hvac vav select-ash

7/28/2019 HVAC Vav Select-Ash

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Selecting VAV Terminals for

Acceptable IAQ

Selecting VAV Terminals for

Acceptable IAQ

Dan Int-HoutChief Engineer, Krueger

Richardson, Texas


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Where We Are Today:


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The Rules


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Air Distribution Device

Selection Guidelines The ASHRAE fundamentals handbook, chapter 20 (Air

Distribution), provides guidance on several methods of air

distribution. Additional guidance can be found in the recently updated Air

Distribution sections in Applications and SystemsHandbooks.

Air distribution methods include overhead fully mixed, as well

as fully stratified and partially mixed systems from below, andeven task /ambient personal air delivery systems.

Described delivery systems include constant and variablevolume, UFAD, displacement ventilation and chilled beamsystems.

All have advantages and disadvantages, which must beunderstood by the design engineer and architect.


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Air Distribution Device

Selection Guidelines

For overhead well mixed systems, one should select a unitwith throw at max, and minimum, flow that meets ADPI

guidelines based on diffuser spacing and T50 (throw to 50fpm).

Additionally, select for maximum mixing: Noise can be good. Dirt on the ceiling is not bad.

Air Distribution Effectiveness (ADE) is a new termdescribing room air mixing, and is a parameter with alldelivery methods.

There is increasing evidence that typical loads in buildingsare far below those that used to require 1 cfm/sf in the past.

I suspect most spaces only require about 0.4 cfm/sf, or less. Good design will require fewer diffusers, further apart, in

order to provide adequate performance at these lower airflow rates.


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Air Distribution, Poor Pattern

Example

THERMOSTAT

POORLY ADJUSTED / SELECTED DIFFUSER


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PROPERLY ADJUSTED DIFFUSER

THERMOSTAT

Good Pattern Example


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Perforated 24X24, 10 inlet, 4 way, 20 Delta-T

2

4

6

8

10

12

14

16

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

CFM/Sq.Ft.

1/2UnitSepara

tionDistance

420

350

300250

160

Range

Spacing for 80% ADPI

NC=35

CFM


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ADPI and LEED Standard 55 requires air speed 80%, roomaverage air speed will be < 40fpm, andvertical stratification will be


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ADPI and LEED


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THERMOSTATCold

Outside

Window

Common Overhead Heating Design


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Perimeter Considerations:

Maximum delta-t for effective mixing when heating,per ASHRAE handbook = ?.

= 15F (90F discharge), continuous operation.

Throw toward and away from glass.

150 FPM should reach 4-5 feet from the floor.

ASHRAE 62.1 requires that ventilation be increasedby 25% when heating, if the above rules are notfollowed.

Typical perimeters require only 8F

delta-t @ 1 cfm/sq.Ft.


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Air Distribution Summary Diffuser selection should be based on throw and

room size.

Select at both max and minimum occupied flowrates. Rates are likely less than 0.5 cfm/sf.

Noisy diffusers are good.

Dirt on the ceiling aint all bad.

Use good design when heating.

Poor air distribution yields poor comfort, poor airquality, slow system response.


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Thermal Comfort:

Latest THERMAL COMFORT STANDARD:

ASHRAE 55-2010 ASHRAE Fundamentals, Chapter 7

PMV - predicted mean vote - a single number

rating. Standard 55 mandates a maximum 5oF vertical

temperature stratification.

A program is available, based on a programlisting in ASHRAE 55, which allows plotting ofthe comfort envelope.


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Graphical Method, ASHRAE 55-2010


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Typical Executive Worker

Profile


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ASHRAE J ournal, J une 2008

Comfort Economics


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Design Issues:Diffuser Selection

Perimeter

Thermal Comfort

Acoustics

Ventilation & IAQ

LEED 09 ASHRAE Std. 55 Checklist


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Comfort Summary Thermal comfort is #1 reason for not renewing the

lease (dont fix the wrong thing).

Many IAQ and air distribution complaints can becured with temperature changes. The ratio of productivity to energy savings is

2000/1. First costs are minor compared toproductivity.

