hvac duct calculator for engineers

8/10/2019 HVAC Duct Calculator for Engineers

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1Taylor Engineering, LLC

HVAC System DesignMark Hydeman, P.E., FASHRAETaylor Engineering, LLC

[email protected]


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it takes 2,000 to 3,000 times the

volume of air to cool what you can

with water!

How do you effectively fight a fire?

With air, or with water?


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State of the present: with air


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Air system design overview

Data center layout

Airflow configurations

Distribution: overhead or underfloorControl: constant or variable volume

Airflow issues

Economizers Humidity control issues


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Server airflow front toback or front to back andtop are recommended

2004, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Reprinted by permission from

ASHRAE Thermal Guidelines for Data Processing Environments. This material may not be copied nor distributed in either paper or digital form

without ASHRAEs permission.

Data center layout

Cold Aisle

Hot Aisle


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Data center layout




Underfloor Supply

Cold Aisle

Hot Aisle

Only 1 pressurezone for UF!


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Data center layout




Overhead Supply

Cold Aisle

Hot Aisle

You canincorporate VAVon each branch


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Elevation at a cold aisle looking at racks

Typical temperature profile with UF supply

Too hot Too hot

Just right

Too cold

There are numerous references in ASHRAE. See for example V. Sorell et al; Comparison of

Overhead and Underfloor Air Delivery Systems in a Data Center Environment Using CFD

Modeling; ASHRAE Symposium Paper DE-05-11-5; 2005


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Elevation at a cold aisle looking at racks

Too warm Too warm

Just right

Typical temperature profile with OH supply


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Aisle capping




Cold Aisle Caps

End cap

Hot aisle lid

APC reprinted with permission

Cold Aisle

Hot Aisle


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Aisle capping




Cold Aisle Caps

LBNL has recently performed

research on aisle capping

Cold Aisle

Hot Aisle


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Overhead (OH) vs. Underfloor (UF)

Issue Overhead (OH) Supply Underfloor (UF) Supply

Capacity Limited by space and aisle velocity. Limited by free area of floor tiles.Balancing Continuous on both outlet and branch. Usually limited to incremental changes by

diffuser type. Some tiles have balancing

dampers. Also underfloor velocities canstarve floor grilles!

Control Up to one pressure zone by branch. Only one pressure zone per floor, canprovide multiple temperature zones.

Temperature

Control

Most uniform. Commonly cold at bottom and hot at top.

First Cost Best (if you eliminate the floor). Generally worse.

Energy Cost Best. Worst.Aisle Capping Hot or cold aisle possible. Hot or cold aisle possible.


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Airflow design disjoint

IT departments select servers and racks

Engineers size the fans and cooling

capacity

Whats missingin this picture?


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Airflow with constant volume systems

Hot spots Higher hot aisle

temperature

Possible equipment

failure or degradation

ServersSupplyHVAC VV _


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Least hot spots Higher air velocities

Higher fan energy

Reduced economizer

effectiveness (due tolower returntemperatures)




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Note most of these observations apply tooverhead and underfloor distribution

With constant volume fans on the serversyou can only be right at one condition ofserver loading!

The solution is to employ variable speedserver and distribution fans



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Partial flow condition

Best energy performancebut difficult to control


Airflow with variablevolume systems


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How Do You Balance Airflow?

Spreadsheet

CFD

Monitoring/SiteMeasurements

Image from TileFlow

http://www.inres.com/Products/TileFlow/tileflow.html,

Used with permission from Innovative Research, Inc.


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Thermal report

From ASHRAEs Thermal Guidelines for Data Processing Environments


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Whats the server airflow?

SUN SUN DELL DELLV490 V240 2850 6850

num fans 9 3 n/a n/atotal CFM (max) 150 55.65 42 185total CFM (min) 27 126fan speed single speed variable 2 speed 2 speedfan control n/a inlet temp. 77F inlet 77F inletForm Factor (in U's) 5 2 2 4heat min config (btuh) 798 454heat max config (btuh) 5,459 1,639 2,222 4,236heat max (watts) 1,599 480 651 1,241dT min config - 13 - 3dT max config 33 27 48 21servers per rack 8 21 21 10CFM/rack (hi inlet temp) 1,200 1,169 882 1,850CFM/rack (low inlet temp) 1,200 567 1,260max load / rack (kW) 13 10 14 12


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Best air delivery practices Arrange racks in hot aisle/cold aisle configuration Try to match or exceed server airflow by aisle

Get thermal report data from IT if possible Plan for worst case

Get variable speed or two speed fans on servers if possible Provide variable airflow fans for AC unit supply

Also consider using air handlers rather than CRACs for improvedperformance (to be elaborated on later)

Use overhead supply where possible Provide aisle capping (preferably cold aisles, refer to LBNL

presentation for more details) Plug floor leaks and provide blank off plates in racks Draw return from as high as possible Use CFD to inform design and operation


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Air-side economizer


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Air-Side Economizer issues

Hygroscopic dust

LBNL is doing some research on this

Design humidity conditionsSee following slides


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Design conditions at the zone


ASHRAE Design Considerations for Data and Communications Equipment Centers. This material may not be copied nor distributed in either

paper or digital form without ASHRAEs permission.


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San Francisco

BAROMETRIC PRESSURE: 29.904 in. HG

PSYCHROMETRICCHARTNormal TemperatureI-P Units

16 FEET

Weather Hours

360 to 321

320 to 281

280 to 241

240 to 201

200 to 161

160 to 121

120 to 81

80 to 41

40 to 1

30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

DRY BULB TEMPERATURE - F

.001

.002

.003

.004

.005

.006

.007

.008

.009

.010

.011

.012

.013

.014

.015

.016

.017

.018

.019

.020

10

15

20

25

30

35

40

Chart by: HANDS DOWN SOFTWARE, www.handsdownsoftware.com

15%

25%

10%RELATIVEHU

MIDITY

20%

30%

40%

50%

60%

70%

80%

90%

30

35

35

40

40

45

45

50

50

55

55

60

6065

65 70

7075WETBULBTEMPERATURE-F

75

80

13.0

14.0VOLUME-CU.FT.PERLB.DRYAIR

H

UMIDITYRATIO-

POUNDSMOISTUREPER

POUND

DRYA

IR

Class1;Allow

NEBS;Recommend

Class1;Recommend

San Francisco Climate Data Binswith Data Center Guideline Zones



16 FEET


X

Design Target

Upper Allowed Humidity Limit

Lower Allowed Humidity Limit (20%RH)

Negligible time of possibleconcern for humidification


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Los Angeles



105 FEET

Weather Hours

396 to 353

352 to 309

308 to 265

264 to 221

220 to 177

176 to 133

132 to 89

88 to 45

44 to 1

30 35 40 45 50 55 60 65 70 75 80 85 90 95 100


.001

.002

.003

.004

.005

.006

.007

.008

.009

.010

.011

.012

.013

.014

.015

.016

.017

.018

.019

.020

10

15

20

25

30

35

40


15%

25%

10%RELATIVEHUM

IDITY

20%

30%

40%

50%

60%

70%

80%

90%

30

35

35

40

40

45

45

50

50

55

55

60

6065

65 70


75

80

13.0

14.0VOLUME-C

U.FT.P

ERLB.D

RYAIR

H

UMIDITYRATIO-

POUNDSMOISTUREPER

POUND

DRYAIR

Class1;Allow

NEBS;Recommend

Class1;Recommend

Los Angeles Climate Data Binswith Data Center Guideline Zones



105 FEET


X

Design Target



Only a few hours of possibleconcern for humidification


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26 FEET

Weather Hours

270 to 241

240 to 211

210 to 181

180 to 151

150 to 121

120 to 91

90 to 61

60 to 31

30 to 1

30 35 40 45 50 55 60 65 70 75 80 85 90 95 100


.001

.002

.003

.004

.005

.006

.007

.008

.009

.010

.011

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.020

10

15

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25

30

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15%

25%

10%RELATIVEHU

MIDITY

20%

30%

40%

50%

60%

70%

80%

90%

30

35

35

40

40

45

45

50

50

55

55

60

6065

65 70


75

80

13.0

14.0VOLUME-CU.FT.PERLB.DRYAIR

H

UMIDITYRATIO-

POUNDSMOISTUREPER

POUND

DRYA

IR

Class1;Allow

NEBS;Recommend

Class1;Recommend

Sacramento Climate Data Binswith Data Center Guideline Zones



26 FEET


Sacramento

X

Design Target



Negligible time of possibleconcern for humidification


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Lower humidity limit

Mitigate electrostatic discharge (ESD) Recommended procedures

Personnel grounding Cable grounding

Recommended equipment Grounding wrist straps on racks Grounded plate for cables Grounded flooring Servers rated for ESD resistance

Industry practices Telecom industry has no lower limit The Electrostatic Discharge Association has removed humidity control as a primary

ESD control measure in their ESD/ANSI S20.20 standard

Humidity controls are a point of failure and are hard to maintain Many data centers operate without humidification This needs more research

And for some physical media (tape storage, printing and bursting) Old technology not found in most data centers It is best to segregate these items rather than humidify the entire data center


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ESD control: floor grounding

Image from Panduit, reprinted with permission

W t Sid E i


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Water-Side Economizer

Integrated

Heat

Exchanger in

series with

chillers on

CHW side


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Economizer Summary

Air-Side Economizers Provides free cooling when

dry-bulb temperatures arebelow 78F-80F.

May increase particulates(LBNL research indicates thisis of little concern).

Should be integrated to bemost effective.

Improves plant redundancy! Can work in conjunction with

water-side economizers ondata centers!

Need to incorporate relief.

Water-Side Economizers Provides low energy cooling

when wet-bulb temperaturesare below 55F-60F.

Avoids increased particulates(and low humidity if thatconcerns you).

Should be integrated to bemost effective (see previousslide).

Improves plant redundancy! Can work in conjunction with

air-side economizers on datacenters!

Both are proven technologies on data centers!


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A case study of two designs

Collocation facility in theBay Area

Side by side designs in

same facility over twophases

Motivation for the seconddesign was to reducecost

Case study wasdeveloped by LawrenceBerkeley National

Laboratory (LBNL) Data Centers 8.1 and 8.2

Both sections at ~30%build-out during

monitoring


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A tale of two designs: overview

Phase 1 Data Center (8.1)

26,200 ft2

27 W/ft2 design

Traditional under-floor designwith CRAC units

Air-cooled DX

Humidity controls (45%-55%)


73,000 ft2

50 W/ft2 design

Under-floor supply fromcentral AHUs with CHW coils

Water-cooled plant

Air-side economizers

No humidity controls


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A tale of two designs: a closer look

Normalized efficiency metric:servers

systemscooling

coolingkW

kW_

Data normalized to computer loads

-

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

Normalizedenergy

Computer Loads UPS Losses HVAC Lighting

~1/4 of the

normalized energy




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A tale of two designs: results


Around 2x the HVACinstalled cost ($/ft2)

Around 4x the energy bills(when normalized to serverload)

Acoustical problems

Higher maintenance costs

Lost floor space in datacenter due to CRACs


Preferred by the facilityoperators and data centerpersonnel


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Two data centers: summary

What made the difference?

Airside economizers

No humidity controlsWater-cooled chilled water system

AHUs instead of CRAC units


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Custom CRAH Unit (Large)


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Example CRAH Unit Comparison

Option 1

Model Std CRAC Custom Model 1 Custom Model 2Budget Cost 16,235$ 23,000$ 41,000$Number of units 21 13 4

net total cooling (btuh) 434,900 410,000 841,000net sensible (btuh) 397,400 399,000 818,000

sensible (tons) 33.1 33.3 68.2CFM 16,500 25,000 50,000

SAT 49.90 59.30 59.00airside dT 25.10 15.70 16.00

Internal SP 2 0.8 0.8 1.8 1.8no. fans 3 3 2

fan type Centrifugal Plenum Plenumno. motors 1 3 2

HP/motor 15 5 15total HP 15 15 30

BHP/motor 15 4.7 11.5Unit BHP 15 14.1 23

unit width 122 122 122

depth 35 36 72height 76 156 168filter type ASHRAE 20% MERV 13 MERV 13Water PD (ft) 13.5 ft 11.1 11.1

CHW dT 14F 20 20GPM 66.80 44.00 88.00

Total GPM 1,403 924 66%Total BHP 315 275 87%

Option 2


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34% less water flow 13% less fan energy

More if you consider the supply air temperature and airflow issues

Excess fan capacity on new units

36% higher cost for units, but Fewer piping connections Fewer electrical connections Fewer control panels No need for control gateway Can use the existing distribution piping and pumps (case study) Can use high quality sensors and place them where they make sense

Possibly less turbulence at discharge?

Example CRAH Unit Comparison


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Air cooling issues

Limitations on the data densities served (~200w/sf) Air delivery limitations Real estate

Working conditions Hot aisles are approaching OSHA limits Costly infrastructure High energy costs Management over time

Reliability Loss of power recovery Particulates


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Take Aways

Use air- or water-side economizers where possible Consider personal grounding in lieu of humidification Consider AHUs as an alternative to CRACs Consider VSDs on fans, pumps, chillers and towers Refer to ASHRAE, LBNL and Uptime Institute for more

recommendations


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State of the future: cooling with liquid

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