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Air Mixing Handbook

How to Identify and Eliminate Air Stratification Problemswith Properly Designed HVAC Mixing Systems

Published by Blender Products, Inc.

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Section Page

1. Introduction ..............................................................2The Company ............................................................2

Why this Manual Was Written...................................2

2. Glossary of Basic Terminology ................................3

3. Discussion of Stratification and Mixing..................4

How Stratification Develops .....................................4

How to Define Mixing Effectiveness ........................4

4. Thermal-Stratification Problems and Solutions .....5

Nuisance Freeze-Stat Trips........................................5

Poor Mixed-Air Control ............................................5

Wasted Energy Due to Sensor Error..........................5

Averaging-Bulb Inaccuracy.......................................5Poor Economizer Control ..........................................7

Excessive Reheat Coil Use........................................7

Stratification Downstream of Face-and-Bypass

Sections .................................................................7

Ineffective Refrigeration-Coil Control ......................8

Stratified Steam-Coil Discharge................................8

Uneven Heating with Direct-Fired Burners ..............8

Uneven Moisture Absorption Across Humidifiers ....8

Improper Filter Loading ............................................8

5. Water-Coil Freeze-Up Problems and Solutions ......9

Glycol Solutions........................................................9

Unit Heaters in Mixed-Air Plenum ...........................9Heat Recovery ...........................................................9

Draining Water Coils ...............................................10

Pumping Fluid at Subfreezing Outside

Air Temperatures.................................................10

Static Mixing Devices .............................................10

Section Page

6. Indoor Air Quality (IAQ) Concerns.......................11Outside Air Dampers Blocked Shut to Eliminate

Stratification Problems........................................11

Dilution of Fresh Ventilation Air Into the

Supply-Air Stream...............................................11

Maintaining Minimum Ventilation Air Under

VAV Conditions...................................................11

7. Application Guidelines for Static Mixing

Devices ....................................................................12

How Much Mixing is Enough? ...............................12

VAV Concerns .........................................................12

Proper Placement of Mixers in HVAC Systems.....12

Effect of a Combination Filter/Mixing Box onMixing Performance............................................14

8. Velocity-Stratification Problems.............................15

Coil Condensate Carryover .....................................15

Coil Inefficiency......................................................15

Filter Loading, Damage, and Particle Bypass .........15

9. Application Guidelines for Velocity-Equalizing

Diffusers..................................................................16

Distance Requirements............................................16

System-Effect Considerations .................................16

Bibliography.................................................................17

1

Table of Contents

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The Company

Since 1962, Blender Products, Inc. (formerly known

as RM Products) has pioneered stratification

abatement developments in air-handler mixing

plenums and has been an innovator in the design of

static mixers.

Over the years, the applications personnel at Blender

Products have developed mixing systems for

thousands of applications in HVAC systems and

industrial process systems. As a result, they have

accumulated a large storehouse of knowledge and

valuable expertise in the disciplines of stratification

and static mixing.

Why This Manual Was Written

During its history, Blender Products has published a

great deal of material dealing with stratification and

mixing in HVAC systems. This material has appeared

primarily in technical bulletins, catalogs and articles.

To date, very little about stratification in the HVAC

environment has appeared in textbooks or journals.

Now, due to increased interest in adding ventilation

air to combat IAQ problems, there is an increasing

need to solve the associated stratification problems.

To help meet this demand, Blender Products has

compiled a significant body of technical information

and outside source articles into a single manual. This

manual will prove to be an invaluable resource in the

design and troubleshooting of stratification systems.

Although this reference is a relatively complete text

in itself, a comprehensive study of stratification

would, of necessity, include an investigation of the

references listed in the bibliography.

2

Introduction1

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Average Temperature (tavg): The average of the

downstream temperatures. It is found using the

following equation:

t avg= 1

n •

Blow Through System: Usually refers to a system

where the filter or coil is installed downstream of the

supply fan. Therefore the fan is “blowing” through

the coil or filter.

Combination Mixing/Filter Box: A factory-made

section that includes filters within the mixing box.

Economizer System: An air-side economizer systemuses the outside air for “free” cooling during times

when the outside air conditions permit.

Freeze Stat: A low-limit thermostat used to protect

hydronic coils from freezing by activating freeze

protection sequences, such as air-handler shut down,

in the air-handler control system.

Inlet Temperature Difference (dT): The difference

in the temperatures between the two incoming air

streams. It is found using the following equation:

dT = t Hot – t Cold

Mass Flow Rate (m): The mass flow rate of an air

stream. It is found using the following equation:

m = ρ • A • v

Mixing Box: A factory-made section that includes

control dampers and may include one or more static

air mixers.

Mixing Effectiveness: A means of quantifying

mixing in terms of “Range” or “Standard Deviation”

reduction across a static mixing device or other

mixing system (see section 3).

Mixing Section: A factory-made enclosure, which

includes one or more static air mixers, that is designed

to be mounted between sections of an air handling unitand is supplied separately from the mixing box.

Mixing System: The entire mixing system consisting

of the mixing/control dampers and any other mixing

devices such as air mixers.

Modified Range Mixing Effectiveness (ERdT): The

absolute amount, expressed as a percentage, that a

mixing box or static air mixer reduces the temperature

spread entering the mixing device. It is typically used

in determining the freeze protection afforded by a

mixing system. See Section 3, page 4.

Range: The maximum temperature less the minimum

temperature for a set of readings.

Range = (T max – T min)Standard Deviation: A statistical calculation applied

to a set of data. In essence it is an average variation

from the average data point. For thermal mixing

analysis the expression is as follows:

SD =1

n – 1

Static mixing device: A mixing device fixed in

position generally consisting of baffles and blades

arranged to induce mixing to a flow of water or air.

Temperature Averaging Bulb: A control sensor

designed to read the average temperature across a

plenum by using a capillary tube or a special

averaging wire approximately 15 to 25 feet long.

Theoretical Mixed Air Temperature: The mixed air

temperature based on a calculation of the combined

mass flow of two air streams. The theoretical mixedair temperature is found using the following equation:

T mix =

Where:mhot = Hot air mass flow (lbs/min)mcold = Cold air mass flowT hot = Hot air temperature (degrees F)T cold = Cold air temperature

3

Glossary of Basic Terminology

∑i=1

n

t i

Where:n = number of temperature readingst i = Individual temperature readings

Where:t Hot = Temperature of hot air

t Cold = Temperature of cold air

Where:ρ = Air density (lbs/ft 3)

A = Area of duct (ft 2)v = Velocity of air stream (ft/min)

∑i=1

n

(T i - T m)2][

.5 Where:

n = Number of samplesT m = Average temperature

T i = Temperature measurement

SD= Standard deviation

mhot • T hot + mcold • T cold

mhot + mcold

2

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How Stratification Develops

Stratification is normally thought of in terms of

vertical temperature gradients. However, stratificationwithin an air handler is different. Momentum in the

entering air streams as well as the combined flow

serves to hold the uneven profile of temperatures or

constituents as they are introduced into the combined

flow (see Figure 5, page 8).

One typical misconception about mixing box design

is the idea that cold air introduced high will drop due

to density difference and mix with the warmer air

below. In reality, the momentum of the main air

stream is typically so much greater than the force due

to the density difference that the cold air introducedon top is simply carried downstream before it has

time to drop. In short, any potential mixing action

due to buoyancy forces are far less than the

momentum of the stratified air stream.

The primary conditions which produce stratification

in an air handler are as follows: The mixing-box layout and damper size, relative

to the total plenum, greatly affect the degree of

stratification for a given mixing system. As the opposing air streams approach a 50/50

proportion, the mixing becomes more difficult. Most importantly, as the temperature differential

between the two streams increases, the range of

downstream air temperatures widens.

Stratification is relatively easy to develop, however

mixing two air streams together is neither automatic

nor easy. Without some form of agitation or mixing

a stratified air stream can continue many duct

diameters downstream.

How to Define Mixing Effectiveness

In the absence of independently established standards

that define how effectively a mixing system is

performing, many “rule-of-thumb” standards have

been used. At a minimum, the HVAC mixing system

should mix two air temperatures sufficiently to

eliminate freeze-stat trips and frozen coils, thus at

least ensuring continued operation of the air handler.

A more stringent performance standard defines how

close the mixed-air temperature range approaches the

actual, or theoretical, mixed-air temperature in the

mixed-air plenum. The latter is important to the proper

functioning of mixed-air thermostats and controls.To simply state that a mixing system should provide

a set range of temperatures for all entering air-stream

conditions fails to recognize the variables mentioned

above. This manual recommends using the “Mixing

Effectiveness” method of rating static mixing systems.

This method takes the entering temperature

differential into account when rating a particular

mixing system. The equation is as follows:

E RdT = 1–

Furthermore, it is important to state the mixing

effectiveness required in relation to various outside

air ratios and temperatures. An example of the

characteristic curve of required mixing effectiveness

as a function of percent outside air for various mixed

air temperatures is shown in Figure 1.

Note: Chart based on the following conditions:

TRA = 75°F, TFZ = 37° F TOA = Outside air temperature

Figure 1 – Required mixing effectiveness vs. percent outsideair for various temperatures to eliminate freeze-stat tripswhen freeze-stat is set at T FZ = 37°F.

Discussion Of Stratification And Mixing

Range DS

dT ( )

Percent Outside Air (By Mass)

0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75

6 0

° F

5 5

° F

5 0

° F

T M i x = 4 5

° F

3 0 ° F

2 5 ° F

2 0 ° F

1 5 ° F

1 0 ° F

5 ° F 0 °

F

R e

q u i r e d M i x i n g E f f e c t i v e n e s s ( E R d T )

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

T OA = 3 5 ° F

3

Where: E RdT = Mixing effectiveness based onrange

Range DS = Maximum temperaturedifference at testing grid (°F,°C)dT = Temperature difference betweenreturn and outside air streams (°F,°C)

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Nuisance Freeze-Stat TripsThe low-level thermostat, also called a freeze stat,

is designed with a long element that will trip if any

portion of the element is exposed to a temperature

below the set point. The set point is typically between

35 and 38 degrees F. Although the coil protection

provided by a low-limit thermostat is often beneficial,

the continuous trip out and resulting air-handler shut

down can be a headache for the building facilities

people.

