Let's dive into the detailed workings of chillers, often referred to as water chillers, which are essential cooling systems in commercial and industrial applications. Chillers are responsible for removing heat from a liquid (usually water or a water-glycol solution) to produce cooled water that can be distributed to air handlers throughout a building for air conditioning. We will explore their major components, operational principles, various compressor types, and the advanced absorption chillers that use chemical reactions rather than mechanical compression.
Chiller Fundamentals
A chiller system absorbs heat from a water solution, which may contain glycol to prevent freezing. When glycol is present, the system can operate below freezing; without glycol, it must operate above freezing. The main cooling process takes place in the evaporator, which absorbs heat from the circulating water solution and then pumps this chilled water to air handlers in different building zones.
The primary refrigerants used in chillers are R134a, R22, and R123. These refrigerants interact with a secondary refrigerant (the water or glycol solution) that flows to remote coils in air handlers for cooling. This configuration allows for flexibility, as chilled water is used to cool different areas without directly exposing them to refrigerants, which simplifies the cooling process.
Flooded Evaporator Design
Many chillers have a flooded evaporator where the water tubes are completely submerged in liquid refrigerant within a large tank. A liquid level sensor inside the evaporator maintains the refrigerant level above the tubes, which is crucial for efficient heat transfer. As the warm water or water-glycol solution from the air handlers flows through these tubes, the refrigerant absorbs its heat, causing the refrigerant to vaporize and cool the water in the process. This cooled solution is then pumped back to the air handlers to continue the cooling cycle.
Key Chiller Components and Operational Flow
Evaporator: Here, the refrigerant absorbs heat from the warm water-glycol mixture returning from the building’s cooling coils. As the refrigerant boils, it cools the water, which then circulates back to the air handlers.
Compressor: The vaporized refrigerant is drawn into the compressor, where it undergoes compression. Compressors vary in design and function based on the chiller’s capacity and application, with centrifugal compressors being common for large-scale systems and scroll compressors for smaller or modular systems.
Condenser: After compression, the refrigerant vapor moves to the condenser, where it releases its heat to cooler water from a cooling tower. This water-cooled condenser setup is typical for large systems. Smaller systems may use air-cooled condensers with fans to dissipate heat.
The refrigerant’s pressure and temperature values are essential for effective operation. For example, an R134a refrigerant system may have a high-side discharge pressure of 135 psi with a corresponding condensing temperature of 105°F, while the low side operates at around 35 psi, producing an evaporating temperature of 40°F. These parameters maintain the flow of heat from the chilled water circuit to the condenser.
Types of Compressors
Chillers can utilize various types of compressors based on their cooling capacity:
Centrifugal Compressors: These compressors use a high-speed impeller to compress refrigerant vapors, making them efficient for large-capacity chillers (over 100 tons).
Screw Compressors: Common in high-pressure systems, screw compressors have slide valves or variable frequency drives (VFDs) to modulate capacity. They’re ideal for applications needing stable cooling across a wide range of load demands.
Reciprocating and Scroll Compressors: Both types are often used in high-pressure systems where modular or incremental capacity control is necessary. Due to their smaller capacity, chillers typically have multiple scroll compressors, which can be turned on or off as needed.
Each type of compressor plays a unique role in accommodating different cooling needs. A centrifugal compressor, for example, has an impeller that spins at high RPMs, while a screw compressor might use slide valves to adjust its output dynamically.
Chilled and Condenser Water Circuits
Chilled water circuits transport cooled water from the evaporator to the building’s air handlers, where it absorbs heat from the air. The temperature difference between the evaporator and chilled water—known as the approach temperature—is a diagnostic metric used by technicians. A narrow approach temperature indicates effective heat transfer, while a widening approach suggests scale or mineral deposits that impede cooling efficiency. When approach temperatures increase, this signals that the tubes may need cleaning to remove build-up.
On the condenser side, the refrigerant vapor condenses at about 105°F. This heat is transferred to the condenser water, which leaves the condenser at around 95°F. The cooling tower then cools this water through evaporative cooling, where a fan draws air across water droplets, causing some water to evaporate and, in turn, cool the remaining water. The cooled water, now around 85°F, returns to the condenser, completing the cycle.
Safety Devices in Chillers
Several safety mechanisms are integrated into chiller systems to protect against operational issues:
- Flow Switches: These detect interruptions in water flow and shut down the chiller to prevent freezing.
- Low Temperature Cutout: Prevents the system from operating below a set temperature (typically 34-36°F), which could risk freezing the evaporator water.
- High Temperature Motor and Bearing Cutouts: These shut down the compressor if critical temperatures are exceeded, safeguarding the motor and bearings.
- Pressure Relief Devices: Located on the evaporator barrel, these devices relieve excess pressure, protecting the system from potential damage due to hydrostatic expansion.
Additional devices, like anti-short cycle timers, delay the chiller’s restart after shutdown, allowing for system stabilization and reducing wear on components.
Absorption Chillers
Absorption chillers differ significantly from traditional chillers by using a chemical reaction instead of mechanical compression. These chillers typically use ammonia (R717) or lithium bromide with distilled water as a refrigerant. Ammonia systems are common in industrial applications but require strict safety measures due to ammonia's hazardous nature. Lithium bromide systems are widely used because of their safer, more manageable properties.
The main components and process in absorption chillers are:
- Evaporator: Distilled water acts as a refrigerant under vacuum conditions, boiling at around 40°F and absorbing heat from the chilled water circuit.
- Absorber: The evaporated water vapor is absorbed into a lithium bromide solution, which is then pumped to the generator as a weak solution.
- Generator: Using a heat source, such as steam, gas, or hot water, the water vapor is driven out of the lithium bromide solution, re-concentrating it before it returns to the absorber.
- Condenser: The water vapor is then condensed with cooling tower water, returning it to liquid form, ready to recirculate through the evaporator.
The evaporator in an absorption chiller operates under a high vacuum (about 13,000 microns), allowing water to boil at lower temperatures. This cycle continues as the chilled water circulates to building air handlers, while condenser water flows to the cooling tower for heat removal.
Approach temperatures in absorption chillers also provide diagnostic information. As mineral or scale deposits accumulate in the tubes, heat transfer efficiency declines, widening the approach temperature and signaling the need for maintenance.
By understanding these fundamental principles, technicians can effectively monitor and maintain both traditional and absorption chillers, ensuring reliable, efficient cooling across various applications.
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