Air Cooled Exchangers Auto-variable Variable Speed Fans Winterization

1. Purpose

This procedure describes Inflection Point Engineering's policy concerning the use of Auto-Variable (AV) type blade pitch angle control, Variable Speed (variable frequency) Fan Drivers and Winterization requirements for air cooled exchangers.

2. Criteria

2.1 Variable Speed Fan Drivers

Specify one-half of the fans in an air cooled exchanger bay as Variable Speed (Variable Frequency) Fan Drivers when any of the following criteria applies:

a. If the total motor horsepower requirement for a single service exceeds 50 HP (38 kW), then 50% Variable Speed fans are provided as a means to reduce the overall electric utility consumption. Exceptions to this guideline occur for a tropical climate where winter/summer and day/night temperature variations are small or where the customer has provided different instructions in the BEDQ.

b. If a process reason exists, such as pour point/viscosity considerations or the desire to avoid over-cooling of a recycled process stream which may have a negative influence on the heat balance.

c. For services when close temperature control is critical.

Consult the appropriate Technology Specialist and Equipment Specialist to determine where to apply the criteria.

2.2 Auto-Variable Type Fans

Specify Auto-Variable (AV) type fans as an alternative to Variable Speed (Variable Frequency) Fan Drivers for air cooled exchangers. This type of driver has an increased maintenance cost that must be economically justified unless the customer has provided specific instructions in the BEDQ sheets as to the use of these drivers. In general, the same criteria are used for Auto-Variable type fans as applies to the use of Variable Speed Fan Drivers. Variable Speed fans or Auto-Variable Type fans are used on 50% of the fans while the other 50% use single speed, manually adjusted pitch fans.

2.3 Winterizing Conditions

Consult the appropriate Technology Specialist and Equipment Specialist to determine the degree of winterization required.

Before internal P&ID checks, the project engineer shall identify those air cooled heat exchangers that may require winterization. Process conditions shall be made available to the equipment specialist for review. If there is more than one unit in a project, consistency for winterization protection among units should be maintained. The project engineer should consult with other project engineers for the complex, the project manager and the equipment specialist. Selection shall comply with API Standard 661/ISO 13706 and consider non-standard operating conditions such as start up, shutdown, regeneration, etc. as well as unit turndown requirements. The list below assumes a concentration of freezing components of 10 weight percent or more. Concentration of freezing components less than 10 weight percent may impact the selection. The difference between the pour point (Tpp) or the freezing point (Tfp), as appropriate, and the winterization temperature (Twint) is used to make the selection.

T = Tpp – Twint or T = Tfp – Twint

In general, the following should be used as an initial estimate:

T 0F(0C)

No winterization required.

0F(0C) < T 20F(11C)

Specify manual louvers and either AV or Variable Speed fans. A steam coil is to be provided for startup and/or shutdown.

20F(11C) < T 40F(22C)

Specify manual louvers, either AV or Variable Speed fans on 100% of the fans and a TI with low temperature alarm on the outlet of each bay. A steam coil is provided for startup and/or shutdown.

40F(22C) < T

Specify External Air Recirculation with a steam coil, manual and automatic louvers and either AV or Variable Speed fans on 50% of the fans. See attachments 4 and 5.

The above should be considered as a general guideline only. Increasing the degree of winterization will increase equipment costs significantly; therefore, the equipment specialist should determine the actual minimum requirements for the service.

Internal recirculation systems are not as protective as the external recirculation system. However, it is less expensive. This system is not recommended but can be used if the customer insists on it and assumes the risk.

3. Project Specifications

The following attachments are examples of how the project specifications will reflect the use of AV pitch fans, variable speed drivers, and external air recirculation for air cooled exchangers.

The critical temperature for winterization, such as freezing point, pour point, etc. shall be identified in a note for the item’s data sheet in the project specification.

These project specification sheets show which air cooled exchangers shall be specified with AV pitch fans. Note, list the item numbers on this sheet or designate "All" if all air cooled heat exchangers shall have AV pitch fans.

In the event where there are no AV or variable speed fans required in the entire process unit, do not include this sheet and add the following note in the “General Information” section:

"Fans to be (SP) Standard Pitch type with single speed electric motor drives".

This is a sheet from a project specification showing which air cooled exchangers shall be specified with Variable Speed Fan Drivers. Note, list the item numbers on this sheet or designate “All” if all air cooled exchangers shall have variable speed drivers.

: These are two project specification sheets for a benzene column condenser where the process fluid freeze point is 41.9F (5.5C) and winterizing temperature is -13F (-25C). In this case, the T is equal to 54.9F (30.5C).

4. References

GPSA Engineering Data Book: Air Cooled Exchanger Section

API Standard 661/ISO 13706 Air Cooled Heat Exchangers for General Refinery Service

DM-EQUIP-2073-402; “Air Cooled Heat Exchanger Equipment Design Manual”