Whenever a “nuisance” trip of the freeze-stat occurs

the building facilities staff gets the job of modifying

the system to eliminate the pesky problem. Often, in

an attempt to solve the problem, the outside air

dampers are wired shut or otherwise disabled to

eliminate cold, outside air altogether. Obviously, this

so-called solution will not allow the proper amount of

ventilation air into the building. Major problems with

the indoor air quality will result (see section 6). The

other common method of eliminating the problem is

to disable or move the freeze-stat. This “solution”

can, of course, meet with catastrophic results should

a freezing condition occur.

Poor Mixed-Air Control

In the past, HVAC mixing applications did not

demand strict performance requirements. As long as

the air-handling unit did not shut down, or a water coil

did not freeze due to stratification, the mixing was

assumed to be satisfactory. However, with the advent

of DDC and as control systems become more precise,

the problems of an improper sensor read become more

pronounced. The root cause of sensor error can often

be traced to a stratification problem in the mixed airplenum1. The following sections address some of the

problems created by temperature stratification in the

control system of a typical HVAC installation.

Wasted Energy Due to Sensor ErrorSubstantial energy is wasted by undetected errors

originating at the mixed-air temperature sensor.

The most common cause for error is temperature

stratification within the mixing plenum2. This

potential error has been accentuated by the use of

single-point electronic temperature sensors in large

mixing plenums. It is very difficult to place a single

temperature sensor in a position where it will read

the average temperature of the ever-changing

temperature profile downstream of mixing dampers.

Another source of error is a thermostat that is outof calibration.

Averaging-Bulb Inaccuracy

A common solution to the stratification problem

has been to place an averaging style temperature

sensor on the face of the filter or coil downstream

of the mixing plenum. This solution has some merit,

provided the sensor capillary or wire is properly

mounted so that it will record a representative

sampling of temperatures. However, taking only

the temperature value at a series of points ignores

the effect of varying velocity or mass flow across

the sample area. In the ASHRAE Fundamentals

Handbook, Chapter 13, titled Measurements and

Instruments3, the Sampling and Averaging section

warns against assuming that a sampling of

temperatures accurately represents the average

temperature at a cross-section of a flowing stream.

The Section goes on to suggest that a weighted

mass-flow average be taken across the stream to

determine the true temperature of the air stream.

In an effort to show the potential error for an

averaging-bulb sensor applied to a mixing box

with a non-uniform velocity profile, a study was

performed at Blender Products. A 25-foot capillary-

Thermal-Stratification Problems andSolutions

1 Co gg an, Dona ld, P.E., “Mixed Air Control with DDC, ” He a ting/P iping/Air Co nditioning, Ma y 1986.

2 Kato, J ame s Y. “S enso r Errors,” AHSRAE Journal , Volume 27, No. 1, America n S oc ity of Hea ting, Refrigera tion and Air Co nditioning

Engineers, Inc., Atlanta , Ge orgia – J anua ry, 1985.

3ASHRAE Fundamentals Handbook, 1985 – Chapte r 13, “Mea surements a nd Instruments,” America n S ociety of Hea ting, Refrige ration a nd

Air Conditioning Engineers, Inc., Atlanta, Georgia – 1985.

4

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tube averaging-bulb sensor was mounted to a test

grid as shown on the test setup drawing. Individual

temperature and velocity readings were taken at 25

points across the grid, providing data to calculate the

weighted mass average using the following equation4:

Equation 1a Equation 1bMass Flow Based Simplified Velocity Based

T mix = miT i T mix =

Where: Where:T mix = Mixed air temperature T mix = Mixed air temperature

mt = Total mass flow (lbs/min) T i = Individual temperature

mi = Mass flow in partial areas V i = Individual velocity

T i = Individual temperature sample in equal areas

n = Number of readings

The first test was run for a standard mixing box.

A second test was run with a static mixer installed as

shown in Figure 2. The results are shown in Table I.

Figure 2 – Test setup to determine error in an averaging-

bulb sensor.

In these tests, the averaging bulb was placed in

12 different paths. From Table II (see page 7), it

can be seen that some paths were more accurate

than others. Overall, the error was greatest when

the velocity profile across the bulb was non-uniform.

The study clearly demonstrated that non-uniform

velocity across an averaging bulb will cause the

mixed-air sensor to falsely represent the mixed-air

temperature. In short, an averaging bulb can only

sense the temperature. It cannot sense the mass flow

across a section. Therefore, it is important to mix

the air-stream temperatures and diffuse the velocity

evenly across an averaging bulb to provide the most

accurate temperature reading.

Averaging-Bulb Test Results

Temperature and Velocity Traverse Without Air Blender Units

F° 90.8 103.5 109.2 100.1 87.1 77.2

FPM 125 50 50 38 50 162F° 86.7 104.0 105 90.4 86.5 78.4

FPM 125 150 200 125 250 500

F° 79.4 84.9 86.1 82.2 77.3 75.4FPM 650 600 475 675 625 925

F° 69.3 71.1 71.2 73.2 72.7 69.4FPM 900 1000 975 1200 1050 1075

To ta l Flo w = 12,500 C FM Ta vg = 84.6° F

THo t = 133° F Tmix = 76.9° F

TCold = 68° F Error = Ta vg - Tmix = 7.7°F

DT= (TH-TC) = 65° F

Temperature and Velocity Traverse With Air Blender Units

F° 90.7 89.8 81.5 83.3 91.3 91.1FPM 475 525 500 650 575 525

F° 91.7 92.9 84.3 80.6 95.0 89.2FPM 450 375 525 575 400 450

F° 102.7 93.2 84.7 81.8 88.1 89.5FPM 500 300 500 525 300 300

F° 83.6 83.3 75.6 75.4 79.3 75.2

FPM 425 400 350 325 350 425

To ta l Flo w = 12,500 C FM Ta vg = 86.6° F

THo t = 139° F Tmix = 86.5° F

TCold = 68° F Error = Ta vg - Tmix = 0.1°FDT= (TH-TC) = 71° F

Tavg = Arithmetic avera ge tem perature ba se d on multipoint

or averag ing b ulb sens or.

Tmix = Velocity weighted mixed air temperature ba sed on

equations 1a and 1b as show n above.

Table I – Averaging-bulb test results.

4ASHRAE Standard, 41.1-1986 “S tand ard Method for Temperature Meas urement,” America n S ociety o f Hea ting, Refrige ration a nd

Air Conditioning Engineers, Inc., Atlanta, Georgia – 1987.

Return Air Da mpers

Pa tented Air Blender

TestSensorGrid

Outside AirDampers

36"

∑i=1

n

1mt

∑i=1

n

T iV i

∑i=1

n

V i

®

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Path1

No Mixer With Static Mixer

Tavg Tmix Error Tavg Tmix Error

1 88.10 76.90 11.20 85.50 86.50 -1.00

2 82.40 76.90 5.50 88.00 86.50 1.503 90.10 76.90 13.20 84.30 86.50 -2.20

4 87.30 76.90 10.40 87.80 86.50 1.30

5 87.30 76.90 10.40 83.50 86.50 -3.00

6 80.20 76.90 3.30 85.80 86.50 -0.70

7 91.80 76.90 14.90 89.90 86.50 3.40

8 80.90 76.90 4.00 90.00 86.50 3.50

9 88.40 76.90 11.50 88.90 86.50 2.40

10 84.30 76.90 7.40 90.90 86.50 4.40

11 87.80 76.90 10.90 89.40 86.50 2.90

12 82.70 76.90 5.80 84.10 86.50 -2.40

Average Error 9.04 0.84

1 The 12 pa ths use d a re represe ntative of paths used in a

typica l system.

2 Error is d etermined b y using this eq uation: Error= Tavg -Tmix

Table 2 – Error comparison with and without static mixer.

Poor Economizer Control

Generally, an air-side economizer offers free cooling

when outside air temperatures fall below 50 degrees F,provided the dew-point temperature is low enough.

Most economizers operate well down to about

35 degrees F. However, as the outside air temperature

approaches freezing, the freeze stat is likely to trip.

When this happens, the economizer is often abandoned

in the interest of keeping the unit operational. In the

absence of free cooling, the mechanical cooling

system is often used. This not only wastes energy, it

also robs the building of fresh air during sub-freezing

temperatures. A properly operating mixing system

extends the economizer operation time.

Economizer systems may also be abandoned for

ineffective operation as the outside air temperature

approaches the mixed-air set point. The problem often

stems from a leaky return-air damper. Leaking return-

air dampers can allow up to 15% of the warmer return

air into the mixing plenum, thereby causing the

mixed-air temperature to rise 5 degrees F or more.

For an economizer to function properly, it is important

to squeeze as much free cooling from the outside air

by using only cool air and exhausting all the return air.

For instance if the return-air damper leaks 12% into

the mixing plenum, the mixed air will be 55 degrees F

when the outside air is only 51 degrees F. This

effectively shuts down the economizer cycle at 51

degrees, or 4 degrees higher than necessary. In a citylike Omaha, this premature shut-down results in a loss

of 580 hours of potential free cooling. It also causes

earlier start-up of mechanical cooling equipment.

In order to keep economizers operating at maximum

efficiency, mixing systems must have tight seals on

both outside- and return-air dampers to insure positive

shut-off of either air stream.

Excessive Reheat Coil Use

Reheat coils are routinely used to heat the airdownstream of the mixed-air plenum. Often these

coils are controlled by an outside-air sensor and

will run continuously at subfreezing temperatures.

Obviously, this wastes large amounts of energy.

Additionally, if the heating-coil valve cannot be

turned down sufficiently, the cooling-coil valve may

open, or outside air may be introduced, wasting

additional energy.

Stratification Downstream of

Face-and-Bypass SectionsFace-and-bypass sections provide a good way to

control temperature and humidity in the air handling

system. However, control of the discharge temperature

downstream of a face-and-bypass coil is difficult

when the bypass air is located at the top or bottom of

the coil. A means of mixing the two air streams should

be incorporated to assure proper control of the damper

modulation, as shown in Figure 3.

Figure 3 – Internal face-and-bypass with Air Blender static

mixer to mix bypass air with coil discharge air, or to diffuse

bypass air across a plenum.

