Indirect-Expansion Evaporators
An indirect-expansion evaporator does not cool the occupied space or product directly. Instead, the refrigerant cools a secondary fluid such as water or glycol, and that fluid carries the cooling effect to another heat exchanger.
Indirect systems separate the refrigerant circuit from the air coils or process loads being cooled. This allows one refrigeration system to serve multiple air handlers, fan-coil units, or process loads through a circulating fluid loop.
Chilled-water systems are the most common example of indirect expansion in comfort air conditioning.
What You Will Learn
By the end of this lesson you should be able to:
Describe how refrigerant cools a secondary fluid instead of directly cooling the conditioned air or product.
Follow heat from the conditioned space into the secondary fluid and then into the refrigerant.
Describe how chilled water circulates between a refrigeration machine and remote air coils.
Recognize that secondary-fluid circulation requires pumps to move water or glycol through the system.
Recognize that glycol solutions can provide freeze protection where water alone would be unsuitable.
Understand how the refrigerant path and heat-transfer path differ between DX and secondary-fluid systems.
What Is Indirect Expansion?
In a direct-expansion system, refrigerant evaporates in the same coil that directly cools the air or product.
In an indirect-expansion system, refrigerant cools an intermediate fluid. That secondary fluid then carries heat between the conditioned space and the refrigerant evaporator.
Direct Expansion
Refrigerant flows through the coil serving the conditioned air.
Air → Evaporator Surface → Refrigerant
Indirect Expansion
A secondary fluid carries heat between the conditioned load and the refrigeration system.
Air → Secondary Fluid → Refrigerant
The secondary fluid separates the refrigerant circuit from the remote cooling coils.
Follow the Heat
Heat exists in the air or process load.
Heat transfers from the air into the secondary fluid.
The fluid carries the absorbed heat back to the refrigeration equipment.
Heat transfers from the secondary fluid into the refrigerant.
Room Air → Chilled Water or Glycol → Refrigerant → Condenser → Outdoors
Chilled-Water Systems
In a chilled-water system, the refrigeration machine cools water rather than directly cooling the building air.
The chilled water is then pumped through piping to air-handling units, fan-coil units, or other heat exchangers located throughout the building.
Water Returns Warm
Water returning from the building has absorbed heat from the conditioned spaces.
The Refrigerant Evaporator Cools the Water
Heat transfers from the returning water into the low-pressure refrigerant.
Chilled Water Leaves
The cooled water leaves the refrigeration equipment at a lower temperature.
Pumps Move the Water Through the Building
The chilled water is circulated to remote cooling coils.
The Water Absorbs Building Heat
Air passing across the chilled-water coils transfers heat into the water.
Warm Water Returns
The water returns to the refrigeration equipment and the cycle repeats.
The Remote Air Coil Is Not a Refrigerant Evaporator
In a chilled-water system, the coil located in the air handler normally contains chilled water rather than refrigerant.
The refrigerant evaporator is located at the chiller or refrigeration machine where refrigerant removes heat from the water.
Air-Handler Coil
Contains chilled water or another secondary fluid and removes heat from the air.
Refrigerant Evaporator
Contains refrigerant and removes heat from the chilled-water loop.
Do Not Confuse the Two Heat Exchangers
The air-handler coil may look similar to a DX evaporator coil, but the fluid inside it is water or glycol rather than refrigerant.
Secondary-Fluid Pumps
Water and glycol do not circulate through the building by themselves. Pumps provide the pressure difference needed to move the fluid through piping, valves, coils, fittings, and heat exchangers.
Supply Side
The pump moves cooled fluid from the refrigeration equipment toward the remote cooling loads.
Return Side
Warmer fluid returns after absorbing heat from the building or process.
A refrigeration system can be operating correctly while cooling performance is poor because the secondary-fluid flow is inadequate.
Fluid Flow Affects Capacity
Just as airflow affects a DX evaporator, secondary-fluid flow affects an indirect evaporator system.
Low Fluid Flow
Reduces the amount of heat carried between the building and refrigeration equipment.
Proper Fluid Flow
Allows the designed quantity of heat to be transferred through the secondary-fluid loop.
Excessive Flow Resistance
Dirty strainers, closed valves, fouled coils, or restricted piping can reduce circulation.
Pump Problems
Incorrect pump operation, air in the system, or loss of prime can reduce or stop secondary-fluid circulation.
Water and Glycol
Water is an effective heat-transfer fluid and is widely used in comfort-cooling systems where freezing is not expected.
Where the secondary fluid may be exposed to temperatures below the freezing point of water, a glycol solution may be used to provide freeze protection.
Water
Provides good heat-transfer performance and relatively low pumping resistance.
Water-Glycol Solution
Provides freeze protection but changes heat-transfer characteristics, fluid density, viscosity, and pumping requirements.
More Glycol Is Not Automatically Better
The glycol concentration should match the required freeze protection and system design. Excessive glycol concentration can reduce heat-transfer performance and increase pumping requirements.
Why Freeze Protection Matters
Water expands when it freezes. Freezing inside coils, piping, pumps, or heat exchangers can cause serious mechanical damage.
Fluid Temperature Falls
The secondary fluid may be exposed to low ambient or process temperatures.
Water Approaches Freezing
If the system contains plain water and temperature falls far enough, ice can begin forming.
Expansion Can Damage Components
Frozen water can split tubing, crack coils, damage heat exchangers, or block circulation.
