Industrial Refrigeration Application Handbook – The Complete Guide to Commercial Refrigeration

Understanding Heat Transfer and Industrial Chiller Operation

Heat transfer is the fundamental process of moving thermal energy from a substance at a higher temperature to another substance at a lower temperature.¹ In the context of a chiller, temperature measures the energy level, while heat is the energy itself. Heat energy cannot be destroyed; it can only be transferred, always flowing from a warmer substance to a cooler one.² This rate of heat flow is commonly expressed in British Thermal Units per hour (³ $Btu/hr$ ), representing the quantity of heat (in Btus) transferred over one hour.⁴

Industrial water chillers leverage heat transfer in two primary locations: the evaporator and the condenser. In the evaporator, heat moves from the warmer process recirculating fluid into the cooler refrigerant.⁵ The condenser then transfers this heat from the now warmer refrigerant to a cooler external cooling source (either air or water).⁶ Both of these components are integral to the refrigeration cycle, which is explained in detail below.

How Industrial Chillers Utilize Circulation

Industrial water chillers employ fluid circulation to transport heat from the industrial process to the chiller itself. This circulating fluid is typically water or a mixture of water and glycol.⁷ The fluid absorbs heat from the process, returns to the chiller, transfers this heat to the refrigerant via the evaporator, and then exits the chiller cold to return to the process, ready to absorb more heat.⁸

What is Industrial Refrigeration?

Refrigeration is a thermodynamic cycle.⁹ Chillers use refrigeration to extract heat from a process circulation fluid and ultimately reject it to the atmosphere.¹⁰ This system operates using a chemical compound known as a refrigerant.¹¹ Various types of refrigerants are used depending on the specific temperature requirements of the application.¹² However, they all function on the basic principle of compression and the phase-change of the refrigerant—from a liquid to a gas and then back to a liquid.¹³ This continuous process of heating and cooling the refrigerant, coupled with its phase changes, constitutes the refrigeration cycle.¹⁴ Changing the physical state of a compound (e.g., from gas to liquid) is an exceptionally efficient method for absorbing or expelling energy.

The refrigeration cycle within a chiller comprises four essential components: the compressor, condenser, expansion valve, and evaporator.¹⁵

As a foundational principle, heat always flows from a higher-temperature substance to a lower-temperature substance.¹⁶

The refrigeration cycle is the process of heating and cooling the refrigerant and changing it from a gas to a liquid and back again.¹⁷

Compressor: This component is engineered to increase the pressure (and consequently the temperature) of the refrigerant and circulate it throughout the system.¹⁸ By elevating the refrigerant’s pressure, its saturation temperature increases.¹⁹ With this elevated saturation temperature, the condenser can easily subcool the refrigerant.
Condenser: A heat exchanger that transfers heat from the hot refrigerant gas to an external cooling source, typically water or air.²⁰ This heat transfer causes the refrigerant to undergo a phase change from gas to liquid (condensation).²¹
Expansion Valve: This device reduces the high-temperature, high-pressure liquid refrigerant to a low-temperature, low-pressure liquid/vapor mixture. This small amount of phase change cools the mixture, providing a low-temperature refrigerant supply to the evaporator. The expansion valve controls the quantity of refrigerant delivered to the evaporator by maintaining a specific superheat at the evaporator’s outlet.²²
Evaporator: A heat exchanger that facilitates the transfer of heat from the process fluid into the refrigerant, causing the refrigerant to undergo a phase change (evaporation).²³

Other common components found in a refrigeration circuit include: a liquid line solenoid valve, filter dryer, hot gas bypass valve, and sight glass.²⁴

Fluid Coolers as an Alternative

With growing concerns about resource limitations and the need to reduce energy and water consumption, the use of fluid coolers has emerged as a popular alternative to traditional evaporative cooling towers.

A fluid cooler also uses ambient air to cool the process water, but it does so through a cooling coil without exposing the water directly to the atmosphere. While effective, this method is limited by the temperature of the ambient air. In most cases, the practical limit is a process water temperature leaving the dry fluid cooler that is approximately ²⁵ $10°F$ warmer than the entering air temperature.²⁶