2 2

Coil

Bypass

AirFlow

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Ineffective Refrigeration-Coil Control

A staggered DX refrigeration coil-array will often

create stratified temperatures downstream, making

it difficult for the discharge thermostat to sense the

average temperature. A high reading will cause the

compressor to operate excessively and can lead to acoil condensate freeze-up. A cold reading will result

in unnecessarily high supply-air temperatures and

resulting loss of the cooling load. Furthermore, the

resulting stratified air streams will often persist into

and through a tee fitting downstream resulting in

cooler air flowing down one duct and warmer air

flowing down the other duct. This condition creates

significant control and comfort problems.

Stratified Steam-Coil Discharge

Steam coils often result in a 20 to 50 degrees Fstratification. This stratified state creates problems

when a single downstream sensor is used to control

the coil valve. Control hunting is the usual result. The

addition of a static mixing device downstream has

proven effective in providing more precise steam-coil

control. In addition, it results in more homogenous

temperatures across processes downstream.

Uneven Heating with Direct-FiredBurners

The temperatures downstream of a direct-fired burner,

which combines hot combustion gases with the air to

be heated (Figure 4), are highly stratified. The air

stream requires some form of mixing to ensure homo-

geneous temperatures at the controlling temperature

sensor, as well as for any process downstream. Static

mixers are ideal choices for these applications. They

have been used with much success on many projects.

Figure 4 – Direct-fired burner with Air Blender device to

mix air with products of combustion.

Uneven Moisture Absorption AcrossHumidifiers

Temperature stratification can create problems with all

plenum-mounted humidification systems. Normally,

the cooler the air stream the longer the vapor travel

downstream as shown in Figure 5. This conditionmay cause problems such as air-filter saturation or

condensation on turning vanes or duct walls. The

best solution is to insure that the temperature of the

air stream is homogeneous and sufficient to absorb

the moisture.

Figure 5 – Extended vapor travel in a stratified plenum.

Improper Filter Loading

A potential problem with stratified air streams in themixing plenum is uneven filter loading. This occurs

when one air stream has a greater particulate

concentration than another. An example would be

a street-level outside air intake that would introduce

larger amounts of particulate than the indoor air

stream. If the outside air stream is not adequately

mixed with the return air stream, the filter will have

a higher concentration of outside air particulate in

one localized area. This uneven loading may cause

excessive velocities across the balance of the filter

face, which will result in inconsistent velocity acrossthe downstream coil. To insure proper filter loading,

some form of mixing device should be used ahead of

the filter.

8

Air

Flow

Flame

TOA

Cold

Hot TRA

Extend ed Vap or Travel in

Cold Area of P lenum

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The most troublesome problem arising from

stratification is hydronic-coil freeze-up. If the air

handler is running and the mixing-damper-control

sequence is operational when the coil freezes, the

freeze-up is most likely due to thermal stratification.

In addition to a functioning low-limit freeze stat,

there are a number of ways to protect coils from

freezing. This section will explore the various options

to provide coil-freeze-up protection.

Glycol Solutions

Mixtures of ethylene or propylene glycol with water

are often used to protect hydronic coils during

subfreezing weather conditions. The advantage of

glycol solutions is their ability to protect all

components under any situation. For example, if the

Figure 6 – Effects of various concentrations of ethylene

glycol.

power shuts down, and the outside air damper staysopen, the glycol mixture will protect the coil and

associated piping. However, there are a number of

disadvantages of using glycol as a freeze-protection

additive. The higher viscosity requires increased

pump horsepower. In addition, the lower specific heat

constant inhibits heat transfer, which requires higher

water flow and/or larger heat-transfer surfaces. It is

also important to point out that some provision for

volume expansion must be included to protect the

heat exchangers in the system. Figure 6 shows coil

derating as ethylene glycol concentration is increased.

Unit Heaters in Mixed-Air Plenum

Unit heaters are sometimes placed in mixing plenums,

as shown in Figure 7, in an attempt to maintain a

minimum mixed-air temperature and thus protect

against coil freeze up or freeze-stat trips. Although a

unit heater may be a reasonable way to heat the mixed

air plenum during air-handler shut down, it is largely

ineffective when the air handler is operational. Theprimary reason for this failure is inherent in the unit-

heater design. They are designed to heat a relatively

static air space. However, a mixed-air plenum has air

flowing at 400 to 500 FPM. This leaves little time for

the unit heater to turbulate the air stream. Furthermore,

any heat introduced into the air stream serves only to

create more stratification downstream, thus creating

problems for mixed-air sensors, freeze stats, and coils.

The only reasonable use for unit heaters in mixed-air

plenums is during air handler shut-down to protect

water coils from cold outside-air migration through

leaky outside-air dampers, as shown in Figure 8.

Figure 7 – Typical unit heater in plenum configuration,

without Air Blender unit.

Heat Recovery

Heat-recovery systems can be beneficial in reducing

the problems of stratification by preheating the cold

outside air with warm exhaust air. These systems

Water-Coil Freeze-Up Problems andSolutions

Ethylene Glycol Concentrationin % by Weight

C a p a c i t y a s % o f

C a p a c i t y a t 0 %

E t h y l e n e G l y c o l

C o n c e n t r a t i o n

0 10 20 30 40 50 60

100

110

120

130

140

150

40

50

60

70

80

90

100

P r e s s u r e D r o p a s %

o f P r e s s u r e D r o p

a t 0 % E

t y l e n e G l y c o l C o n c e n t r a t i o n

Pressure

Capacity

5

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tend to be costly and

are often difficult to

design into a

restricted mechanical

layout because the

exhaust-air streams

must be brought to asingle point and in

close proximity to the

outside air stream.

The benefits of this

type of system are in

their potential to

recover otherwise

wasted energy.

Draining Water CoilsA widely used practice to guard against water-coil

freezing, is to drain the water and isolate the coil

during periods of subfreezing weather. This method is

obviously effective when all the required maintenance

is done in a “timely” manner. Potential problems occur

when warmer-than-normal temperatures occur in early

spring or late fall (when the cooling coil is drained)

and the building requires mechanical cooling.

Pumping Fluid at Subfreezing

Outside Air Temperatures

Control systems are often designed with a control

sequence to turn on the water pumps at subfreezing

outside-air temperatures to prevent water-coil

freezing. This sequence is effective as long as the

pump is properly maintained and the control system

is functioning. The primary downside is the use of

additional pump horsepower and run time.

Static Mixing Devices

Static mixing devices, such as the one shown in

Figure 9, provide excellent freeze protection for

hydronic coils by continuously mixing the cold

outside air into the warm

return air whenever air

is flowing across the

device. A static mixer

is the only option that

provides excellent freeze

protection without the

need for maintenance.

Furthermore, the device

provides a high degree

of flexibility during seasonal changeovers, when

chilled water cooling may be needed. In addition to

freeze protection, the static mixer provides a solution

to problems created by stratified temperatures in the

mixed-air plenum. These include mixed-air control-

sensor error and uneven air distribution across filters

and coils.

A summary of various coil-protection alternatives is

shown in Table III.

10

Figure 9 – Typical static

mixing device.

Consideration

Maintenance Reliability Flexibility Protection

Protection Attention Operating For Freeze During During AHUAlternative First Cost Required Cost Protection Changeover Shutdown

Add Glycol High High High Med High Yes

Unit Heaters High Med Med Low Low Yes

Heat Recovery High Low Med Med High No

Drain Coil Low High Low Med Low Yes

Pump Fluid Low High Med Med High YesBelow 32°F

Static Mixing Med None Med High High NoDevice

Table III – Summary of coil-protection alternatives.

Figure 8 – Large bank of

outside air dampers could cause

outside air migration.

7/23/2019 airmix


IAQ has become one of the most heavily discussed

topics in the HVAC industry. This issue will affect

the HVAC industry, and the design of systems, foryears to come. The following section discuss how the

application of ASHRAE Standard 62-1989 creates

new challenges for HVAC designers and building

engineers relative to stratification in the mixing

plenum.

Outside Air Dampers Blocked Shut toEliminate Stratification Problems

Control malfunctions due to equipment failure or

improper maintenance procedures can cause IAQ

problems. As discussed earlier, a typical practice

involves disabling the outside air dampers, or blocking

the damper shut, to eliminate freeze-stat trips or

frozen-coil problems. One situation was documented

in which an outside-air intake was blocked shut by

a 2x4 to prevent the water coil from freezing in the

winter. However, the 2x4 was left in place for years,

preventing the controls from functioning5.

Dilution of Fresh Ventilation Air IntoThe Supply-Air Stream

One of the principal concerns in the IAQ discussion

is the ability to prove that adequate and proportional

ventilation air is delivered to each building occupant.

Without a static mixing device there is concern that

outside ventilation air may stratify and favor one side

of a centrifugal fan or duct branch fitting. The endresult can be a greater percentage of outside air

delivery to one portion of the building at the expense

of the rest of the building. Obviously, the ventilation

system should be designed to properly diffuse the

ventilation air into the return air stream. Figure 10

shows such a design.

Maintaining Minimum VentilationAir Under VAV Conditions

ASHRAE Standard 62-1989 requires a minimum of

15-20 CFM of ventilation air per person. This

translates to about 15-20% fixed outside air based on

maximum airflow. If the 15% rate is used, and the

supply airflow turns down to 30%, the proportion of

fixed outside air increases to 50% of the total air

supply, as shown in Figure 11. This increased

percentage of sub-freezing outside air gives rise to

freeze-stat trip and potential frozen coils. To guard

against freeze problems the heating coil is often

activated as soon as the outside air drops below

freezing.

Figure 11 – Outside air percentage based on fixed outside

air with varying supply air.

11

Indoor Air Quality (IAQ) Concerns

5 Rob erso n, Wa yne K. “Troublesho oting IAQ Troublesp ots ,” Engineered Systems , J uly/Aug us t 1993.

Return air

Fresh outside air

Fresh air is evenlydistributed in supply air

Without Air Blender,fresh air and return aircan stratify throughcoils, fans and branchtake-offs downstream

Air Blender static mixer insures that fresh air isevenly distributed throughout supp ly airstream

Figure 10 – Design toeffectively diffuse

ventilation air into thereturn air stream.

Varying Supply Air (SA) Flow

100% 90% 80% 70% 60% 50% 40% 30% 20% 10%

100%

90%

80%

70%

60%

50%

40%

30%

20%

10%

0%

Full Flow

Turn Do wnPercentage A

c t u a l O u t i s d e A i r ( O A ) F l o w

P e r c e n t a g e

1 5 %

M i n

O A

.

