Lesson Notes By Weeks and Term v3 - Senior Secondary 2
Evaporators and condensers
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Evaporators and condensers
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Subject: Air Conditioning And Refrigeration
Class: Senior Secondary 2
Term: 2nd Term
Week: 4
Theme: Compressors And Evaporators
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design, constructand serviceevaporators and condenser. estimate the loadon evaporatorsand condenserusing the for mula Q = Ax Ux T.D estimate the diameter and length of refrigeration piping
An evaporator is a heat exchanger in a refrigeration system where the liquid refrigerant absorbs heat from the conditioned space or substance (e.g., air, water, food products) and changes its phase from liquid to vapour (evaporation). This process of absorbing latent heat of vaporization is what produces the cooling effect.
Function: To absorb heat from the load and transfer it to the refrigerant, thereby cooling the space or product.
Location in System: Placed inside the cold space or where cooling is required, typically after the expansion device and before the compressor.
Operating Principle: Low-pressure, low-temperature liquid refrigerant enters the evaporator, absorbs heat, and boils into a low-pressure, low-temperature superheated vapour.
Materials of Construction: Copper: Most common due to excellent thermal conductivity, malleability for tube bending, and corrosion resistance. Suitable for most refrigerants.
Aluminium: Lighter and less expensive than copper. Used for fins and sometimes tubes, especially in domestic refrigerators. Requires specific brazing techniques.
Steel (Carbon Steel): Used for larger industrial evaporators (e.g., shell and tube) where strength is paramount and cost-effectiveness is desired, especially with ammonia refrigerant, which corrodes copper.
Types of Evaporators: Bare Tube Evaporators: Consist of simple copper or steel tubing. Used for direct contact cooling, e.g., shelves in freezers or liquid chillers. Limited heat transfer surface area. Finned Tube Evaporators (Coil Evaporators): Most common. Tubes have metallic fins (aluminium or copper) attached to increase the heat transfer surface area.
Natural Convection: Air circulates by natural buoyancy. Used in domestic refrigerators (freezer compartment).
Forced Convection: A fan is used to force air over the coil, significantly increasing heat transfer. Used in air conditioners, cold rooms, and display cases.
Plate Evaporators: Made of two metal plates (e.g., aluminium) pressed or welded together to form channels for refrigerant flow. Flat and easy to clean. Used in domestic freezers, beverage coolers, and ice makers.
Shell and Tube Evaporators: Refrigerant flows through tubes, while the substance to be cooled (e.g., water, brine) flows through the shell around the tubes, or vice-versa. Used for large capacity liquid chilling applications (chillers). A condenser is a heat exchanger in a refrigeration system where the high-pressure, high-temperature refrigerant vapour rejects the heat absorbed from the evaporator (and the heat added by the compressor) to a cooling medium (e.g., air or water) and changes its phase from vapour to liquid (condensation).
Function: To dissipate the heat absorbed from the conditioned space and the heat of compression to the surroundings.
Location in System: Placed outside the cold space, typically after the compressor and before the expansion device.
Operating Principle: High-pressure, high-temperature superheated refrigerant vapour enters the condenser, desuperheats, condenses into a high-pressure, high-temperature liquid, and may subcool slightly.
Materials of Construction: Copper: Excellent thermal conductivity, used for tubes.
Aluminium: Used for fins due to lightweight and cost-effectiveness.
Steel (Carbon Steel): Used for shells and larger components, especially in water-cooled or industrial condensers.
Types of Condensers: Air-Cooled Condensers: Reject heat directly to the ambient air.
Natural Convection: No fan. Used in domestic refrigerators, water coolers. Limited capacity.
Forced Convection: A fan draws or pushes air over the condenser coil. Most common type for residential and light commercial air conditioners and refrigeration units. Highly effective.
Water-Cooled Condensers: Reject heat to a circulating water supply. More efficient than air-cooled for larger capacities, especially in hot climates.
Double-Tube (Tube-in-Tube): Inner tube carries refrigerant, outer tube carries water.
Shell and Coil: Refrigerant in a coil inside a shell through which water flows.
Shell and Tube: Water flows through tubes, refrigerant flows in the shell, or vice-versa. Used for large industrial applications. Requires a cooling tower for water recirculation.
Evaporative Condensers: Combine principles of air-cooled and water-cooled. Refrigerant tubes are sprayed with water, and air is forced over them. The evaporation of water removes significant heat, making them very efficient for large industrial systems. This formula is fundamental for estimating the rate of heat transfer across a heat exchanger surface. Q: Heat transfer rate (Heat Load) in Watts (W) or British Thermal Units per hour (BTU/hr). A: Total heat transfer surface area of the evaporator or condenser in square meters (m2) or square feet (ft2). This is the area exposed to heat exchange. U: Overall Heat Transfer Coefficient in W/(m2·K) or BTU/(hr·ft2·°F). This value accounts for the thermal resistance of the materials, fouling, and the heat transfer coefficients of the fluids on both sides. It's often determined experimentally or found in engineering handbooks for specific heat exchanger types and fluid combinations. ΔT (Delta T or T.D): Temperature Difference (also known as Mean Temperature Difference, MTD, or Log Mean Temperature Difference, LMTD, for more precise calculations in complex flow scenarios). For simpler applications, it can be the difference between the average temperature of the refrigerant and the average temperature of the fluid being cooled/heated. In simple terms, it's the driving force for heat transfer, in Kelvin (K) or Celsius (°C) for SI units, or Fahrenheit (°F) for imperial units.
Worked Example for Evaporator Heat Load: A finned-tube evaporator in a cold room has a total effective surface area of 15 m
2. The overall heat transfer coefficient (U) for this evaporator is given as 15 W/(m2·K). The average temperature difference (ΔT) between the cold room air and the evaporating refrigerant is 10
K. Calculate the heat load on the evaporator.
Solution: Given: A = 15 m2 U = 15 W/(m2·K) ΔT = 10 K Using the formula Q = A × U × ΔT: Q = 15 m2 × 15 W/(m2·K) × 10 K Q = 2250 W Therefore, the heat load on the evaporator is 2250 Watts (or 2.25 kW). This represents the rate at which heat is removed from the cold room by the evaporator.
Worked Example for Condenser Heat Load: A forced-convection air-cooled condenser has an effective surface area of 20 m
2. The overall heat transfer coefficient (U) is 30 W/(m2·K). The average temperature difference (ΔT) between the condensing refrigerant and the ambient air is 15
K. Calculate the heat rejected by the condenser.
Solution: Given: A = 20 m2 U = 30 W/(m2·K) ΔT = 15 K Using the formula Q = A × U × ΔT: Q = 20 m2 × 30 W/(m2·K) × 15 K Q = 9000 W Therefore, the heat rejected by the condenser is 9000 Watts (or 9 kW). This represents the total heat absorbed from the conditioned space plus the heat of compression, rejected to the surroundings. Correct sizing of refrigeration piping (suction, liquid, and discharge lines) is critical for efficient and reliable system operation. Incorrect sizing leads to performance issues, increased energy consumption, and potential component damage.
Importance of Correct Sizing: Minimize Pressure Drop: Excessive pressure drop reduces system capacity and efficiency (especially in the suction line, leading to lower evaporator pressure and temperature).
Ensure Proper Oil Return: Refrigerant oil circulates with the refrigerant. Velocities must be high enough to entrain and return oil to the compressor. Prevent Excessive Velocity Noise and Erosion: Too high velocity can cause noise and premature wear of piping.
Optimize System Capacity: Correct sizing ensures the designed cooling/heating capacity is achieved.
Cost-effectiveness: Avoids oversized pipes (expensive) or undersized pipes (inefficient, requires more energy).
Factors Influencing Pipe Diameter: Refrigerant Type: Different refrigerants have different densities and flow characteristics.
System Capacity: Larger capacity systems require larger pipes to handle higher mass flow rates.
