1.

Air Handling Unit

2. Types of AHU
3. Cooling tower
4. Cooling load
An Air Handling Unit (AHU) is a central component of most HVAC (Heating, Ventilation, and Air Conditioning) systems,
responsible for conditioning and circulating air throughout a building. Here’s a breakdown of its key aspects:

1. Core Functions
1.Ventilation
•Introduces fresh outdoor air to maintain indoor air quality.
•Dilutes and removes pollutants, odors, and CO₂.
2.Air Filtration
•Removes particulates (dust, pollen) via filters of various MERV ratings.
3.Thermal Conditioning
•Heating coils (hot water, steam, or electric) warm incoming air.
•Cooling coils (chilled water or refrigerant) cool and dehumidify.
4.Air Distribution
•A fan/blower moves conditioned air into ductwork for delivery to occupied spaces.
5.Exhaust/Air Return
•Collects stale return air; can recirculate part of it to save energy.
2. Main Components
1. Housing/Casing
1. Usually double-skinned, insulated panels to minimize heat loss/gain and noise.
2. Filters
1. Pre-filters (e.g., coarse-rated) capture large debris.
2. Fine filters (e.g., HEPA, MERV 13–16) for high indoor-air-quality needs.
3. Fans/Blowers
1. Centrifugal (plenum/forward/backward-curved) or axial types.
2. Sized to overcome system static pressure and meet required airflow (cfm).
4. Heating/Cooling Coils
1. Materials: copper tubes with aluminum fins; coil face area sized for heat load.
5. Humidifier/Dehumidifier (optional)
1. Controls indoor relative humidity (e.g., steam humidifiers, desiccant wheels).
6. Mixing Section
1. Dampers to modulate proportions of fresh vs. return air for energy optimization.
7. Sound Attenuators
1. Acoustic liners or silencers to reduce fan- and airflow-generated noise.
8. Controls & Instrumentation
1. Temperature sensors, differential pressure gauges, airflow stations.
2. Integrated into building management systems (BMS) for scheduling and alarms.
3. Types & Configurations
• Packaged AHU: Factory-built “plug-and-play,” often rooftop-mounted.
• Built-up AHU: Field-assembled, highly customizable for large or
complex facilities.
• Variable Air Volume (VAV) versus Constant Air Volume (CAV) systems:
• VAV modulates airflow to zones based on demand, improving efficiency.
• CAV supplies constant airflow; temperature is varied to meet loads.
4. Design & Sizing Considerations
1.Thermal Load Calculation
1. Estimate sensible and latent loads from occupants, equipment, envelope.
2.Airflow Requirements
1. cfm per person or per sqft per relevant standards (e.g., ASHRAE 62.1).
3.Pressure Drop
1. Sum of filter, coil, duct, and diffuser losses; fan selection must overcome this.
4.Energy Efficiency
1. High-efficiency motors (ECM/PM), heat recovery wheels, economizer cycles.
5.Maintenance Access
1. Space for filter changes, coil cleaning, fan servicing; hinged access doors.
5. Operation & Maintenance

• Routine Tasks (monthly/quarterly):

• Inspect and replace filters; check belt tension; verify damper operation.
• Periodic Tasks (annually/biennially):
• Coil cleaning; fan balancing; motor bearing lubrication; calibration of sensors.
• Fault Detection:
• Monitor differential pressures, temperature differentials, unusual
vibration/noise.
• Use BMS alarms for deviations (e.g., frozen coils, stuck dampers).
6. Common Applications

• Commercial office buildings

• Hospitals & laboratories (with HEPA filtration)
• Industrial processes requiring precise temperature/humidity control
• Educational campuses, shopping malls, airports
Types of AHU
2. By Airflow Control Strategy

• Constant Air Volume (CAV)

• Supplies a fixed airflow rate; temperature is modulated (via coil control) to meet
loads.
• Simpler controls; suitable for spaces with fairly constant occupancy or process loads.
• Variable Air Volume (VAV)
• Varies airflow to zones based on demand, using VAV boxes. Temperature setpoint
held constant at the AHU coil.
• Energy‐efficient in part-load conditions; common in offices, mixed-use buildings.
• Dual‐Duct
• Two parallel air streams (hot and cold) carried to terminal units. Mixing happens at
zone level.
• Allows simultaneous heating/cooling to different zones; more ductwork and cost.
3. By Mounting Location
• Rooftop Units (RTU)
• Packaged AHUs installed outdoors on a roof curb. Weather-proof casing and
access hatches.
• Frees up interior floor space; common in low-rise commercial.
• Indoor/Basement Units
• Installed within mechanical rooms. Typically built-up for large installations.
• Easier access for maintenance but requires dedicated floor/ceiling space.
• Ceiling-Plenum Units
• Slim “slab-type” AHUs that fit within a dropped ceiling cavity.
• Ideal for small retail, offices, or lab fume-hood make-up air.
By Control & Modulation

• Single-Path AHU
• One airflow path: filter → coil → fan → supply. Simple and compact.
• Multi-Path (Split-Flow) AHU
• Air stream split into parallel paths through coils/filters then recombined.
Reduces face velocity, pressure drop, and unit length.
• Modular/Split Coil AHU
• Coil and fan modules can be added or removed to adjust capacity; useful for
phased expansions.
cooling tower
• A cooling tower is a heat-rejection device that extracts waste heat
from a building or industrial process and releases it to the
atmosphere. They’re critical in large-scale HVAC systems, power
plants, manufacturing, and process industries where chilled water
systems or process fluids must be kept cool. Here’s a structured
overview:
1. Basic Principle