2 0 %

M i n

O A .

2 5 %

M i n

O A .

1 0 %

M i n

O A

.

5 % M i n

O A

6

7/23/2019 airmix


How Much Mixing is Enough?

Mixing performance levels are defined in various

ways. Blender Products has chosen the range-mixing-

effectiveness method as the most definitive way to

rate a mixer’s ability to mix. Generally effectiveness

can be grouped into three categories. Low-efficiency

mixing includes standard mixing boxes available

on typical air handlers. Medium-efficiency mixing

systems include some form of static mixing device

mounted after the mixing box. High-efficiency

systems include a specially designed damper

configuration in combination with a static mixing

device to provide the highest possible level of mixing

performance. Figure 12, taken from the article “How

Much Mixing is Enough?”6 provides a way to

determine the level of mixing required to prevent

freeze-stat problems. It should be stressed that these

levels represent the minimum mixing required. It is

also necessary to provide good mixing to ensure

proper mixed-air control performance.

Figure 12 – Mixing required to prevent freeze-stat

problems.

VAV Concerns

The mixing performance of the Air Blender static

mixing device requires a minimum amount of airflow

to maintain adequate mixing action. Generally, if the

velocity through the Air Blender mixer is 400 FPM

or above, the mixer will perform properly, as shown

in Figure 13. Therefore, if a 3 to 1 turndown is desired,

the design velocity through the mixing device should

be no less than 1200 FPM.

Figure 13 – Air Blender performance under variable air-

volume conditions.

Proper Placement of Mixers in HVAC

Systems

There are a number of constraints on the proper

placement of static mixers in an air-handling system.

The first is to ensure that each mixing unit is

positioned so that a proportionate amount of each

stream is introduced into each mixer. It is also

important to provide the proper mixing distance

downstream so there is adequate time for the airstream to turbulate after flowing through the mixing

blades. Normally the static mixing units are centered

in the plenum top to bottom and side to side. Figure

14 shows correct and incorrect placements of static

mixers.

12

Application Guidelines For Static MixingDevices

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A A

A A

A A

A A

A A

A A

A A

A A

A A

A A

A A

A A

A A

A A

A A

A A

6 Robinson, Keith D., P.E., “How Much Air Mixing is Enough?” Heating/Piping/Air Conditioning, November 1995.

Feet Per Minute (FPM) Velocity

E f f e c t i v e n e s s

200 400 600 800 1000 1200 1400

30%

40%

50%

60%

70%

80%

90%

7

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Patented

Air B lender

Units

Turning

Vanes

AirFlow

PatentedAir Blende rUnits Filter

AirHandler

Side Elevation Front View

FilterAirHandler

Top View

OA

RA

OA

RA

FilterAirHandler

Top View

OA

RA

RA

OA

Air Flow

Air Flow

• Use only in one single line.

• Each Air Blender device must see eq ual amo unts of da mper area a nd air flow.

• Inlet ducts c an c ause an uneq ual effect on air flow. Fresh a ir and return air dampers should be the sa me length. Use turning vanes ,

as shown in (A), to equalize flow across Air Blender units when air enters from the side.

• Mixing dampers must open so that air streams face eac h other.

• . When a minimum outside da mper is us ed it must be the full width of the return air damper and supply equal amo unts of a ir to eac h

mixer. Note that a minimum air damper is available as an option on the BlenderBox™ mixing bo x.

• Blender Products tes ts, plus independent lab tes ts, s how that multiple mixer configurations d o perform when a pplied properly.• When looking a t ava ilab le mixers, a lwa ys q uestion whether they have been tested under all sta nda rd operating and installation

co nditions , including:

– Multiple mixer co nfigurations ,

– VAV Sys tems to 33% turn-down.

• Sketch 2 show s tw o misapplica tions o f multiple Air Blender units.

• P arallel or opposed b lad e da mpers with blad e axles ac ross the short dimensions of the dampe r opening w ill ca use a ir flow to fa vor

the side of the mixing plenum and should not be used with multiple Air Blender applications. Blade axles should be oriented across

the long d imensions of the da mper opening a s s hown in Sketch 1.

A

Sketc h 1 – Corr ect Placement

Sketc h 2 – Inco rrec t Placem ent

Figure 14 – Comparison of correct and incorrect configurations when using static mixers.

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Effect of a Combination Filter/MixingBox on Mixing Performance

Filters are sometimes placed in mixing boxes to

save space in the air handler. Placing air filters in

this seemingly “unused” space causes a significant

reduction in mixing effectiveness, as shown in

Figure 15. The preferred method is to place the

static mixer before the filter section. This provides

better mixing and diffusion across the filter and the

coil downstream.

14

Figure 15 – Comparisons of mixing effectiveness with various filter configurations.

Combination Filter/Mixing BoxWithout Air BlenderMixing Efficienc y = 45%

Lowes t Tempera ture

at Coil Face = 28°F

Standard Mixing Box withFilter Section Downstream

No Air BlenderMixing Efficienc y = 55%


at Coil Face = 32°F

Standard Mixing Box withBlender Section and FilterSection DownstreamMixing Efficienc y = 80%


at Co il Face = 45° F

BlenderBox Mixing Box withFilter Section Downstream

Mixing Efficienc y = 85%Lowes t Tempera ture

at Co il Face = 47° F

Notes:Lowes t temperature at c oil fac e is b as ed on return air tempera ture = 75°F, outside a ir = 15°F, 55°F mixed a ir tempera ture.

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This section defines the problems that occur with an

uneven velocity profile and suggests practical

remedies to rectify these problems.

Coil Condensate Carryover

When a coil begins to remove moisture from the air

stream, water is formed as droplets on the coil fins.

Normally the droplets fall to a condensate drain pan

at the bottom of the coil and are drained away.

However, if the velocity profile across the coil is

uneven some of the condensate will “spit off” and

miss the drain pan. This carryover causes problems

by wetting the downstream areas such as duct liner

and other equipment. Equalizing the velocity profileis very important to ensure that the coil condensate

stays on the face of the coil and drains harmlessly

into the drain pan.

Coil Inefficiency

Non-homogeneous velocity through a coil also

degrades coil performance. Coils are generally tested

with even velocity across the face. However, when a

coil is applied in a system, the velocity across the coil

face is far from homogeneous. This is especially true

if the coil is applied in a blow-through configurationdown-stream of a fan discharge. The effect of this

velocity variation is difficult to quantify, but it is

obvious that performance will not be the same as that

shown by the test data.

Filter Loading, Damage and Particle

Bypass

Velocity variations across a bank of filters often

cause the filters to load unevenly or in some cases

to be damaged. This improper loading, coupled with

the high localized velocity, will often cause particle

bypass. Again, this condition occurs downstream of

a fan discharge.

15

Velocity-Stratification Problems8

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In blow-through applications, coils and/or filtersare located downstream of a fan discharge. These

components require relatively low velocities to

operate correctly. This typically requires a large

change in area downstream of a fan.

Distance Requirements

To minimize the pressure loss of this expansion, it is

necessary to install a full-length outlet duct ahead of

the expansion, as shown in Figure 16. Although this

duct minimizes the pressure loss, its length can bequite large, especially with large fans. An additional

drawback is that it provides no diffusion of the fan

discharge jet. There is still a need for a diffusion

device or an extremely long diffusion distance

between the fan outlet and downstream coil. A long

distance is needed because the discharge jet expands

at an angle of about 15°.

Figure 16 – Full-length outlet duct, providing an ideal

installation for system effect.

System-Effect Considerations

The “No-outlet duct arrangement,” shown in Figure

17, may be used to save space. Like the full-length

outlet-duct arrangement, this configuration does not

provide additional diffusion of the air stream. Thus

the distance between the fan outlet and downstream

component must be sufficient for the jet to naturally

expand and fill the plenum. This arrangement has a

large system-effect penalty.

To enhance the diffusion of the no-outlet duct

arrangement, a diffuser plate or target plate can be

mounted between the fan outlet and the downstream

Figure 17 – No-outlet duct arrangement.

coil/filter, as shown in Figure 18. Like the no-outlet

duct arrangement, this configuration has a substantial

system-effect penalty. It can provide velocity diffusion,

but it is difficult to predict how well a system will

work until after system startup. In general, the closera diffuser plate is to the fan discharge, the better the

velocity profile. Unfortunately, as the diffuser plate

is moved closer to the fan, the system-effect penalty

increases rapidly.

Figure 18 – Target plate, vertical diffuser plate and angled diffuser plate configurations.

The use of the BlenderConeTM fan diffuser/mixer

(Figure 19) provides an alternative approach. The

pressure drop of the BlenderCone is greater than the

pressure drop of the full outlet duct, but is less than

half the pressure drop of the other arrangements.

The distance required by the BlenderCone is less

than the distance required by the full-outlet duct and

is essentially the same as that required by the other

arrangements.

Figure 19 – Short BlenderCone, for retrofit applications in

tight spaces, and the more efficient long BlenderCone for

new construction.

16

Application Guidelines For Velocity-Equalizing Diffusers9

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Bibliography

Amoroso Jr., Victor, P.E. & Gjestvang, Ryan, P.E., “Air Handling Units for Hospitals,” Consulting/Specifying

Engineer, October 1989.

Faison Jr., T.K., Davis, J.C., & Achenbach, P.R., “Performance of Louvered Devices as Air Mixers,” Building

Science Series 27, US Dept. of Commerce, National Bureau of Standards, March 1970.

Haines, Roger W., “HVAC Systems Design Handbook,” Section 4.8 (pp 109-111) TAB Books, 1988.

Haines, Roger W., “Stratification,” Heating/Piping/Air Conditioning, November 1980.

Hallstrom, Arthur D., “Fixing the Frozen Coil Problem,” Contracting Business, October 1986.

Robinson, Keith D., P.E., “Blender Cone Basics,” Blender Products, Inc., December 1992.

Robinson, Keith D., P.E., “Fan Basics,” Blender Products, Inc., December 1992.

Application Guidelines For Velocity-Equalizing Diffusers

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5010 Cook Street, Denver, CO 80216

Toll-Free: 800-523-5705

Fax: 303-296-1520

Phone: 303-295-6111

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