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Chiller Plant Efficiency Series Chiller Efficiency – Part 1

This article focuses on the evaporator and condensers, specifically the flooded shell-and-tubes used in most large centrifugal chillers, and answers a fundamental question: What actually makes an evaporator “efficient”?

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Evaporator Heat Transfer: What Really Drives Efficiency

In the previous articles, we focused on cooling tower efficiency and how air flow, water flow, and the L/G ratio determine how effectively heat is rejected to the atmosphere.

Now we move inside the chiller.

This article focuses on the evaporator and condensers, specifically the flooded shell-and-tubes used in most large centrifugal chillers, and answers a fundamental question:

What actually makes an evaporator “efficient”?

Two Concepts That Are Often Confused

Before going further, we need to clearly separate two terms that are often used interchangeably—but should not be.

Heat Exchanger Effectiveness

Heat exchanger effectiveness describes how much of the maximum possible heat transfer is actually achieved.

ε=Qactual/Qmax

Dimensionless (a ratio)
Purely thermodynamic
Answers the question:

“How close did we get to ideal heat transfer?”

Effectiveness says nothing about electrical energy.

2. Heat Exchanger Efficiency (Practical / Plant Context)

In real chiller plants, when operators talk about “efficiency,” they usually mean:

How much cooling is produced for a given electrical input

Ultimately expressed as kW/ton.

A heat exchanger can be:

Highly effective thermodynamically
And still electrically inefficient if achieving that effectiveness increases compressor lift

This distinction is critical.

The Fundamental Heat Transfer Equation

At the heat exchanger, heat transfer is governed by:

Q=U⋅A⋅ΔTlm

Where:

Q = heat transfer rate (cooling capacity)
U = overall heat transfer coefficient
A = heat transfer surface area (fixed by design)
ΔTₗₘ = log mean temperature difference between refrigerant and water (driving force)

This equation allows us to understand leverage.

Decomposing U: Where Flow Actually Matters

The overall heat transfer coefficient is defined as:

Where:

hw (water-side coefficient) → driven by water flow and turbulence
hᵣ (refrigerant-side coefficient) → driven by boiling quality, oil presence, refrigerant distribution
Rₜᵤᵦₑ → tube wall resistance (fixed)
Rf → fouling resistance

Key Insight

For a new, clean flooded evaporator:

Refrigerant-side boiling coefficients are already very high
Tube resistance is fixed
Fouling is minimal

This means:

Water flow affects only one term inside U—and with diminishing returns. We have already discussed the behavior of diminishing returns.

Relative Potency: What Actually Moves Q on a New Chiller

For a new evaporator/condenser, the relative influence on heat transfer looks approximately like this:

ΔTₗₘ (temperature driving force): ~50% (Can be controlled)
Refrigerant-side boiling quality: ~20–25% (Can not be controlled deteriorates over time if too much oil is introduced to the refrigerant circuit).
Water flow / turbulence (hw): ~15–20% (Can be controlled)
Tube wall & geometry: ~5–10% (Can not be controlled)
Fouling: ~0–5% (initially) (Can not be controlled but can be monitored for triggering maintenance)

ΔTₗₘ is the single most powerful lever in the equation.

What ΔT Really Means in a Flooded Evaporator

In a flooded evaporator:

Refrigerant is boiling at a nearly constant saturation temperature.
For analysis, we can treat the refrigerant as a “fluid” at that saturation temperature or at saturation minus a few degrees to consider evaporation effect.

So the driving force is approximately:

ΔT ≈ Tentering water−Tevaporator saturation

Increasing this difference increases heat transfer linearly.

Controlled vs. Uncontrolled Variables

Let’s clearly mark what operators can influence.

Variables You Can Influence

Chilled water/Condenser water flow rate→ affects turbulence → affects hw
Chilled water/Condenser water temperature setpoint→ affects ΔT

A Critical Warning About “Artificial” ΔT

There are two ways to increase ΔT:

Artificial (Electrically Costly)

Lower chilled water setpoint
Raise condenser water temperature

These actions:

Reduce evaporator saturation temperature
Increase condenser saturation temperature

This results in Increased compressor lift thus Increase kW/ton (more on article 6)

Thermodynamically valid but Electrically inefficient.

2. Physical (Correct)

Improve heat transfer on the water side by ensuring the highest possible chilled water return temperature on the evaporator or the coldest entering condenser water temperature that optimizes tower fans Kw vs compressor Kw.

Maintain the highest refrigerant saturation temperature on the evaporator that still produces water at the temperature needed to satisfy the load. These preserve low compressor lift
Maintain heat transfer effectiveness by regularly cleaning heat transfer surfaces and by maintaining a good water treatment program.

ΔT improvements must come from the water side—not from forcing the compressor to work harder.

What About Flow vs. ΔT?

Increasing chilled water flow:

Improves U But with diminishing returns. If going from low flow to nominal flow then important gains could be made however once in nominal flow increasing it yields diminishing returns. Just like the cooling towers relationship between evaporation rate and fan speed.
Typical gains in U are often 15–25%

Increasing ΔT:

Directly multiplies Q
Often produces 40–60% changes in heat transfer for the same exchanger

This is why:

ΔT is more potent than flow.

Why Fouling Changes Everything Over Time

Fouling inside evaporator or condenser increases the resistance term within the U factor and often becomes the dominant factor even outweighing the Delta T factor. Just like in a cooling tower where the maintenance status of the equipment – particularly nozzles and fill media – can have a significant effect on the levers that influence efficiency.

As fouling develops:

Rf grows rapidly
Becomes a dominant term in U
Flow increases stop working

This is why maintenance eventually outweighs control strategy.

The Key Takeaway

A highly effective evaporator and/or condenser is not automatically an efficient chiller.

Heat exchanger effectiveness tells you how well heat is transferred
Chiller efficiency (kW/ton) tells you how much electrical energy was required to do it

Optimizing heat transfer without regard to compressor lift can make kW/ton worse—not better.

Written by SIMA Intelligent Buildings

Articles in this Series: