Effects of Altitude - PDH Source · • Gage pressure at altitude has a lower operating pressure...

Copyright Carrier Corporation 2001

Technical Technical Program

Effects of

Altitude

The Carrier Technical

Training World of HVAC

OUTLINE• INTRODUCTION

• REVIEW OF ENGINEERING GUIDE

• CENTRIFUGAL FANS

• DERATION METHOD

• DIRECT DRIVE FANS

• ALTITUDE VS. ARID CLIMITE

• CONCLUSION

REVIEW OF ENGINEERING GUIDE

• Carrier’s Engineering Guide for

Altitude Effects

• Published 1967

• The first thing I read in 1991

• Effects of altitude.pdf

Three Effected Areas

• Psychrometric Properties

• Temperature Levels of Steam Coils

• Air Density

• All effected by increases in altitude

Psychrometric Air Properties

• Enthalpy Values

• Increases at higher altitudes

• Dew Point

• Decreases at higher altitudes

• Specific Humidity

• Increases at higher altitudes

• Different Psychrometric Charts are

available for various altitudes

2,500 Ft Altitude

5,000 Ft Altitude

7,500 Ft Altitude

Steam Heating Coils

• Higher altitudes reduces heating capacity

• Actual operating pressure is lower at higher

altitudes for the identical gage pressure

• Saturated temperature is therefore lower at

higher altitudes for identical gage pressures.

• Heating coil’s heating performance is therefore

lower for identical gage pressures

Air Density

• Air density is lower at higher altitudes

• Most common property effected on HVAC

equipment

• Effect directly correlates to air density

• Table is published on page 12

Air Density Ratio Table

Air Density

• HVAC systems depends on heating and

cooling the air molecules through the coil or

heat exchanger,

• Higher altitudes have less mass (air

molecules) per CFM

• To provide the same capacity as sea level,

the CFM be increased at a rate inversely

proportional to the ADR

• Many times that is not possible

A Practical Guide

• Carrier’s Engineering Guide For Altitude

Effects is a practical guide to determine the

effects (typically negative effects) of

altitude on HVAC systems you are

designing.

Centrifugal Fans

• Air density – Yes• RPM Constant (Direct Drive)

» Airflow volume is reduced

» BHP of motor is reduced

• RPM Increased (Belt drive)

» Airflow can be adjusted to equal volume flow as SL

» BHP of motor is reduced

» Divide SL TSP by ADR to calculate equivalent TSP

» Look up in fan curve RPM & BHP. This is the actual RPM

» Multiply BHP by ADR for actual BHP required

• Psychrometrics – No

• Steam Heating - No

Direct Drive

Furnace/Fan Coil Fan

Belt Drive

Direct Drive

Plenum Fan w/ VFD

Centrifugal Fans• Fan performance/selection at

altitude overview

• Determine the air density ratio

• Divide required TSP by ADR

• Look up fan performance at this

pseudo point

• This determines the required RPM

• Multiply BHP at pseudo point by

ADR to get actual BHP

• ..\2011\AHU-5000 CFM.pdf

Centrifugal Fan Selection

Centrifugal Fans• Volume (CFM) Vs. Mass flow

(Lbs/min)

• Previous example was 5000 CFM

• At 6,000 ft altitude, RPM was 1,309

• At sea level, RPM needed was 1,180

• At 6,000 ft, 1,180 RPM can not achieve

3.0” (5,000 CFM @ 2.4”)

• At 1,309 RPM

• Volume flow is 5,000 CFM

• Mass flow is equal to 4,000 CFM @ SL

Effects of lower mass flow

• Pros

• Lower BHP to deliver the same volume

• Less energy required to meet required

ventilation rates Getting away with

murder)

• Cons

• Lower delivered cooling capacity of

» Especially chilled water coils

• Lowers suction temperatures

» Freezing coils

» Wasted energy on latent capavcity

Direct Drive Fans

• Direct drive fans at altitude really hurt

• High speed is high speed

• Lowers Volumetric flow (CFM)

• Double Wammy when taking into account Air

Density Ratio (1400 CFM sl on 5 ton)

• Oversize blowers when possible

• Rooftops can’t be done

• Split systems up to 4 tons can (2 ton ACU / 2.5

ton furnace)

Effects on Compression

• Air Density – No

• Steam Heating – No

• No Effects

Water Cooled Condensers

• No Effects

Air Cooled Condensers

• Air Density – Yes

• Typically prop fans

• Airflow is held constant

• Air mass is decreased @ higher altitudes

• Performance is reduced

Evaporative Condensers

• Psychrometrics – Yes

• Typically Prop Fans

• Enthalpy increases at altitude

• An increase in performance is caused

Evaporative Condensers

Chiller Barrells

• No Effects

Absorption Chillers

• Steam Heating – Yes

• Gage pressure at altitude has a lower operating

pressure compared to identical gage pressure at

seal level

• Lower actual pressure gives lower performance

• Typically corrected by setting at higher gage

pressure.

Absorption Chillers

Air Cooled Condensing Units

• Compressor / Condenser cross plot reduces the

effects compared to a straight condenser

Air Cooled Condensing Units

Air Cooled Chillers

• Compressor / Condenser cross plot reduces the

effects compared to a straight condenser

Air Cooled Chillers

Chilled Water Coils

• Centrifugal fans can be adjusted to deliver

identical airflows

• Face areas of coils typically limit increasing

CFM (Increasing to published max helps)

• Coil ratings typically are done by comparing

identical air mass rates for altitude

• Multiply the altitude CFM by the ADR to

obtain seal level CFM rates

• Obtain coil performance from sea level

tables/charts

Chilled Water Coils

• If coil performance includes significant

dehumidification, higher altitudes will release

more moisture and increase the capacity of the

cooling coil performance.

• Less CW flow is required to obtains the same

SL performance.

• If coil is 95% sensible (like our climate), CW

flows are identical

Chilled Water Coils

DX Coils

• Centrifugal fans can be adjusted to deliver

identical airflows

• Face areas of coils typically limit increasing

CFM (Increasing to published max helps)

• Coil ratings typically are done by comparing

identical air mass rates for altitude

• Multiply the altitude CFM by the ADR to

obtain seal level CFM rates

• Obtain coil performance from sea level

tables/charts

DX Coils

• If coil performance includes significant

dehumidification, higher altitudes will release

more moisture and increase the capacity of the

cooling coil performance.

• Less CW flow is required to obtains the same

SL performance.

• If coil is 95% sensible (like our climate), CW

flows are identical

DX Coils

Cooling Coil Face Velocities

• Less air molecules to cause condensate water

carry-over

• Increases the published maximum air velocities

Cooling Coil Face Velocities

Steam Coils

• Air Density – Yes• Centrifugal fans can be adjusted to deliver identical

airflows

• Air mass is reduced

• Capacity is reduced

• Steam Heating – Yes• Gage pressure at altitude has a lower operating pressure

compared to identical gage pressure at seal level

• Lower actual pressure gives lower performance

• Typically corrected by setting at higher gage pressure.

Steam Heating Coils

Hot Water heating Coils

• Centrifugal fans typically can be adjusted to

give identical airflow

• Face area of coils typically limit increasing

• Less mass flow reduces capacity

• Coil ratings obtained by using correction factor

on performance at seal level

Hot Water heating Coils

Electric Heaters

• Capacity is not reduced since the capacity is

based on power consumption of the heating

element (Higher LAT’s)

• Minimum published airflows must be increased

to avoid tripping thermal overloads

Electric Heaters

Min Airflow = published min / ADR

Gas Heat Exchangers

• Heat exchanger and flue systems are designed

for constant air volume

• Lower air density causes reduction in

combustible air at higher altitudes

• Reduces capacity 4% / 1,000 FT of elevation

Direct Combustion Gas Heat& Power Burners

• Direct Combustion MUA’s and Power Burner

Boiler system state that they do not have any

reduction in output capacity at higher elevations.

• This is true if the natural gas is not de-rated by the

utility company – also called ‘Hot Gas.”

• Most utility companies derate their gas, so

installing contractors do not need to change the

gas orifices.

• Since they do derate the gas, the same 4%/1000 ft

should be used for direct fired and power burned

applications.

Room Fan Coils

• Typically direct drive centrifugal fans – Air

volume is decreased!

• Air mass is decreased (lower CFM & ADR)

DXRoom Fan Coils

CW Room Fan Coils

Package Rooftops

• Air Density – Yes• Centrifugal fans typically can be adjusted to give

identical airflow

• Face area of coils typically limit increasing CFM.

• Cooling ratings are typically done by comparing

identical air flow rates for SL at Altitude

• Obtain coil performance from sea level tables/charts

and multiply by correction factors for total and sensible

capacity

Package Rooftops

Air Friction Loss

• Reduces friction loss at same velocities

• Multiply ACFM by ADR to determine SCFM

• Look up SL friction loss in SL table

• The SL friction loss at the SCFM is the actual

loss for the ACFM at altitude

Effects of Altitude - PDH Source · • Gage pressure at altitude has a lower operating pressure...

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