How Does a Chiller Work?

How Does a Chiller Work?

A chiller does not create cold; it removes unwanted heat from a process and transfers that heat somewhere else.

That distinction is fundamental to understanding how chillers work. It comes directly from the first law of thermodynamics: energy cannot be created or destroyed; it can only be transferred or converted from one form into another.

Industrial chillers use this principle to control temperatures in food and beverage production, plastics manufacturing, pharmaceuticals, engineering and many other industrial processes.

What does a chiller do?

A chiller cools water or a water-and-glycol mixture, which is pumped around a closed circuit to remove heat from machinery, production equipment or another process.

The heated fluid returns to the chiller, where the absorbed heat is transferred into refrigerant. The cooled fluid is then circulated back to the process, and the cycle continues.

A typical system therefore contains two connected circuits:

• The process-water or glycol circuit, which collects heat from the process

• The refrigerant circuit, which transfers that heat away and rejects it to the atmosphere or a separate water system

The refrigerant and process fluid remain separate. Heat passes between them through a heat exchanger called the evaporator.

What is the first law of thermodynamics?

The first law of thermodynamics is the principle of energy conservation.

In simple terms:

Energy cannot be created or destroyed. It can only be transferred or changed from one form into another.

This means a chiller cannot make heat disappear.

The heat removed from a production process must be transferred somewhere else. In an air-cooled chiller, it is normally discharged into the outside air. In a water-cooled system, it is transferred into condenser water and then rejected through a cooling tower, dry air cooler or another heat-rejection system.

The chiller also uses electrical energy to operate its compressor, fans, pumps and controls. Much of this electrical energy ultimately becomes additional heat that must also be rejected.

The basic energy balance is:

Heat rejected at the condenser = heat absorbed at the evaporator + work added to the refrigerant by the compressor.

In practical calculations, the compressor’s absorbed electrical power is often used as an approximation, particularly for hermetic and semi-hermetic compressors.

A practical thermodynamics example

Imagine a chiller providing 100 kW of cooling and absorbing approximately 30 kW at the compressor will reject roughly 130 kW through the condenser, subject to motor losses and heat transferred through the compressor casing. The chiller may be described as a 100 kW chiller, but its condenser must reject approximately 130 kW under those operating conditions.

Depending on where the system boundary is drawn, fan motors, pumps and other electrical components may add further heat.

This is why condenser capacity cannot be assumed to be the same as the stated cooling capacity.

Why is thermodynamics important to the whole system?

The first law affects almost every part of chiller design and operation.

Condenser sizing

The condenser must reject both the heat removed from the process and the energy used by the compressor.

If the condenser is undersized, dirty or has insufficient airflow, the heat cannot leave the system effectively. Refrigerant pressure then rises, compressor power increases and the chiller may eventually stop on a high-pressure alarm.

Hot-weather performance

An air-cooled chiller rejects heat into the outside air.

As the ambient temperature rises, it becomes more difficult for the condenser to transfer heat into the surrounding air. The compressor must operate at a higher pressure to move the same amount of heat.

This can cause:

• Reduced cooling capacity

• Increased electrical consumption

• Higher compressor temperatures

• Longer operating periods

• High-pressure alarms

• Increased stress on components

The heat has not increased simply because the weather is hot. The problem is that the chiller has a less effective route for transferring that heat away.

Indoor installations

An air-cooled chiller installed inside a building will discharge the process heat and compressor energy into that building unless the hot condenser air is properly ducted outside.

A 100 kW chiller consuming 30 kW could release around 130 kW of heat into the room.

Without suitable ventilation, the room temperature rises. This makes the condenser less effective, which raises the refrigerant pressure and causes the chiller to release heat less efficiently.

The system can end up heating its own surroundings and making its operating conditions progressively worse.

Energy efficiency

A more efficient chiller uses less electrical energy to remove the same amount of process heat.

Using the earlier example:

• A less efficient chiller might remove 100 kW of process heat while consuming 30 kW

• A more efficient chiller might remove the same 100 kW while consuming 20 kW

The first chiller must reject around 130 kW at the condenser. The second must reject around 120 kW.

Improved efficiency therefore reduces both electrical consumption and the amount of additional heat that the condenser must reject.

Heat recovery

Because the chiller does not destroy heat, some of or all of that energy may be recoverable.

Rather than rejecting all the condenser heat to the atmosphere, it may be possible to use some of it for:

• Hot-water generation

• Process-water heating

• Space heating

• Product washing

• Preheating boiler feedwater

• Defrosting or cleaning processes

The available heat may be greater than the cooling duty because it can include both the process heat removed and the compressor’s electrical input.

Whether heat recovery is commercially worthwhile depends on temperature requirements, operating hours and whether there is a simultaneous demand for heating and cooling.

Overall system design

A chiller is only one part of a heat-transfer system.

The process creates or absorbs heat. The water or glycol collects it. The evaporator transfers it into the refrigerant. The compressor raises the refrigerant pressure. The condenser transfers the heat to the air or water.

If any part of that route is restricted, the complete system is affected.

This is why problems such as low water flow, excessive glycol concentration, dirty heat exchangers, blocked condenser coils or poor ventilation can all reduce chiller performance.

The four stages of the refrigeration cycle

The refrigerant circuit normally contains four main components:

1. Evaporator

2. Compressor

3. Condenser

4. Expansion valve

Each performs a different part of the heat-transfer process.

1. The evaporator absorbs heat

Warm water or glycol returning from the process enters the evaporator.

Inside the evaporator, heat transfers from the process fluid into the refrigerant. The refrigerant boils at a low temperature because it is operating at low pressure.

As the refrigerant absorbs heat, it changes from a liquid into a vapour. At the same time, the process fluid loses heat and leaves the evaporator at a lower temperature.