Proper system design, controls, insulation, fluid concentration, and operating procedures are used to prevent freezing.
Air in the Secondary-Fluid Loop
Air trapped in a chilled-water or glycol system can interfere with circulation and heat transfer.
Air Pockets
Can reduce fluid flow through coils and create uneven temperature conditions.
Air Removal
Properly designed systems use air separators, vents, and service procedures to remove trapped air.
Fluid Expansion Must Be Accommodated
Water and glycol change volume as temperature changes.
Closed hydronic systems therefore require a method of accommodating fluid expansion and contraction without allowing system pressure to rise or fall excessively.
A properly sized expansion tank provides space for changes in secondary-fluid volume as system temperature changes.
Diagnostic Thinking Is Different
A DX evaporator places the refrigerant directly at the conditioned-air coil. An indirect system adds another heat-transfer loop between the refrigerant and the building.
DX System
Air-side performance and refrigerant-side performance interact directly at the same coil.
Indirect System
The technician must consider the refrigerant side, the secondary-fluid side, and the air or process side.
There Is One More Heat-Transfer Step
Poor cooling can be caused by a refrigerant problem, a secondary-fluid circulation problem, or an airflow/load problem.
Common Indirect-System Problems
Low Fluid Flow
Can result from pump problems, closed valves, clogged strainers, fouled coils, or piping restrictions.
Air in the System
Can reduce circulation and create uneven coil temperatures.
Incorrect Glycol Concentration
Can reduce heat transfer or provide inadequate freeze protection.
Fouled Heat Exchangers
Deposits on the water side or air side can reduce heat-transfer performance.
Incorrect Control-Valve Position
A valve that is closed or improperly controlled can restrict flow through a remote cooling coil.
Poor Insulation
Heat gain into chilled-water piping can reduce delivered cooling and cause unwanted condensation on piping surfaces.
Cold Piping Can Condense Moisture
Chilled-water and glycol piping can operate below the dew-point temperature of the surrounding air.
If the piping or valves are not properly insulated, water can condense on their surfaces just as it does on a cold evaporator coil.
Proper Insulation
Reduces heat gain and helps keep the outer insulation surface above the surrounding air dew point.
Damaged or Missing Insulation
Can allow condensation, dripping water, heat gain, corrosion, and building damage.
Think Like a Technician
When an indirect system is not cooling correctly, do not assume the refrigeration circuit is automatically responsible.
Verify secondary-fluid supply and return temperatures.
Confirm that the circulation pump is operating correctly.
Check valves, strainers, and coils for restrictions to fluid flow.
Look for trapped air and verify that the system is properly vented.
Verify glycol concentration where freeze-protection fluid is used.
Evaluate airflow across the remote cooling coil.
Inspect insulation on cold piping, valves, and fittings.
Evaluate the refrigerant circuit only as one part of the complete heat-transfer system.
Put the Concepts Together
Indirect-expansion systems use a secondary fluid between the refrigerant and the conditioned load.
Chilled-water systems are a common indirect-expansion application.
The refrigerant evaporator removes heat from water or glycol rather than directly from the building air.
Pumps circulate the secondary fluid between the refrigeration equipment and remote cooling coils.
Remote air coils normally contain chilled water or glycol rather than refrigerant.
Fluid flow directly affects the amount of heat the system can transport.
Glycol can provide freeze protection but changes heat-transfer and pumping characteristics.
Air trapped in the secondary-fluid loop can reduce circulation and performance.
Cold piping requires proper insulation to limit heat gain and prevent surface condensation.
Indirect-system diagnosis must consider the refrigerant side, secondary-fluid side, and air or process side.
Can You Explain Indirect-Expansion Evaporators?
You should be able to answer these questions before continuing.
1. What is an indirect-expansion evaporator system?
2. What is the purpose of the secondary fluid?
3. What is a common example of an indirect-expansion comfort-cooling system?
4. Does the air-handler cooling coil in a chilled-water system normally contain refrigerant?
5. What component moves chilled water through the system?
6. Why does fluid flow affect system capacity?
7. Why might glycol be added to a secondary-fluid system?
8. What disadvantage can excessive glycol concentration create?
9. How can trapped air affect chilled-water circulation?
10. Why are expansion tanks used in closed hydronic systems?
11. Why must chilled-water piping be insulated?
12. Why should a technician evaluate the secondary-fluid loop before concluding that a poor-cooling problem is caused by the refrigerant circuit?
What You Should Have Learned
Indirect expansion separates the refrigerant circuit from the remote cooling loads.
The refrigerant evaporator cools a secondary fluid such as water or glycol.
The secondary fluid carries heat between the conditioned space and refrigeration equipment.
Pumps are required to circulate the secondary fluid through the system.
Glycol can provide freeze protection where plain water would be at risk of freezing.
Secondary-fluid flow, air removal, insulation, and heat-exchanger cleanliness all affect system performance.
The air-handler coil in a chilled-water system is not the refrigerant evaporator.
Indirect-system diagnosis requires evaluation of multiple heat-transfer loops rather than the refrigerant circuit alone.
Next: Evaporator Problems and Performance
The final lesson in this subsection brings the evaporator concepts together from a diagnostic perspective.
We will examine dirty coils, restricted airflow, icing, poor refrigerant distribution, excessive pressure drop, oil-return problems, condensate problems, and other conditions that reduce evaporator performance.
Observe the Symptoms · Follow the Heat · Find the Cause