Allowable Pressure Drop: The maximum permissible pressure drop for each line (suction, discharge, liquid) is specified by system designers or manufacturers. Typically, suction line pressure drop should be kept low (e.g., 1-2 psi or 7-14 kPa equivalent saturated temperature drop).
Refrigerant Velocity: Recommended velocity ranges are used to ensure proper oil return and avoid excessive pressure drop or noise.
Suction Line: 3.5 - 12 m/s (700 - 2400 ft/min)
Discharge (Hot Gas)
Line: 7.5 - 18 m/s (1500 - 3500 ft/min)
Liquid Line: 0.5 - 1.5 m/s (100 - 300 ft/min)
Length of Run: Longer runs naturally lead to higher pressure drops for a given diameter, requiring larger diameters or multiple parallel lines.
Number of Fittings: Each elbow, valve, or tee contributes to an "equivalent length" of pipe, increasing total pressure drop.
Estimation of Diameter: For SS2 level, pipe diameter estimation primarily involves using simplified sizing charts/tables provided by manufacturers or in HVAC&R handbooks. These charts typically relate refrigerant type, cooling capacity (TR or kW), and line length to recommended pipe diameters, ensuring acceptable pressure drop and velocity.
Rule of Thumb (Basic understanding): Liquid lines are generally the smallest in diameter because refrigerant is in a dense liquid state. Suction lines are typically the largest because refrigerant is a low-density vapour and requires larger cross-sectional area to maintain acceptable velocity and pressure drop. Discharge lines are intermediate, handling high-pressure vapour.
Estimation of Length: The length of refrigeration piping is determined by the physical distance between components (evaporator, compressor, condenser) in the installation. It involves physically measuring the planned route, including vertical risers and horizontal runs.
Equivalent Length Method: For precise calculations, the actual length is combined with the "equivalent length" of all fittings (elbows, valves, strainers, etc.) to get a total effective length for pressure drop calculations. This is typically found in tables where each fitting is assigned an equivalent straight pipe length.
Example for Pipe Sizing Principle: Consider a 3-ton (approximately 10.5 kW) split air conditioning unit to be installed in a home. The outdoor unit (condenser) is to be placed 15 meters away from the indoor unit (evaporator).
Initial thought for diameter: Based on tables for R22 or R410A (common refrigerants in Nigeria) for a 3-ton capacity, the liquid line might be 3/8 inch (9.52 mm) O.D., and the suction line might be 3/4 inch (19.05 mm) O.
D. These are typical sizes for this capacity.
Length: The actual measured length of 15 meters will be used in conjunction with pressure drop charts. If this length is excessive for the initial chosen diameter, a larger diameter might be required to avoid unacceptable pressure drop, or alternative routing considered.
Food Preservation and Agriculture: Evaporators and condensers are fundamental to cold storage facilities and refrigerated trucks used across Nigeria for preserving perishable goods like fruits, vegetables, meat, fish, and dairy products. This is critical for reducing post-harvest losses, supporting farmers, and ensuring food security. For example, large cold rooms in markets like Mile 12 (Lagos) or perishable goods markets in Kano rely heavily on efficient evaporators and condensers to keep produce fresh for longer. Comfort Cooling in Residential, Commercial, and Public Spaces: Given Nigeria's tropical climate, air conditioning is essential for comfort in homes, offices, hospitals, schools, and public buildings. Split unit ACs (with their indoor evaporators and outdoor condensers), central air conditioning systems, and car ACs are ubiquitous. Understanding these components helps in proper installation, maintenance, and energy-efficient use, crucial for both individual comfort and national energy consumption.
Industrial Processes: Beyond comfort and food, evaporators and condensers play a vital role in various Nigerian industries.
Examples include: Brewing and Beverage Production: Cooling fermentation tanks and chilling products before bottling.
Pharmaceuticals: Maintaining precise temperature control for drug manufacturing and storage.
Oil and Gas: Cooling processes in refineries and gas plants, where heat removal is critical for efficiency and safety.
Ice Production: Commercial ice plants, common throughout Nigeria (especially in fishing communities and beverage industries), rely on robust evaporators for ice making.