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Home , Room air distribution

21 Fans

• Fans help move air through the system. • There are various types of fans, most common of which for HVAC use is the Centrifugal (“Squirrel cage”) type (seen in the image). • Drive types Direct-drive Belt-drive • Controlling how much air a moves. • Fan controls can be on/off or variable speed. • Variable speeds are achieved with multi-speed motors, variable pitch blades, or variable frequency motor control (VFD). • Fan output can also be controlled by inlet dampers (“vanes”) or outlet dampers. 22 Fan Maintenance

• Fan maintenance requires cleaning fan blades, lubricating the motor, checking belts and alignment according to a schedule.

• Where multiple belts are used, all belts should be replaced at the same time.

• Fan belt tension should be set with a belt tensioner instrument that can measure the force and deflection to follow the manufacturers recommendations. If a belt squeals excessively when a fan is started up, it is a sign that the tension is not tight enough. Slight squealing at startup is not uncommon for correctly tensioned belts. Loose belts waste energy.

• Adjustment of the sheaves and pulleys in a belt drift enables the same fan unit to produce different combinations of air volume and pressure. 23 See next slide, Rooftop Exhaust Fan This is a Belt drive and a • A (squirrel cage): very common fans, Pulley & sheave usually connected to a riser duct so that they serve a stack of areas.

• As a single fan might be responsible for a large section of a building so the system needs to be balanced.

• Even if the fan is moving the correct amount of air, if the riser and individual outlet registers are not properly balanced, then areas may not get the right amount of air -- some will get too much and some will get too little.

• We will discuss balancing the system in the next section. Belt-Drive Fan 24

A Pulley and Sheave Fan

• Changing the diameter of the pulley, changes the speed of the fan and therefore how much air it will move.

These two points are typical of any type of fan regardless of the drive The amount of air the fan will move is determined by: • The speed of the fan. • The resistance in the duct system.

If there is more resistance the fan will move less air. Many things effect the resistance, duct size, bends, opening and closing of dampers, dust, dirt, diffusers. 25 Mistakes with Fans

• Problem #1: Do you see anything that might be a problem with this air intake point?

• Answer: The mushroom fan is probably an exhaust. Its usually not a great idea to have an exhaust fan and an air intake right next to each other! What will happen to air circulation if this condition occurs?

• #2: No exhaust fans in rooms! (because the fan may pull the combustion gasses back down the Fixed Air Intake and adjacent mushroom fan stack into the boiler room which would expose people to poisonous carbon monoxide.) Balancing dampers are normally located right before the final delivery device and 26 are used to adjust each zone for the Dampers proper air flow. These are adjustable devices that are locked into position.

Parallel dampers are Dampers with opposing normally used for 2 position blades are used to control control (open or closed), air flow. but do not provide good flow control. controls can be mounted outside the duct and connected with shafts Airflow dampers are used in duct systems to improve and linkages to the damper airflow and direct the air to or mounted directly on the go where we want it. damper. Direct mount is most popular in new Examples of airflow installations. dampers include and air extractors.

Dampers control the flow of air in through the ducts. Control dampers are typically either parallel or opposed blades. 27 Dampers of All Sizes

This giant damper is a whole room! Its called a Mixing Chamber and the set of dampers controls the flow of outside air and return air for a large building. Coils 28

Coils help heat and cool air and remove . They are classified by: • Their size • Number of rows passing through the air stream • The density of their aluminum fins. Aluminum fins are used to increase the effective surface of the coil exposed for . Coils are commonly constructed of copper, aluminum, or combination of both.

• Maintaining the coils affects the overall efficiency of the system. • Dirty coils require hotter (or colder) water temperatures to achieve the same level of heat transfer as clean coils. • Dirty coils also have higher pressure drops which increase system pressures and fan energy consumption. • Coils should be inspected at least once per year and cleaned as required. • Aluminum fins that are bent should be straightened to maintain even airflow across the coil. • A good filter replacement program is imperative to maintain coil heat transfer efficiency. • Cooling coil moisture issues - pans, carry-over, corrosion

As air passes through a heating coil moisture is removed. Humidity should be between 30% - 60%. add moisture to overly dry air. This moisture can present the same type of problems found with cooling coils.

• Heating coils • Humidification requirements to avoid too-dry air • Warm-water pans – Legionella (from last week- Lesson 8) Control Valves 29

Control valves regulate the fluid delivered to the coils, when heating and cooling are delivered to the distribution system. There are two types of control valves: two-way (on-off) and three-way (“by-pass”). Air-handling Unit (AHU) 30

An Air-Handling Unit (AHU) combines all the components (fans, dampers, coils and control valves and filters) into a single assembly. An AHU may have a heating element and/or equipment on-board, instead of coils from an external . A common type of AHU is the packaged rooftop unit (RTU), shown, that is factory-assembled and delivered ready for mounting and connection to building ductwork.

Typical Packaged Rooftop Unit Return fan Filter section Outsi de air Supply fan

Cooling coil

Compressor Retu rn air Suppl Condenser y air unit

Outside air intake (fixed louver, operable damper) 31 AHU Schematic

A simplified AHU schematic.

Air is mixed with fresh air. Coils which can be refrigerated Fresh air coils (DX) or coils. Air is “supplied” to goes in the space. here

Dampers regulate and mix the air.

The air is “returned” from the space

Air comes out here (same volume as brought in). AHU: The Unit Ventilator 32

The Unit Ventilator is a small AHU designed for through-the wall installation. Unit-ventilators or Uni-Vents are common in classrooms and hotel rooms. They are designed to bring in and condition outdoor air. They are different from a fan coil, which only recirculates the air in the room. Zone Controls 33

Zone controls regulate how much heat or cooling a particular part of a building (or zone) gets. They either add heat or cooling to the air or regulate the volume of the air.

