, this is limited by:

Motor amps
Surge limits
Manufacturer operating envelope

But within safe boundaries, lower lift can increase both efficiency and available capacity.

That’s the power of lift.

Now the Critical Question

If lower lift improves compressor kW/ton so much…

Why not always chase the lowest possible condenser water temperature?

Because Lift Is Not Reduced for Free

To reduce lift, we usually:

Increase cooling tower fan speed
Increase airflow
Reduce condenser water temperature

But fan power increases roughly with the cube of speed.

That means:

Lowering condenser water temperature reduces compressor kW
But increasing airflow increases tower kW rapidly

There comes a point where:

Additional lift reduction produces small compressor gains
But tower fan power increases significantly

This is exactly what we learned in Article 4.

The Plant-Level Reality

At the plant level:

Total kW = Compressor kW + Tower Fan kW + Pumps kW

Lower lift reduces compressor kW.

But excessive pursuit of minimum condenser water temperature can increase tower kW enough to offset those gains.

Therefore:

Minimum lift does not necessarily equal minimum plant kW/ton.

There exists an operating point where:

Lift is reasonably low
Fan power is reasonable
Combined kW is minimized

That is the real target.

How This Connects to Article 5

In Article 5, we learned:

ΔT (water-side driving force) is more potent than flow.
Artificially lowering evaporator temperature increases lift.
Increasing condenser temperature increases lift.

This reinforces a key rule:

ΔT improvements must come from the water side — not by forcing lower saturation temperatures.

Because forcing saturation temperatures directly increases lift and hurts compressor efficiency.

The Integrated View

By now we understand that:

Entering Condenser Water Flow and Temperature are intrinsically related to Cooling Tower Efficiency, Chiller Lift (and Efficiency), Condenser Heat Transfer Efficiency and Condenser Pumps Efficiency.
Likewise Leaving Chilled Water Temperature and Flow are intrinsically related to Chiller Lift (and Efficiency), Evaporator Heat Transfer Efficiency and Chilled Water Pumps Efficiency.

These are the CONTROLLABLE variables that affect the entire chiller plant efficiency.

You cannot optimize the cooling tower in isolation. You cannot optimize the evaporator in isolation. You cannot optimize the compressor in isolation.

They are mechanically and thermodynamically linked.

It is important to mention that equipment maintenance and water treatment quality can outweight the effect of controlling the above mentioned variables.

The Summary

Lower lift significantly improves compressor efficiency but at a cost.

The Bigger Picture

At this stage of the series, we have identified the primary levers:

Airflow (tower fans)
Water flow (CHW and CW pumps)
Lift (saturation pressures)
ΔT (water-side driving force)

These variables move continuously with:

Wet-bulb temperature
Load
Fouling
Time of day
Tower condition

Which means:

The optimal operating point is forever moving.

And that is precisely why static setpoints struggle to maintain peak efficiency.

What Comes Next

In the next article, we will bring everything together:

Cooling tower electrical efficiency
Evaporator and condenser heat transfer efficiency
Compressor lift
And total plant kW/ton

And we will show how optimization does not require solving thermodynamic equations in real time — but instead managing a few measurable variables intelligently within safe operating limits (already in place on most chiller plants).

Because ultimately:

The best chiller plant is not the one with the lowest lift. It is the one with the lowest total kW/ton.

Written by SIMA Intelligent Buildings

Articles in this Series: