hvac vavselect-90min part1

7/28/2019 HVAC Vavselect-90min Part1

1/20

Selecting VAV Terminals for

Acceptable IAQ

Selecting VAV Terminals forSelecting VAV Terminals for

Acceptable IAQAcceptable IAQ

Dan Int-HoutChief Engineer, Krueger

Richardson, Texas


2/20

Where We Are Today:Where We Are Today:


3/20

The Rules


4/20

Air Distribution DeviceAir Distribution Device

Selection GuidelinesSelection Guidelines

The ASHRAE fundamentals handbook, chapter 20 (AirDistribution), provides guidance on several methods of airdistribution.

Methods include overhead fully mixed, as well as fullystratified and partially mixed systems from below, and even

task /ambient personal air delivery systems. Described delivery systems include constant and variable

volume, UFAD, displacement ventilation and chilled beamsystems.

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


5/20

Well Mixed system design


6/20

Air Distribution DeviceAir Distribution Device

Selection GuidelinesSelection Guidelines

For overhead well mixed systems, one shouldselect a unit with throw at max, and minimum, flowthat meets ADPI guidelines based on diffuserspacing and T50 (throw to 50 fpm).

Additionally, select for maximum mixing:

Noise can be good.

Dirt on the ceiling is not bad.

Air Distribution Effectiveness (ADE) is a new term

describing room air mixing, and is a parameterwith all delivery methods.


7/20

ADE (Ez)ADE (Ez)

Air Distribution Effectiveness replaces ventilationefficiency and other terms.

Older ASHRAE 62-99 minimum outside air quantitieswere based on an Ez = 1.

Tests using ASHRAE 129 show that with ceilingcooling systems, Ez always = 1.

Heating system performance, however, can reduce Ezto less than 70% (as low as 20%).

Current Std 62 (from addendum N) wording sets Ez =0.8 when heating when rules not followed.


8/20

POORLY ADJUSTED / SELECTED DIFFUSER

THERMOSTAT

Air Distribution, Poor PatternAir Distribution, Poor Pattern

ExampleExample


9/20

Air Distribution, Poor PatternAir Distribution, Poor Pattern

ExampleExample

THERMOSTAT

POORLY ADJUSTED / SELECTED DIFFUSER


10/20

PROPERLY ADJUSTED DIFFUSER

THERMOSTAT

Good Pattern ExampleGood Pattern Example


11/20

ADPI Air DiffusionADPI Air Diffusion

Performance IndexPerformance Index Single Number Rating: % of points in a room that meet a

comfort criteria based on temperature differences andairspeeds.

In practice, a measure of well mixed.

In ASHRAE fundamentals since 1967.

Agrees with ASHRAE & ISO comfort requirements.

Can be predicted from a table in ASHRAE fundamentals,Chapter 20.

Can be measured using ASHRAE 113.

Selection to achieve acceptable ADPI was first recommended inreview draft of std 62-89, and is in the current ASHRAE

Fundamentals handbook, Chapter 20. ADPI is the ONLY means of documenting compliance to

Standard 55 in the design phase.


12/20

ASHRAE Journal, 2004ASHRAE Journal, 2004


13/20

2 x L2 x L

(Separation)(Separation)

Area Conditioned by DiffuserArea Conditioned by Diffuser

44--way Diffuserway Diffuser

SelectionSelection


14/20

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.

1/2UnitSeparationDis

tance

420350

300

250

160

Range

Spacing for 80% ADPI

NC=35

CFM


15/20

Prism 24x24

0

4

8

12

16

20

24

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6

cfm/SqFt

1

/2

U

nitS

e p a ra tio n

Dis

ta nc e ,L

600

545

436

273

109

10in. Neck

NC=37


16/20

ADPI and LEED


17/20

THERMOSTATCold

Outside

Window

Common Overhead Heating Design


18/20

Perimeter Considerations: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 8Fdelta-t @ 1 cfm/sq.Ft.


19/20

Perimeter Considerations:Perimeter Considerations:

From 2009 ASHRAE Fundamentals, Chapter 20:

Diffusers should be located such that the published 150 fpm

isothermal throw (which is typically unaffected by t) extends

to within 4.5 ft of the floor.

According to ASHRAE Standard62.1, if throw does not meet

this requirement, and the discharge-to-room temperature

differential exceeds 15F, the ventilation rate must be increased

by 25%. Furthermore, when the room-to-discharge differential

exceeds 15F, it is unlikely that the vertical temperature

limitation of ASHRAE Standard55 will be met.


20/20

ASHRAE Standard 62.1ASHRAE Standard 62.1

Table 6.2Table 6.2

hvac vavselect-90min part1

Documents