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Technology Report

March, 2003

Siemens Industry, Inc. Page 1 of 8

VAV Laboratory Room Airflow

The Lowdown on Turndown

Airflow Turndown

Airflow control ranges are normally expressed in

terms of the required maximum and minimum

attainable airflow rate. When this airflow rate is

expressed as a simple ratio, it is commonly referred

to as the turndown ratio or just turndown. For

instance, if an air terminal and its associated control

system provides fully modulated airflow control from

400 cfm up to 2000 cfm, the maximum-to-minimum

airflow ratio can be expressed as 5-to-1 since the

maximum airflow is 5 times the minimum airflow.

Therefore, the turndown is stated as 5:1.

Turndown is commonly used as a convenient way to

express the airflow control needed for specific types

of applications without the necessity of having to refer

to the specific maximum and minimum airflow rates

involved. For instance, specifying that all VAV fume

hood exhausts must be controlled over a 5-to-1

turndown ratio means the exhaust airflow must be

modulated so that the minimum fume hood exhaust

will be one fifth of the maximum exhaust. This

minimum to maximum exhaust airflow relationship

then applies to every fume hood even though

different sizes of fume hoods have different airflow

ranges.

Turndown Misconceptions

As simple as the concept of turndown may seem, it

has led to considerable misunderstanding in the

HVAC industry, and especially with regard to VAV

laboratory ventilation applications. For instance, if a

specific air terminal has a maximum to minimum

airflow range from 3200 cfm down to 400 cfm, it

would have a turndown ratio of 8-to-1. If another air

terminal has the same maximum airflow of 3200 cfm

but a minimum airflow of 200 cfm, its turndown ratio

would be 16-to-1 or twice that of the 8-to-1 unit. A

common but erroneous assumption is that the 16-to-1

unit provides twice the airflow range that the 8-to-1

unit can provide. Figure 1 shows a graphic

omparison of these two turndown ratios. Note the

small actual airflow difference between them.

3200 CFM

3000 CFM

2800 CFM

2600 CFM

2400 CFM

2200 CFM

2000 CFM

1800 CFM

1600 CFM

1400 CFM

1200 CFM

1000 CFM

800 CFM

600 CFM

400 CFM

200 CFM

0 CFM

16 : 1

TURNDOWN

8 : 1

TURNDOWN

Figure 1. Comparison Between 16:1 and 8:1

Turndown Ratios.

Higher turndown ratios have also been erroneously

assumed to be indicative of better control

performance. However, larger turndown ratios do not

positively impact the main control performance

factors such as accuracy, repeatability, low

hysteresis, fast response, etc. Rather, the best

control performance results when a control device

has a range or turndown ratio that closely matches

actual need. For example, precise control of

temperature over a range from 65 to 75 degrees

would more likely be attained from a controller having

a range of 60 to 80 degrees as opposed to one with a

much wider range such as 0 to 120 degrees.

Consider a need to control shaft rotational speed at

1200 rpm. A controller with a range of 500 to 2000

rpm (a 4-to-1 turndown ratio) would most likely

provide better control than one with a range of 200 to

5000 rpm (a 25-to-1 turndown ratio).

Document No. 1

49-978

Summary of content (8 pages)

PAGE 1
Technology Report March, 2003 VAV Laboratory Room Airflow The Lowdown on Turndown Airflow Turndown Airflow control ranges are normally expressed in terms of the required maximum and minimum attainable airflow rate. When this airflow rate is expressed as a simple ratio, it is commonly referred to as the turndown ratio or just turndown.
PAGE 2
Thus, it is important not to equate higher turndown ratios with superior control performance. Rather, control devices should be compared on the basis of how closely their range or turndown ratio matches the application. In fact, the range or turndown ratio of a controller or control system should ideally be only slightly more than what the control application calls for. Turndown ratios greater than the actual need are generally undesirable.
PAGE 3
VAV Laboratory Room Supply Air VAV laboratory room supply air fulfills several functions:  It provides makeup air for fume hood exhaust and the miscellaneous room exhausts.  It maintains adequate room ventilation regarding the maximum amount of room supply air that must be provided. Therefore, in laboratories with VAV fume hoods the supply air turndown requirements will quite often be close to the fume hood turndown requirements of 5-to-1. It maintains the room ambient temperature and humidity.
PAGE 4
Table 1. Air Driven Laboratory Rooms with Fume Hoods - Supply Airflow Turndown Ratios. Laboratory Room Size: (Length) (Width) (Height) R O O M Floor Area Excess Room Exhaust Airflow For Negative Pressure Total Fume Hood Exhaust VAV Fume Hoods in Room 4 Total Miscellaneous Room Exhaust Total Room Exhaust 5 6 8 Foot Widths Room Minimum Air Changes Per Hour Room Supply Airflow Room Supply Turndown Ratio Room Volume 4' 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 6.
PAGE 5
Laboratory Room Size Total Miscellaneous Room Exhaust Table 1 covers three different sizes of laboratory rooms separated by the double line divisions of the table. Each group has eight identically sized laboratory rooms. Rooms 1 through 8 represent single module laboratory rooms that are 25 feet long, 10 feet wide, and 9 feet high. Rooms 9 through 16 represent double module laboratory rooms with twice the area of the single module rooms.
PAGE 6
Room Supply Airflow The Room Supply Airflow Min. CFM equals the Total Room Exhaust Min. CFM minus the Excess Room Exhaust Airflow For Negative Pressure. The Room Supply Airflow Max. CFM equals the Total Room Exhaust Max. CFM minus the Excess Room Exhaust Airflow For Negative Pressure. Room Supply Turndown Ratio The room examples in Table 1 show the supply airflow turndown ratios (right most column) vary from a high of 7-to-1 for Room 1 down to a low of 4.3-to-1 for Room 17.
PAGE 7
and maximum airflows required for each airflow control application in an appropriate airflow control schedule. The turndown ratios required for each individual airflow control application will be inherent when these maximum and minimum airflows are specified. This puts the responsibility for providing the required airflow control turndown ratios on the control system provider.
PAGE 8
Product or company names mentioned herein may be the trademarks of their respective owners. © 2009 Siemens Industry, Inc. Siemens Industry, Inc. Building Technologies Division 1000 Deerfield Parkway Buffalo Grove, IL 60089-4513 USA Printed in the USA Document No.