Internal heat loads cost 2X power consumption. Mandated designs and set points often waste

energy. People will make themselves comfortable!

Vertical Stratification requirement is often ignored Thermal comfort program available online!


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Acoustics:

AHRI 885-08 acoustical application standard.

AHRI 880-08 air terminal test standard. AHRI 260-01 ducted equipment except air terminals.

ASHRAE 70, air diffuser performance.

Acoustical quality suggests the use of RC (or newer

measures) rather than NC. Many acousticians areheading back to dBA!

LEED 2012 will include acoustical credits andrequirements.

A new ruling by AHRI will change everyonesdischarge sound ratings considerably.


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End Reflection Low frequency sound traveling down a duct willpartially reflect back when encountering a rapid

change in area. The smaller the duct, the greater the effect.

It can be as much as 10dB at 125Hz. It is much

less at higher frequencies. Since NC is usually set at 125Hz, reported NC

can go up as much as 10NC.

Most importantly, Specifying Engineers shouldbe modifying their discharge soundrequirements to reflect the new data.


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End Reflection


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End Reflection An end reflection happens when air travels from a main

to a branch duct, and especially when it enters a flex

duct connection. Another happens when a duct terminates at the room, at

the diffuser.

AHRI 885 includes one 8 end reflection in Appendix E.

This change only affects reported discharge soundpower levels.

Discharge sound is the same, but will be reported a bit

louder. Engineers will need to update their discharge sound

specifications.


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Sound Specifications

Should be based on clearly stated assumptions.

Should reflect real project needs, not anymanufacturers data and use currently accepted

application factors. Discharge sound specs need to be modified to account

for the louder reported sound power.

If duct lining is used requireNC shall be determinedin accordance with AHRI 885-08, Appendix E,otherwise specify octave band sound power.

Over-silencing increases both initial costs andoperating costs, and may hinder proper IAQperformance.


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Classroom Acoustics:


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LEED 2012 Proposed criteria

LEED for schools presently gives a point for soundlevels < 45 dBA (about 36 NC).

For 2012, it is proposed that 40 dBA be aprerequisite, with a point for achieving 35 dBA.

Instead of requiring resultant sound levels, the newcriteria says that calculated HVAC system noise

shall not exceed the specified values, using eitherthe ASHRAE Handbooks (which are inadequate) orAHRI 885.


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Indoor Air Quality

Standing Standard ProjectCommittee 62.1

Residential Committee is 62.2

Current Standard is62.1-10, and all previous addendahave been incorporated (and nolonger available on the web).

Several additional Addenda havebeen approved and more are inprogress.

Addenda are now published 18months after the release of astandard.


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Terminal Units


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Single Duct

Retro-Fit Dampers

Bypass Dampers

Dual Duct

Parallel Fan Powered

Series Fan Powered

Terminal Units


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The Construction Specifier,

July 04


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Single Duct Terminal Units


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When to Use Single

Duct Terminals

Interior spaces Where only cooling is required. Heating not needed

due to climate and heating from perimeter areas.

Perimeter with reheat Cooling along outer walls with some small reheat on

the coldest days.

Interior with reheat to maintain ventilation minimums Only cooling is required but reheating for minimum

airflows

Supply return tracking Airflow measurement and control into and out of a

laboratory space.

Standard

Single Duct


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Retro-Fit Single Duct

Terminals

Retrofitting old terminal units. Mechanical constant volume dual duct dampers

for high pressure systems replaced with roundunits for measuring and controlling airflow.

Adding a control zone. Round units added to a round duct or square

ones to a square duct to measure and controlairflow in a new occupied zone monitored with

Tstat.

Replacing VVT-type zone dampers

replace existing high pressure, high velocity zone

controlled units with low pressure VAV units.

As a return damper Added to existing ductwork to measure and

control leaving airflow as required from controls.

