Chiller Energy Auditing Series: Part 2/8
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Victor Karisa
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Chiller Energy Auditing Series: Part 2/8

Understanding the Chiller System

Reading Time: 2-3 minutes:

Where Part 1 Ended


We built the audit attitude in Part 1:
  • Describe the purpose.
  • Draw the boundaries of the system.
  • Understand the operational context
The next logical step is:
You must comprehend how the system functions after you know what you're auditing and where it begins and ends. This is because a system you don't understand cannot be audited.

What You’re Really Auditing:
When you stand in front of a chiller, you’re not auditing a single machine. You’re auditing a heat removal system - one that:
  • Absorbs heat from a load
  • Moves it using energy
  • Rejects it somewhere else
The core audit question becomes: "How is heat being removed, and where is the energy going?"

The Four Essential Components

There are four essential parts to every vapor-compression chiller system. Your audit is not complete if you miss one.


1️⃣ Evaporator - Where Heat Is Absorbed

Role: Absorbs heat from chilled water or process fluid
What to assess:
  • Is heat transfer effective?
  • What’s the supply vs return temperature?
  • Is fouling reducing performance?
Typical ranges:
  • Supply: 6–8 °C
  • Return: 12–15 °C
  • ΔT: 5–7 °C
Red flag:
  • Low ΔT often signals poor heat transfer, low load, or flow issues.


2️⃣ Compressor - Where Energy Is Consumed

Role: Raises refrigerant pressure and temperature. This is where most electrical energy is used.
What to assess:
  • Is it oversized for the load?
  • Is it running efficiently?
  • What compressor type is used?
Key indicators:
  • Power input (kW)
  • Current draw
  • Discharge pressure and temperature
Red flag:
  • High power at low load = wasted energy.


3️⃣ Condenser - Where Heat Is Rejected

Role: Rejects absorbed heat to air or water
What to assess:
  • Heat rejection effectiveness
  • Coil or tube cleanliness
  • Adequate airflow or water flow
Typical ranges:
  • Condensing temperature: 35–50 °C
  • Low approach temperature is desirable
Red flag:
  • High condensing temperature drives up compressor energy.


4️⃣ Expansion Device - Where Pressure Drops

Role: Reduces refrigerant pressure before the evaporator
What to assess:
  • Flow control stability
  • Refrigerant charge health
Key indicators:
  • Suction pressure
  • Superheat (if accessible)
Red flag:
  • Poor control = affects the entire cycle.

The Refrigeration Cycle: Why It's Important

These four parts, when combined, make up the refrigeration cycle. The cycle repeats after heat is absorbed in the evaporator, energy is added at the compressor, heat is rejected at the condenser, and pressure is lowered at the expansion device.

Every stage either transfers heat or consumes energy from the standpoint of an energy audit. By understanding this cycle, the auditor can identify the underlying causes of inefficiencies rather than treating their symptoms in isolation.

 The System Beyond the Chiller

The system boundary goes beyond the chiller itself, as was determined in Part 1. Water circulation to the load requires chilled water pumps, which can consume 10–30% of the system's energy, especially if they are large or run constantly. Condenser water pumps add an additional layer of energy consumption to water-cooled systems that is often disregarded.

Heat rejection is largely dependent on cooling towers or condenser fans, and inadequate maintenance or blocked airflow can greatly raise energy consumption.

Lastly, control systems frequently offer the biggest potential for cost savings through setpoints, staging, and scheduling. Ineffective control strategies can cause even the most efficient equipment to perform poorly.

The True Meaning of Efficiency

The Coefficient of Performance (COP), which is the ratio of cooling output to electrical input, is frequently used to express chiller efficiency.

Air-cooled chillers typically have COP values between 2.5 and 4.5, while water-cooled chillers typically have COP values between 4.0 and 7.0. But it's important to remember that system COP and chiller COP are not the same.

Pump energy, fan energy, control performance, and the chiller are all components of true system efficiency. If auxiliary systems use too much energy, a chiller with a high COP may still have low overall efficiency.

There are two main types you will encounter

Air-cooled chillers reject heat directly into the surrounding air and are typically easier to install and maintain; however, they consume more energy per unit of cooling and are sensitive to high outside temperatures.

Water-cooled chillers reject heat via cooling towers and provide higher efficiency, particularly in large or industrial applications, but they require additional system complexity, such as water treatment and pumping energy. 

Understanding the type you're dealing with is critical before interpreting performance data.

✔️Key Takeaway

Part 1 focused on developing auditor-like thinking skills. Part 2 focuses on understanding exactly what is being audited. System knowledge transforms measurements into insights. Without it, numbers are simply numbers.

What’s Next?

In Part 3/8, we step back even further - covering the data that should be gathered before the site visit. This preparation is what separates rushed audits from efficient, high-value ones.

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