• Mixing boxes – thermostatically controlled to mix hot and cool air from the separate ducts in a dual-duct system. • Re-heat coils - electric or hydronic coil in zone ductwork, heats the conditioned air as necessary for zone requirements indicated by the zone . • (VAV) boxes – a damper device that controls the volume of air delivered to a space based on zone thermostat and minimum ventilation requirements. There are several different VAV box designs. 34 Air Supply Terminal Units: Diffusers

• Diffusers are more than just an opening to let air into a room: they distribute air to rooms and are designed to create room air circulation with sufficient “spread” and “throw” and with gradual mixing of cool air, called the Coanda Effect. Air must have sufficient pressure and velocity to create these effects for good room air distribution. • Relative positioning of supply diffusers and return registers is also important for good room distribution. Watch out for return register short-circuiting. • A newer design for air supply is called “Underfloor Air Distribution”.

Coanda Effect – cold air along ceiling and mixes gradually 35 Air-Supply Terminal Units: Diffusers

• What happens when dampers, VAV throttle? As air volume is reduced (in variable air volume systems), room air distribution may deteriorate.

If the velocity is too low the cold air will drop on peoples heads and make them feel cold.

The spread and throw of diffusers influence effective air mixing at the point of use. In addition, if there are occupants in the high velocity area of the spread and throw, their comfort will be affected by the air stream.

Source: Tao & Janis Mechanical and Electrical Systems in Buildings Air-Supply Terminal Units: Diffusers 36

This is not an issue with a constant velocity system. Variable air volume VAV diffusers that throttle air flow to maintain the exit velocity of the reduced quantity of air Fan-powered VAV boxes that re-circulate room air to maintain total volume and velocity systems have dampers that slow / throttle the air and need something to regulate the air flow characteristics.

• Without adequate velocity, air is “dumped” rather than distributed. There are several methods of maintaining diffuser exit velocity to maintain good room air circulation: they’re self-acting when the primary air supply is reduced.

Accutherm™ self-acting diffuser

Fan-powered VAV boxes recirculate Diffuser is restricting air to maintain room air to maintain total volume and velocity and room mixing velocity when the primary air supply is reduced. 37 Filters

Conditioned air has to be both filtered and cleaned. • Air-filters remove by trapping them in the filter medium. • The most common type of is a Mechanical Filter that contain a fiber that has two purposes: 1. To trap particulate matter, and 2. For the particles to remain attached to the filter medium. • Filters do not remove gaseous contaminants, like VOCs, unless activated charcoal or other chemical process are added. • The Minimum Efficiency Reporting Value, also known simply as MERV, measures the performance of filters. The rating are from 1 - 20, with 20 removing the smallest particles. Ratings from 5 – 8 remove particles from 3 – 10 microns and can be from 1 to 6 inches thick. This rating will remove mold spores and dust mites.

• It is important to use the filter specified for your system. If the filter has a rating higher than specified it may make the fans work harder and air flow may be restricted. Also, if the filter does not fit correctly it will let air leak past it and defeat the purpose of the filter. HVAC Maintenance: Changing Air 38 Filters

• If the filter is dirty it makes the fan motor work harder & use more energy. • When is the right time to change the air filters? 1. When the delta P across the filter is higher than specified in the O&M manual {delta P = change in pressure} use a Manometer to test for the drop in pressure across the filter. Many ducts have a manometer permanently mounted to the duct to monitor the pressure drop at the filters. 2. Based on a predetermined time period, i.e. quarterly. 3. Follow the Owners Manual – O&M Manual for the equipment 39 HVAC Maintenance: Balancing Goals of Balancing: • Adequate air supply for heating/cooling • Minimum ventilation • Conservation of energy • TAB: Test - Adjust - Balance The engineer determines how many BTU of heating or cooling are required: these loads determine both equipment sizing and how many cubic feet per minute (CFM) are required for each space / zone. CFM is the volume of air to be delivered to that space by the fan for heating or cooling. In the design process, equipment and CFM are put on Mechanical Equipment Schedules which are used to design the sizing of the ductwork and diffuser layout for mixing of air in the spaces. After construction, the drawings are used by a balancer to fine tune the system to ensure that each space has the required air flow : this is called TAB Test Adjust & Balance. Hopefully you can access your building’s design drawing for the design/specs originally supposed to be installed. That design may have changed prior to construction; use and loads may have changed. Ventilation standard (ASHRAE Standard 62) and requirements also have changed over time- your initial installation may now be above/below current standards! Building drawings can serve as a starting point verify your current system and by the balancer for the TAB process. The system needs to be balanced to verify total volume and ensure that some spaces aren't starved for air while other have too much. Each zone is tested, adjusted and the system is balanced along with the flow of each HVAC component . The TAB process will effect comfort and energy consumption. Any time a facility is modified for a new load or occupant, the technician should balance the system to match the new conditions. Do the CFM on those schedules that tell you how much ventilation air/ outside air/ fresh air is being delivered? No. The CFM tell you the volume of air required by the load. To determine ventilation air you need to know what percentage of outside air is mixed in with the total volume of air delivered to the space. 40 HVAC Maintenance: Measuring Air Flow

• A balometer - also known as a capture hood - can measure air volume by holding it over a supply or return diffuser (not inside ducts). It is the only instrument that reads cubic feet per minute / cfm. It is an essential tool for balancing a system, it can help you determine total air flow to a space.

• There are other Instruments to Measure Airflow, but not CFM: • Pressure: Manometer, Pitot Tubes; used in ducts • Velocity: Anemometer (Rotating, Hot-wire)