Square Slip In Retro-Fit

Round Retro-Fit


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Dual Duct Terminal Units

Wh t U D l D t


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When to Use Dual Duct

Terminals

Perimeter Heating & Cooling

Positive ventilation control (measuredventilation to the zone)

Laboratories / Constant volume applications

Supply / Return tracking

Underfloor Air Conditioning Be sure you understand the mixing / control

requirements

Upstream vs. downstream flow sensing issues

N Mi i D l D t


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Non-Mixing Dual Duct

No Mixing

Two single duct Units J oined S&D Discharge

Inlet Sensing Only

Standard Dual Duct

2 Single

duct units

Joining Strip

Mi i D l D t


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Mixing Dual Duct

Attenuated Dual Duct

May have inlet or total flowdischarge sensing

Unequal inlet sizecombinations are available

Many types of linings are

available Mixing baffle can have up to

20:1 Mixing Ratio. Typically have high minimum

inlet pressure requirementPremium Dual Duct


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Opportunities for Dual Duct

Systems Dual Duct can meet the most stringent

requirements for classroom acoustics, aswell as 90.1 requirements.

Providing 100 OA (DOAS?) through oneinlet allows validation of ventilation rates

Supplemental heating and even sensible

cooling coils are being considered in somedesigns.


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Fan Powered Terminal Units

When to use fan powered


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When to use fan powered

terminals Originally designed for perimeter heating and

cooling - to avoid reheat

Constant volume applications have a fan nearthe discharge - series boxes

Series Boxes provide relatively constantinterior sound levels and air flow rates and areoften used in interior zones.

Because of energy concerns, the ECM motor

has been employed in many applications. Texas A&M recently completed a study,

donated to ASHRAE and AHRI at the summermeeting in Montreal, on Series vs. Parallel, andECM vs PSC motors.


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The use of ECM motors can reduce the true energyconsumption of Fan Powered Terminals

An ECM motor is a brushless DC motor, with an

integral computer controlled inverter. At reduced rpm and load, an ECM motor can use as

little as 60% of the watts of a standard Permanent Split/ Capacitor Start (PSC) motor

An ECM motor can be programmed to be pressureindependent, resulting in true constant volumeperformance, easier balancing, and even DDC

controlled performance.

ECM Motors


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Some Cautions, however:An ECM motor can add $350 to the purchase

price of a Fan Powered Terminal.Power consumption CAN be greater than anequivalent PSC motor, and the connected load

requirement is usually higher because of this.There are issues with the power factor of thesetypes of devices which may cloud true energy

savings.

ECM Motors


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Series Fan Boxes

SERIES FAN POWERED BOXES

Fan is on discharge of unit - all air

goes through the fan (primary airflowcannot exceed fan airflow)

Fan runs at all times during occupancy

Fan must be started before primary airis supplied.

Note: there is no such thing as an anti-backward rotation device.

Radiated sound is critical soundapplication factor

Parallel Fan Boxes


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Parallel Fan Boxes

PARALLEL FAN POWERED BOXES

Fan is in parallel with VAV damper Fan airflow independent of primary

airflow

Fan is essentially first stage of heat Backdraft damper is standard

Discharge sound is criticalapplication factor


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Texas A&M Study Results

Data was generated for entire building

systems, not just terminals. Parameters were developed for detailed

full building energy analysis

Results are being presented in ASHRAEPapers, and full study results have been

donated to ASHRAE and AHRI.


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Texas A&M Study Results

The results showed that both system operation and climate had agreat effect on final energy use.

In general, however there were a few important observations:

ECM and PSC motors had the same building energy consumption withParallel boxes.

Parallel boxes could be more efficient than series boxes, except forback-draft damper leakage, which was found to be very significant.

Series boxes, with ECM motors, are the most energy efficient choice,

(at least until someone develops a leak proof parallel box). Series Boxes, with DDC system variable volume fan flow, are likely

more efficient than any other air distribution method, at low part loadoperation.


