Air Conditioning & AHU Systems

• Air conditioning systems control dry bulb temperature, cleanliness, and relative humidity
• A zone is an area with temperature controlled by a thermostat
• Basic types of comfort air conditioning include all air, all water, direct refrigerant, and air water systems
• Centrifugal chillers are for 100 - 30,000 tons
• A cooler and shell on a liquid chiller is the evaporator and casing
• The condenser is above the cooler
• Water is a cooling medium for chiller refrigeration
• Two chiller categories include absorption and compression cycles
• Centrifugal compressors are in low pressure chillers
• R11 and R123 are refrigerants for low pressure centrifugal chillers
• "Draw-thru" AHU fans are downstream, pushing air through the coil
• AHU casings downstream of the cooling coil prevent condensation
• CARRIER AHU coil fin designs include plate and spiral
• Fan capacity reduction reduces drive power input, energy, and cost

AHU

• AHU face and bypass dampers allow air to bypass the cooling coil for part load control
• Mixing boxes mix return and outside air
• Pneumatic operators modulate damper settings
• HEPA filters are efficient air filters
• Multi-zone AHUs heat/separate areas independently

Condensers

• Dual duct systems have separate hot and cold supply ducts
• Water cooled condensers offer better system efficiency due to lower saturated condensing temperature
• Waterside cleaning of water cooled condensers maintains system energy efficiency
• High head pressure usually indicates blockage
• Head pressure control is also called condenser capacity control
• Head pressure control systems maintain high side pressure as outdoor temperature decreases
• Head Pressure control is needed when there is low cooling capacity at cool outdoor temperatures

Cooling Towers

• Cooling towers in chilled water systems reject heat absorption
• Water is spread out to enhance evaporation
• Cooling towers require makeup water systems to replenish evaporated water
• Blowdown relieves the mineral concentration in the sump water

Compressors

• Minimum load is likely to cause compressor instability
• Inlet vanes extend the compressor stability range
• 3-stage centrifugal designs offer greater stability than single stage
• Condensing pressure reduction extends compressor unloading range
• A purge unit controls the amount of non-condensable gases and moisture
• Surge causes include excessive head pressure and inadequate cooler pressure
• Marine boxes assist condenser maintenance by providing easy access to clean tubes

Other

• CenTraVac uses an orifice metering device
• The trane CenTraVac cooler is a flooded type
• Centrifugal compressors are used in low pressure chillers
• 3-way valves allow coil control while maintaining constant system water flow rate
• Decoupler systems balance chilled water production and pumping capacity with load
• Condenser water temperature control allows condensing pressure to ride with load, increasing compressor modulating range
• Assuming the leaving evaporator water temperature is 42F*, the minimum entering condenser water temperature 62F*
• The design temperature CHW -10*
• The design temperature CW -10*
• A 4 pipe system can independently cool and heat separate zones
• Materials for piping include black steel, hard copper, and plastics
• Pipe sizing determines against what the pump must work
• Excessive water velocity can cause noise and erosion
• The preferred pump and motor RPM is 1750 RPM
• Pump packing types include ring and mechanical
• A decrease in head causes increased GPM & horsepower
• Centrifugal pumps require discharge throttling valves because overestimating the system head needs a throttling valve to balance a flow to design GPM
• Vortexing in a cooling tower causes a whirlpool type action
• Pump/motor alignment types include angular and parallel
• Condenser water pumps are generally centrifugal
• Pump Cavitation will result if NPSHA is less than NPSHR, the suction valve is partially closed, the water source is too far below the inlet, or the strainer is clogged
• Pump head is a force measure (units of feet) representing pump discharge
• Pump head is calculated by multiplying PSI times 2.31, or dividing PSI by .433
• Centrifugal pump types include horizontal split case and end suction
• CW flow switches protect the condenser from high pressure
• CHW flow switches protect the cooler from high pressure
• Backflow preventers prevent potable water contamination
• CW treatment control systems protect CW from scale and algae
• Freeze thermostats protects CW and CHW systems by automatically starting the CW and CHW pumps?
• Direct Action pneumatic thermostats create variable increases and output increases
• The definition of Offset control is the amount of difference between the control point and set point in a proportional control system
• Modulating control allows controlled devices to assume any position from fully closed to fully open
• Controlled variables include temperature, humidity, and pressure
• Spring range is the start point to finish point of an actuator