The cooled water or glycol is then pumped back to the process.

2. The compressor raises the refrigerant pressure

The low-pressure refrigerant vapour enters the compressor.

The compressor raises the pressure and temperature of the refrigerant, producing a hot, superheated high-pressure gas.

The compressor does not directly produce cooling. It supplies the energy needed to move heat from the low-temperature evaporator to the higher-temperature condenser.

Industrial chillers may use several compressor technologies, including:

• Scroll compressors

• Screw compressors

• Reciprocating compressors

• Centrifugal compressors

• Oil-free magnetic-bearing compressors

The most suitable type depends on the cooling duty, operating temperatures, required resilience and expected load profile.

3. The condenser rejects the heat

The hot refrigerant gas flows into the condenser.

Here, the heat collected from the process together with the compressor energy is rejected from the chiller.

As the refrigerant loses heat, it condenses from a gas back into a high-pressure sub cooled liquid.

How this happens depends on the chiller design.

Air-cooled chillers

An air-cooled chiller uses fans to draw outside air across condenser coils.

Heat transfers from the refrigerant into the surrounding air, and the warmer air is discharged to the atmosphere.

Air-cooled chillers are widely used because they do not require a cooling tower or separate condenser-water circuit. However, their performance is affected by outside temperature and condenser cleanliness.

Water-cooled chillers

A water-cooled chiller transfers heat into a separate condenser-water circuit.

That heat is normally rejected through a cooling tower, dry air cooler or another heat-rejection system.

Water-cooled chillers can offer high efficiency, particularly on larger systems, but require additional pumps, water treatment and heat-rejection equipment.

4. The expansion valve reduces the pressure

The high-pressure liquid refrigerant leaves the condenser and passes through the expansion valve.

The valve controls refrigerant flow into the evaporator and creates a sharp reduction in pressure.

This lowers the refrigerant’s boiling temperature, allowing it to absorb heat again when it enters the evaporator.

The refrigeration cycle then repeats continuously while cooling is required.

What happens on the water or glycol side?

A pump circulates process fluid between the chiller and the equipment being cooled.

The system may also contain:

• A buffer tank

• Duty and standby pumps

• Strainers and filters

• Expansion vessels

• Flow switches

• Pressure gauges and temperature sensors

• Control valves

• Plate heat exchangers

• Water-treatment or glycol-monitoring equipment

The cooling capacity delivered depends on the fluid flow rate and the temperature difference between the fluid entering and leaving the process.

For water, the approximate calculation is:

Cooling duty in kW = flow rate in litres per second × 4.186* × temperature difference in °C

Glycol mixtures have different *specific-heat capacities and viscosities, so their concentration and temperature must be considered when calculating system performance.

Insufficient flow, excessive glycol concentration, blocked strainers or poor water quality can reduce cooling capacity even when the refrigerant circuit is working correctly.

How does a chiller control capacity?

Industrial cooling loads are rarely constant.

A chiller must adjust its output as production conditions, ambient temperature and process demand change.

Depending on its design, the chiller may control capacity by:

• Starting and stopping multiple compressors

• Loading or unloading screw compressors

• Varying compressor speed

• Using staged refrigerant circuits

Good capacity control keeps the process temperature stable while avoiding unnecessary energy consumption and excessive compressor cycling.

This is why selecting a chiller solely from its maximum capacity can be a mistake. Its performance under normal and part-load conditions can be just as important.

Common signs that a chiller is not working correctly

Warning signs can include:

• The process temperature gradually increasing

• Repeated high- or low-pressure alarms

• Frequent compressor cycling

• Reduced water flow

• An unexpected fluid-temperature difference

• Ice forming on the evaporator or pipework

• Condenser fans running continuously

• Unusual noise or vibration

• Rising electrical consumption

• Repeated controller resets

• One refrigerant circuit doing more work than another

Resetting an alarm may restart the chiller, but it does not explain why the alarm occurred.

The cause could be within the refrigerant circuit, but it may also involve water flow, glycol condition, controls, heat-exchanger fouling or changes to the production process.

How can chiller reliability be improved?

Reliable cooling depends on the complete system,, not just the compressor.

Effective maintenance should include:

• Recording refrigerant pressures and temperatures

• Checking water or glycol flow

• Measuring entering and leaving fluid temperatures

• Inspecting and cleaning condenser coils

• Testing glycol condition and concentration

• Checking strainers, pumps and expansion vessels

• Inspecting electrical connections and contactors

• Reviewing alarm history

• Checking compressor and fan operation

• Identifying trends before they become breakdowns

These measurements should be compared over time. A single set of figures may show that the chiller is running, but trending can reveal gradual deterioration.

Choosing the right industrial chiller

A correctly selected chiller must suit more than the headline cooling duty.

The selection should consider:

• Required inlet and outlet temperatures

• Fluid type and glycol concentration

• Design flow rate

• Peak and average cooling loads

• Minimum and maximum ambient temperatures

• Part-load operation

• Available electrical supply

• Required resilience

• Noise and space restrictions

• Refrigerant choice

• Maintenance access

• Condenser heat rejection

• Future production requirements

• Opportunities for heat recovery

The correct solution is the chiller that performs reliably under the site’s real operating conditions, not simply the model with the closest capacity figure in a brochure.

Need help with an industrial chiller?

NovaCool Solutions supports industrial process cooling systems across food and beverage, plastics, pharmaceutical and manufacturing environments.

Whether you need breakdown assistance, a second opinion, planned maintenance, temporary cooling or a replacement chiller, we focus on understanding the complete system and finding the root cause.

Call 0808 258 6894 or visit novacoolsolutions.com to speak with our team.