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Electric Reheat Options

120/208/240/277 single phase

208/3 phase (3-wire), 480/3 phase (4-wire)

1-3 Stages (most types) Side access - Leave clearance per NEC

Operation at minimum airflow is unlikely to provide comfort

Consider proportional electric heat with discharge temperature

control for maximum comfort

P ti l El t i R h t


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There are two types of proportional electricheat:

SCR Electric Heat Requires analog input

Somewhat expensive

Time Proportional Electric Heat 1 Stage only required

Infinite adjustment

Low cost

Silent

Allows for effective discharge temperature control

Many different control inputs

Proportional Electric Reheat


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Dedicated Outdoor Air Systems

DOAS D l D t


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DOAS Dual Duct

One inlet provides 100%outside air, dehumidified,typically cold.

Other duct provides 100%return air, either warm or cold,depending on the season.

Supplemental reheat coilsand even a sensible coolingcoil have been considered

A good mixing baffle should

be employed (20:1 MixingRatio recommended)

Mixing Dual Duct

Fresh Air Terminal Unit


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Outside air can be supplied to a Series Fan Terminal through a secondducted system.

This requires two duct systems, but separates ventilation andrecirculated air.

The system allows monitoring of ventilation rates into each zone. A stand-alone Pressure Independent controller on the DOAS inlet,tied to the fan relay, will allow standard DDC zone controls.

Fresh Air Terminal Unit

PrimaryInlet

Fresh Air Inlet

Series FanPowered

Terminal Unit

Induced Air

Inlet

Chilled Fan Powered Terminal Unit


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100% Outside air can be supplied to a Series Fan Terminal througha ducted system, designed to handle ventilation and latent loads.

A sensible cooling coil, on the induced air inlet, cools plenum air foradditional sensible space cooling.

The system allows monitoring of ventilation rates into each zone. A stand-alone Pressure Independent controller for the DOAS inlet,

tied to the fan relay, will allow standard fan coil DDC zone controls.

Both Series and Parallel designs are currently being used dependingon the installation.

Chilled Fan Powered Terminal Unit

DOAS Air Inlet

Series Fan Powered Terminal UnitInduced Air Inlet

Sensiblecooling

coil

Chilled Box In The Interior


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C

Sensible Cooling Coil

Optional MERV 8 Filter

Standard Ceiling Diffusers

Series Fan Powered

Terminal Unit

Dedicated OutdoorAir Supply

Standard Reheat Options

Chilled Box at the Perimeter


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Series DOAS Unit

InteriorChilledBeams

DOAS Air Supply

Discharge Heating Coil

Perimeter Slot Diffusers

Chilled Parallel Box in a Laboratory


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Lab Pressure Supply Valve

Non Condensing Cooling Coil

Optional Merv-8 Filter

Laboratory Diffuser

y

Chilled Parallel Unit


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Dedicated Outdoor Air Duct

Active Chilled Beams

Parallel DOAS Unit

Secondary DOAS Duct

Supplying Chilled Beams

Chilled Box Summary


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Chilled Box Summary

Low first cost

Low system pressure requirement Safe location possible

Easily reconfigurable Minimal Contractor Training

Guaranteed Performance

Several LEED points possible

S


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Summary

LEED 2009 requires meeting Standard 62.1 Documented use of ADPI is the ONLY way to assure

compliance to 55 in the design phase.

Acoustics should be specified on the basis of needwith clear assumptions. Use AHRI 885

Reheat needs to be carefully considered in terms of

discharge temperatures and velocities. Software is available to assist in selecting the best mixof products.

The rules are dynamic - pay attention.

Engineers Checklist


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Engineer s Checklist

Are Specifications current? Real Manufacturers

Current Standards referenced Do we have clear acoustical requirements:

Inlet Ps defined?

Assumptions clearly defined?

Reheat: Sufficient airflow? Discharge Static?

3 or 4 wire 480V? Contactors? Stages?

gpm or btuh? (pick one)

Linings are acoustics and cost understood?

Accessories Transformers?

Filters?

Hanger brackets?

Fuses, Disconnects?

If specified

clearly, you mayget what youwant!

If unclear, youwill get what thelow bidder wants

you to get.


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ANY QUESTIONS?


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Contact

[email protected]

www.krueger-hvac.com

EXCELLENCE IN AIR DISTRIBUTION

KRUEGER

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