hvac design overview

Michael BrandemuehlUniversity of Colorado 1

HVAC Systems: OverviewMichael J. Brandemuehl, Ph.D, P.E.University of ColoradoBoulder, CO, USA

OverviewSystem DescriptionSecondary HVAC Systems

Air distributionRoom diffusers and air terminalsDuct DesignFan characteristicsAir Handling Units

Water distributionCooling coilsPipes and pumps

Primary HVAC SystemsElectric chillers

Air and water cooledCompressor technologiesPerformance

Thermal chillersAbsorptionEngine-driven

Cooling towersOverall Design Process


System Overview

Core Objectives: healthy, productive, comfortable indoor environment

Heating to perimeter spacesCooling to perimeter and core spacesHumidification or dehumidification as neededVentilation to occupied spaces

Deliver over time and space

Time and Space

HVAC needs in each room change over day and over yearAt any time, may need heating and cooling in different rooms of building

Core needs cooling even in winterHVAC system must meet simultaneous diverse loads


System Options

Separate HVAC system for every zoneResidentialMotelStrip mall

One HVAC system for entire buildingDistribute heating, cooling, ventilation to individual zones

Typical Home System

(AIR CONDITIONING)

AIR

FLO

W

AIRFLOW

(WITH FAN AND FILTER)


Typical Large Commercial System

Distributed HVAC Systems

Packaged terminal air conditioner (PTAC)Water loop heat pump (WLHP)Packaged rooftop unit (RTU)


Typical Small Commercial System With Rooftop Units

Packaged Rooftop Unit (RTU)


Slab Installation with Side Discharge

Typical Small Commercial System With “Split System”


Water Loop Heat Pump System

System Characteristics

Rooftop Units (RTU) or Split SystemsOne unit each “zone”Refrigerant in cooling coilVentilationCeiling diffusers and ductworkSimple controls – one thermostat per zoneSeparate billing for each tenantSometimes separate boiler and radiators


Zoning

One thermostat per zoneRooms with similar load profiles

Good: offices on same side of buildingBad: exterior office and interior conf. rm.

Proximity (one thermostat!)Air communication allows larger zonesRecognize local loads in large spaces

Central HVAC System

Terminal devicesFan coil unitsAir and water distribution systemsHeat exchangersCentral heating and cooling sources


Typical Central System

Packaged Central System


Large Central System Equipment

Typical Large Central System


Benefits of Central and Distributed System Designs

Large equipment has higher quality, efficiency, and durabilityMaintenance is concentratedNoise is removed from zoneDiversity allows lower installed capacityCan use thermal storage

Easy to provide zoningDirect control by occupantsEasier independent scheduling for energy savingsGenerally lower capital costs and shorter lead time for equipmentDon’t need dedicated maintenance staff, use service contractCan often install on roof, less useable space for equipment

Central Distributed

Typical Design Approach

Start at the zone and work outLoadsAir diffusers and zone terminalsAir distribution systemAir handlersChilled water distributionCentral cooling and heat rejection


Meeting Zone Loads

)()(

SARApSAsen

SARASAtot

TTcmQhhmQ−=

−=

Given controlled room air temperature, can control airflow or supply temperature to meet changing sensible loads

Supply Air (SA)

Return Air (RA)

Loads Qtot and Qsen

Controlled RoomConditions

Air Handling Systems (All Air)

Constant air volume (CAV) systemsConstant zone airflowMeet varying loads with varying supply air temperature

Variable air volume (VAV) systemsConstant zone supply air temperatureMeet varying loads with varying supply airflow


Air Handling Systems (cont.)

Dual duct (DD) systemsMix hot and cold air at each zoneUse constant or variable supply airflow

Multizone (MZ) systemMix hot and cold air for each zone at the air handler

Typical CAV AHU System


Typical VAV AHU System

Typical Dual Duct System


Typical Multizone System

Air-Water Systems

Use combination of conditioned air and zone water coilsVentilation requires airZone heating and cooling loads can be met with fan coils


Fan Coil

One or two coils (use seasonal valves if one coil for both hot and cold water)Thermostat controls water flowVentilation must be met with conditioned or unconditioned outdoor air

Fan Coil System: 4 Pipe


Fan Coil System: 2 Pipe

Integrated With Central System


General System Classification

Secondary HVAC Equipment and SystemsGenerally in the buildingAir distributionWater distributionAir handlers and fan coils

Primary HVAC Equipment and SystemsPrimary sources of heating and coolingChillers and heat rejectionBoilersEngines and generatorsThermal Storage

Air Delivery to Zone

Fully mixed zoneSupply air is mixed uniformly with room airAir can be introduced at ceiling, walls, floor

Displacement ventilationSupply air is slowly introduced at floorAir rises, absorbing heat and pollutants


Mixed vs. Displacement

Typical Diffusers


Diffuser Selection

Mix air without causing draft, quietly, with low pressure dropVelocities less than 50 fpm (0.25 m/s) in occupied zoneDiffuser manufacturers report throw: distance till velocity is reduced to specified levelBeware of change in throw at reduced airflow

Air Terminals

Control supply airflow entering zone (VAV)Control supply air temperature (CAV & VAV)Interact with zone thermostat


Alternative Air Terminals

Parallel Fan Power Mixing Box

Modulate VAV airflow for coolingDraw warm air from plenum and add heat as necessaryMaintains higher air velocity in heating to overcome stratification


Duct Design

Optimization of initial cost with operating costsLarger ducts have lower velocity, pressure drop, and fan energySmall ducts reduce ducting costs and save building spaceDouble duct size reduces fan power by factor of 32!

Typically use sizing heuristics

Air Handling Units (AHU)

Delivers air to zonesHeats and cools airOften integrates ventilation


AHU Configurations

Fans

Centrifugal Fan Axial Fan


Coils

Filtration


Mixing Dampers

Control airflow rates of outdoor and recirculated airMix air streams

Uniform temperatureUniform concentration

Pressure Control

Hot and Cold Water Distributrion

Deliver heat to the AHUsRemove heat from the AHUs


Hot and Cold Water Distribution

System Configuration


Pumps

Energy Efficient Design

Low pressure drop in piping and fittingsHigh efficiency motors and pumpsVariable speed pumpingProperly sized two-way valves


OverviewSystem DescriptionSecondary HVAC Systems

Air distributionRoom diffusers and air terminalsDuct DesignFan characteristicsAir Handling Units

Water distributionCooling coilsPipes and pumps

Primary HVAC SystemsElectric chillers

Air and water cooledCompressor technologiesPerformance

Thermal chillersAbsorptionEngine-driven

Cooling towersOverall Design Process

Central HVAC Plants


Chillers in Central HVAC Plants

Chillers


Electric Chillers

Expansion ValveEvaporatorCondenser

Air cooledWater cooled

CompressorReciprocatingScrollScrewCentrifugal

Evaporator

Refrigerant to water heat exchangerTypically water in tubes, refrigerant on shell-sideUsually designed for constant water flow


Condensers

Air cooledPlate fin heat exchangerMultiple fans for capacity control

Water cooledShell and tube heat exchangerCooling tower

Reciprocating Chiller

Used to be common at relatively small capacitiesMore recently displaced by scroll and screw compressorsControl capacity with cylinder unloading


Scroll Chiller

Small capacities < 50 tonsMost common in smaller air conditioners and packaged unitary equipmentTypically no capacity modulation

Screw Chiller

Medium capacities, 30-500 tonsRelatively low speed, direct driveCapacity modulation using slide valve or variable speed drive


Centrifugal Chiller

Large capacities, 200-2500 tonsHighest efficienciesOften improved cycle efficienciesCapacity control with inlet vanes or VSD

Chiller Performance

Larger chillers are more efficientCapacity and efficiency increase when compressor lift (pressure differential) is reduced

Higher evaporator temperatureLower condenser temperature


Part Load Performance

0

0.5

1

1.5

2

2.5

3

3.5

0 100 200 300 400 500 600

Energy Input Ratio, kW/Ton, Power per Capacity

Efficiency degrades at part load

Load, Tons

Absorption Chillers

Compressor replaced by pumpAbsorb refrigerant in other liquidPump liquid to higher pressureUse heat to drive refrigerant from solution


Absorption Chillers

No fluorocarbonsEnergy source can be waste heatRelatively low efficiency, COP = 0.6Risk of crystallizing solution

Cooling Tower


Cooling Tower Types

Open towerUse tower water in condenserWater treatmentClosest approach

Closed towerClosed condenser water loop with heat exchangerLess maintenance

Cooling Tower

Reject heat from warm condenser water to outdoor air using evaporation


1819202122232425262728

Temperature

Cooling Tower Concepts

Evaporative process allows heat rejection to temperature below outdoor dry bulbCondenser return water approaches outdoor wet bulbApproach = Tcw,out – Twb

Range = Tcw,in – Tcw,out

Twb,ent

Twb,lvg

Tcw,sup

Tcw,ret

Ran

geA

ppro

ach

Conde

nser

Wate

r

Tower Air Wetbulb

Energy Efficient HVAC Design

High efficiency componentsChillerTower FansPumpsMotors

Design for part load conditionsMultiple chillers, towers, pumpsVariable speed drivesControl system to monitor and adjust operation

Question heuristic design criteria

hvac design overview

Documents

winter hvac system

distributed system

air communication

split systems

space hvac needs

large equipment

benefits of central

zone rooms