Heat Transfer Basics and Common Definitions

BTU (British Thermal Unit): The most common unit of heat measurement in the United States. It is defined as the amount of energy required to heat one pound of water by ²⁷ $1°F$ .²⁸
kcal (kilocalorie): An International System of Units (SI) measurement of heat. It is defined as the amount of energy required to heat ²⁹ $1 kg$ of water by ³⁰ $1°C$ .³¹
Latent Heat: The amount of heat added or removed that results in a phase change of a substance without a change in temperature (e.g., the heat required to melt ice into water).
Sensible Heat: The amount of heat added or removed that results in a change in the temperature of a substance without a change in its phase (e.g., increasing water temperature from $50°F$ to $60°F$ ).
Rate of Heat Transfer: The quantity of heat that flows from one substance to another over a given period of time.³² This is commonly expressed in ³³ $Btu/hr$ (or Btuh) and in kilowatts (³⁴ $kW$ ) for SI units.³⁵
- For example: One ton of refrigeration represents the rate of energy absorbed to melt one ton (³⁶ $2,000 lb$ ) of ice in one day.³⁷ Since one pound of ice requires ³⁸ $144 Btus$ to melt, the amount to melt one ton of ice is ³⁹ $288,000 Btus$ (⁴⁰ $144 Btu/lb \times 2,000 lb$ ).⁴¹ Dividing this over ⁴² $24 hours$ , one ton of refrigeration equals ⁴³ $12,000 Btu/hr$ (⁴⁴ $288,000 Btus / 24 hours$ ).⁴⁵

Heat Transfer Formula

The general heat transfer formula is:

Q=M×Cp×ΔT

Where:

$Q$ is the heating or cooling capacity ( $Btu/hr$ )
$M$ is the mass of the fluid per hour (⁴⁶ $lb/hr$ ) (For water: ⁴⁷ $8.34 pounds/gallon \times 60 minutes/hour = 500.4 lb/hr per GPM$ )⁴⁸
$Cp$ is the specific heat of the fluid (⁴⁹ $Btu/lb/°F$ – the value for water is ⁵⁰ $1 Btu/lb/°F$ )⁵¹
$Δ T$ is the temperature difference between entering and leaving fluid (⁵² $°F$ )⁵³

For water, with a Cp of 1 Btu/lb/°F and considering 500.4 lb/hr per GPM, the heat transfer formula simplifies to:

Btu/hr=GPM×500×ΔT

Key Refrigeration Definitions

Saturation Temperature: The temperature at which a fluid changes phase from liquid to vapor or vapor to liquid. Saturation temperature increases with pressure.⁵⁴
Superheating: The process of raising a fluid’s temperature above its boiling point without actually causing it to boil. This typically occurs under elevated pressures. In the refrigeration circuit, superheating happens after the evaporator and within the compressor.
Subcooling: The process of cooling a liquid below its saturation temperature. In the refrigeration circuit, subcooling occurs within the condenser, before the expansion valve.⁵⁵

Heat Exchangers

Heat exchangers are devices specifically designed to transfer heat from one fluid to another without the fluids physically mixing.⁵⁶ The fluids are separated by the heat exchanger’s structure, allowing only heat energy to pass between them.⁵⁷ These fluids can be various compounds, such as water, oil, or refrigerant.⁵⁸

There are three basic types of heat exchangers commonly used in chiller design:

Plate and Frame Heat Exchangers: These use multiple plates, arranged as “plate packs,” to separate the two fluids.⁵⁹ The plate pack is mounted on a frame, with two end plates mechanically clamping the pack.⁶⁰ Gaskets seal the spaces between the plates.⁶¹ Heat transfer is highly efficient due to the small passages between plates. While the frame design allows for cleaning, these small passages are susceptible to clogging, and the units may require more floor space.⁶²
Brazed Plate Heat Exchangers: A variation of the plate and frame design where the plate pack is permanently brazed together.⁶³ This eliminates the need for the frame, gaskets, and end plates, resulting in improved efficiency and a much more compact size. However, they are not easily cleaned. Brazed plate units are frequently used as evaporators in chillers.⁶⁴
Shell and Tube Heat Exchangers: These consist of an outer shell vessel containing internal tubes that separate the two fluids.⁶⁵ They feature larger passageways between the tubes within the shell, which helps avoid clogging but reduces heat transfer efficiency. Shell and tube heat exchangers are commonly employed in applications where one of the fluids has high levels of contamination. Variations of these are used as condensers in water-cooled chillers.
Coil Heat Exchangers: Most often air-to-water or air-to-refrigerant units. They consist of tubes with fins stacked to form flat pieces. A common example is a car radiator. Forced air passing through the coil’s fins conducts heat from the fluid in the tubes, through the fins, and into the air.⁶⁶ These are frequently used as condensers in air-cooled chillers.