1.Evaporative Cooling
1. Warm water from the building or process is pumped to the top of the tower and distributed
over a “fill” material (a media of sheets, films, or splash bars).
2. As water spreads over the fill, a small portion evaporates—this phase change removes heat
from the remaining water.
3. The cooled water collects in the basin at the tower’s bottom and is recirculated back into the
system.
2.Heat Transfer Mechanisms
1. Evaporation (latent heat): Primary cooling effect; roughly 1 lb of water evaporated removes
about 1,000 BTU.
2. Sensible Cooling: Minor heat loss due to temperature difference between water and air,
outside of evaporative effect.
3.Airflow
1. Moves upward through the fill, counter-current or cross-flow to the falling water, carrying
away the evaporated moisture and latent heat.
 2. Main Components
1. Hot Water Inlet/Distribution System
1. Spray nozzles, troughs, or rotating arms to uniformly wet the fill.
2. Fill Media
1. Splash Fill: Water droplets “splash” over horizontal bars—durable, with lower fouling risk but larger footprint.
2. Film Fill: Thin sheets create a film of water—more efficient per volume but prone to scale/fouling if water treatment
isn’t maintained.
3. Drift Eliminators
1. Capture water droplets entrained in the exiting airflow, minimizing water loss (drift).
4. Air Inlet Louvers
1. Guide incoming air over the fill and block light (to reduce algae growth) and debris.
5. Fan(s)
1. Induced-Draft: Fans at the top pull air up through the fill—more uniform airflow, lower drift loss.
2. Forced-Draft: Fans at the air inlet push air through the fill—simpler but can produce higher fan noise externally.
6. Cold Water Basin
1. Collects cooled water before it’s pumped back into the system.
7. Drains, Bleed-Off, and Blowdown
1. Bleed-Off: Periodic discharge of a small portion of circulating water to control dissolved solids.
2. Blowdown: Manual or automatic larger purges to prevent scaling and biological growth.
8. Structure/Casing
1. Often fiberglass-reinforced polyester (FRP), galvanized steel, or concrete. Designed to resist weather and chemical
corrosion.
3. Types of Cooling Towers
Cooling load
1. Types of Cooling Loads
1.Sensible Load
– Heat gains that change air temperature, but not moisture content (°F or °C).
– Sources:
1. Solar radiation through glazing
2. Conductive gains through walls, roof, floor
3. Internal gains from lighting, equipment, and occupants
2.Latent Load
– Heat gains associated with moisture addition (humidity) to the space (lb H₂O/hr or
kg H₂O/s).
– Sources:
1. Occupant respiration and perspiration
2. Infiltration air that carries outdoor moisture
3. Process or humidification equipment
3.Total Cooling Load
– The sum of sensible and latent loads, typically expressed in Btu/hr (or kW):
4.Qtotal=Qsensible+QlatentQ_{\text{total}} = Q_{\text{sensible}} +
Q_{\text{latent}}Qtotal​=Qsensible​+Qlatent
 3. Heat Gain Calculations
1. Conductive (Envelope) Gains
2. Q=U A ΔTQ = U \, A \, \Delta TQ=UAΔT
1. UUU = overall heat-transfer coefficient (Btu/hr·ft²·°F)
2. AAA = area (ft²)
3. ΔT\Delta TΔT = inside – outside design temperature difference (°F)
3. Solar (Window) Gains
4. Q=A×SHGC×IQ = A \times SHGC \times IQ=A×SHGC×I
1. SHGCSHGCSHGC = Solar Heat Gain Coefficient (fraction)
2. III = solar irradiance on the glass (Btu/hr·ft²)
5. Internal Sensible from People
6. Qsensible,people=N×qsensible,per personQ_{\text{sensible,people}} = N \times
q_{\text{sensible,per person}}Qsensible,people​=N×qsensible,per person​
1. NNN = number of occupants
2. qsensible,per personq_{\text{sensible,per person}}qsensible,per person​ ≈ 230 Btu/hr at office activity levels
7. Latent from People
8. Qlatent,people=N×qlatent,per personQ_{\text{latent,people}} = N \times q_{\text{latent,per person}}Qlatent,people​=N×qlatent,per person​
1. qlatent,per personq_{\text{latent,per person}}qlatent,per person​ ≈ 215 Btu/hr for typical office occupants
9. Infiltration & Ventilation
10.Q=m˙ (houtside−hinside)Q = \dot{m} \, (h_{\text{outside}} - h_{\text{inside}})Q=m˙(houtside​−hinside​)
1. m˙\dot{m}m˙ = mass flow rate of air (lb/hr)
2. hhh = enthalpy (Btu/lb), captures both sensible and latent
Load Calculation Methods

1.Manual J (Residential)
– A standardized method from ACCA for homes.
2.ASHRAE Fundamentals Handbook (Commercial)
– Detailed procedures in Chapters 14–18.
3.Software Tools
– e.g., Trace 700, HAP, EnergyPlus, IES VE for automated hourly or
sub-hourly load profiles.