BEM CLASS 6 Building HVAC Role of HVAC Equipment To maintain a - - PowerPoint PPT Presentation

BEM CLASS 6

Building HVAC

Role of HVAC Equipment

To maintain a comfortable and healthy indoor environment for occupants by controlling the

• Temperature
• Moisture Content (humidity)
• Air Quality
• Air Circulation

Goals

Gain an understanding of:

• Different Types of Space

Conditioning Systems

• Various System

Components

• General Characteristics

About How They Operate

HVAC System Types

• 1. Direct refrigerant systems (DX)
• 2. All-air systems
• 3. Air-and-water systems
• 4. All-water systems

HVAC systems are categorized by the method used to convey energy

Direct-expansion (DX) refrigerant systems

Example: Window AC

• Refrigerant absorbs heat

from a room Changes from liquid to gas

• The gas is compressed and

moves to the Condenser

• In the Condenser, heat is

rejected to the environment and allowed to expand and change from gas back to a liquid

• The cycle repeats until the

room is cooled to the desired set point

Vapor Compression Cycle

Direct-expansion (DX) refrigerant systems

These “unitary systems” are very common in residential buildings New York City. They Require no distribution network (ducts) By their nature, they are individually controlled and not connected to other building operations. Though not used in Modern Commercial buildings the core cooling process (DX vapor compression cooling cycle) is used in more sophisticated air-handling applications, heat pump systems and large chillers.

All-air systems

• Air is the only heat transfer medium used between

the mechanical system and the zone(s) it serves.

Single-duct Constant volume Multi-zone, variable-air volume (VAV) Multi-zone with re-heat Dual-duct systems also possible but uncommon

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All-Air Systems: Overview

• Provides all sensible and latent cooling, preheating, and

humidification required by the zone(s)

• Additional cooling or humidification at the zone rare (industrial

systems)

• Heating is either provided by the main air stream by the central

system (AHU) or locally at a specific zone (Reheat)

• Classified into single- and dual-duct categories as well as constant

and variable volume categories

• Conditioning depends on air mass flow rate and temperature

(enthalpy) difference between supply and room air

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Single-Duct Systems

• Main heating and cooling coils in series arrangement
• Ducts supply air to all terminals at a common temperature
• Capacity varied by varying temperature or flow rate
• Types of single-duct systems
• Constant Volume

Single zone Multiple-zone reheat Bypass VAV

• Variable Air Volume (VAV)

Throttling Fan-powered Reheat Induction Variable diffusers

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Dual-Duct Systems

• Main heating and cooling coils in parallel
• May use separate warm and cold air duct distribution systems, blending air

at the terminal device

• May blend air near the main unit and have separate duct for each zone
• Most vary supply temperature, limited number (around 1% of all installed

systems?) vary flow rate

• Types of dual-duct systems

Single zone (“dual duct”)

Constant volume Variable air volume Dual conduit

Multizone

Constant volume Variable air volume Three-deck multizone

All-air systems

• This class of system is extremely versatile
• Components Include
• Fans (Supply/Return) (Constant/Variable)
• Heating Element (furnace, heat pump, hot water coil)
• Cooling Element (DX coil, heat pump, chilled water coil)
• Control sensors, allow for air-side economizing,

temperature resets, energy recovery

• VAV applications allow for thermal control of multiple

zones

All-air system: Air handling unit

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VAVSystems

• Objective: reduce flow rate when loads are not as high to

save fan energy (fan energy proportional to flow rate cubed)

• Especially effective for perimeter zones that may receive

solar heating

• Temperature is maintain same for all zones, flow rate varied

to each

• Flow rate bounded at lower end by a minimum air fraction
• Concern 1: indoor air quality—outside air may limit lower

bound of VAV flow rate

• Concern 2: humidity—it may also limit lower bound of VAV

flow rate

• Terminal devices used to further reduce cooling or to provide

heating

• From maximum cooling point, VAV first throttles back flow

and then adds reheat (or uses terminal device)

Air and water Systems

• Majority of space heating/cooling is accomplished via the

water distribution network. Ventilation is handled separately and only a “Primary” stream of outside air is supplied to the space.

• Potential for heating without the circulation of air (unoccupied

periods, similar to baseboard heating)

• Main categories/arrangements
• Fan-coil units
• Induction units
• Two-, Three-, and Four-Pipe arrangements
• Packaged Terminal AC (PTAC) units

Air and water Systems

All-water systems

• These systems only heat and cool
• Air quality and ventilation are either handled by other

systems or by fresh air from infiltration

• Typically located in non-regularly occupied spaces.
• Garages
• Vestibules
• IDF rooms

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Heating Equipment

Electric resistance heating Heat pump in heating mode Solar panels Boiler Water Steam Furnace (air)

same basic principle, just a different fluid

Boilers

• Recirculate hot water to maintain meet the heating load of the

building.

• Boilers fuels include:
• Natural Gas
• #2, #4, #6 Oil
• Wood Pellets
• Electricity

Chillers

• Recirculates chilled water

through the building to meet the cooling load.

• Chillers reject their heat to the
• utside environment either by

air exchange (air-cooled) or to cooling towers (water cooled).

• Common chillers use

compression cooling cycles. Absorption chillers are used when an abundant source of waste heat exists

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Compression Cycle: Chilled Water

Condenser Evaporator Compressor Expansion Valve Cooling Coil Air System To Zones… Cooling Tower Direction of heat transfer

Efficiency Characteristics