Evaporators vs. Condensers

Evaporator: In an evaporator, refrigerant enters as a low-pressure liquid/vapor mixture and exits as a low-pressure gas. The change of state from liquid to gas occurs at a constant temperature and absorbs energy. A chiller’s evaporator specifically achieves superheated refrigerant vapor. Superheat refers to the state where all liquid refrigerant has evaporated, and the gas temperature has risen above its saturation temperature.⁶⁷ The process fluid enters the evaporator as a hot liquid and exits at a lower temperature after transferring its energy to the refrigerant.
Condenser: In a condenser, refrigerant enters as a high-temperature vapor and exits as a high-temperature liquid. Condensers are responsible for exhausting the heat from the chiller to the surrounding air or cooling water.⁶⁸ The condenser’s design accounts for the “Total heat of rejection,” meaning it rejects both the heat absorbed by the evaporator and the heat added by the compressor.⁶⁹ The refrigerant exiting the condenser is a subcooled liquid, meaning all the vapor refrigerant has condensed and cooled below its saturation temperature.

Types of Compressors

A compressor’s primary function is to increase the pressure (and temperature) of the refrigerant and circulate it through the process cooling system.⁷⁰ By increasing refrigerant pressure, its saturation temperature rises, enabling the condenser to easily subcool the refrigerant.⁷¹

Scroll Compressors: These use positive displacement to compress refrigerant from a low-pressure gas to a high-pressure gas.⁷² The compressor motor is cooled by refrigerant flowing over it, and oil is vital for lubrication.⁷³ Scroll compressors are hermetically sealed, meaning they are typically replaced rather than repaired.
Screw Compressors: Also employing positive displacement, these compressors use two meshing screw-rotors that rotate in opposite directions to increase refrigerant gas pressure.⁷⁴ Screw compressors require oil for lubrication and can be disassembled for maintenance and repair.⁷⁵
Centrifugal Compressors: These are dynamic compressors that raise refrigerant pressure by rotating an impeller, which creates centrifugal force to compress the gas.⁷⁶ Certain centrifugal compressor designs, such as those in the Thermal Care TC and TCF series, utilize magnetic bearings and therefore do not require any oil for lubrication.⁷⁷ Centrifugal compressors can be disassembled for maintenance and repair.⁷⁸

What Do Expansion Valves Do?

Expansion valves reduce the high-pressure, high-temperature refrigerant liquid to a low-temperature, low-pressure liquid/vapor mixture. These valves are installed in the refrigeration circuit after the condenser and before the evaporator. By sensing the temperature at the evaporator’s outlet (superheat), the valve controls the amount of refrigerant supplied to the evaporator to maintain the desired superheat.

There are two main types:

Thermal Expansion Valves (TXV): These use a bulb and capillary tube to mechanically control the valve’s position. The bulb is mounted at the evaporator’s outlet to sense superheat, and the capillary tube connects back to the valve, adjusting its position.⁷⁹ TXVs are robust, simple, and generally inexpensive.⁸⁰
Electronic Expansion Valves (EEV): These are controlled based on input from a sensor mounted in the refrigeration piping at the evaporator’s outlet. A controller’s algorithm adjusts the valve’s position. EEVs are more expensive but offer a greater level of control within the refrigeration circuit, allowing system designers to optimize performance for enhanced efficiency and reliability.⁸¹

Common Refrigeration Components and Definitions

Compressor: A vessel designed to increase the pressure and temperature of refrigerant gas and circulate it through the process cooling system.⁸²
Condenser: A heat exchanger used to transfer heat from the refrigerant gas to an external cooling source (typically water or air).⁸³ This transfer causes a phase change from gas to liquid (condensation).⁸⁴
Expansion Valve: Reduces the high-pressure, high-temperature refrigerant liquid to a low-temperature, low-pressure liquid/vapor mixture.
Evaporator: A heat exchanger that transfers heat from the process fluid into the refrigerant, causing a phase change (evaporation).⁸⁵
Filter Dryer: Installed in the refrigeration circuit, it filters out particles of dirt, metal, or other debris, protecting components like expansion valves. It also absorbs any residual moisture in the system after evacuation and charging, which is crucial to prevent freezing and the formation of acids when moisture reacts with system oil.
Hot Gas Bypass Valve: Used for chiller capacity control. It creates an artificial load on the evaporator by introducing high-pressure, high-temperature gas to it.⁸⁶ This prevents the compressor from short-cycling when cooling demand is low. Additionally, the hot gas helps prevent the evaporator from freezing and avoids low refrigerant pressure alarms.