Cooling Water Systems - Cooling Towers - Cold Weather Operation
September 18, 2002

Introduction Fan Operation Temperature Control Water Flow Control Cold Weather Startup and Shutdown

Associated Links Cooling Tower Maintenance and Inspection Long Term Shutdown

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Introduction

Ice damage can be a common occurrence to towers subject to severe weather conditions.  Ice will normally build up on the louvre panels at the air inlets to the towers, however, ice can also build up on the tower fill itself under certain conditions.  Counterflow towers are most susceptible to ice build up at the air intake louvres and the exterior portions of the fill in the louvre area.

Factors affecting build up of ice in a tower are:

• Tower heat load

• Air and water flow rates

• Tower design

• Geometric layout

• Location of tower relative to buildings and prevailing winds

• Location relative to other cooling towers

There are several methods to minimize and prevent the formation of ice on a cooling tower.

Fan Operation

The simplest, most direct and effective method of controlling ice formation is through variations in fan operation.   Reduction in the airflow will substantially reduce the heat transfer rate and operating temperatures in the cell or tower will rise.

During cold weather operation, operation of the fan(s) at full speed is generally not required to achieve the desired cold water temperature.  Towers can be equipped with 2-speed fans that will allow a reduction in the airflow through the tower.  In all cases the fan(s) can be turned off complete if required.  The cooling tower would then act like a natural draft-cooling tower.   Airflow will be reduced to 0 to ¼ of the normal airflow. 

A feature often incorporated into cooling towers located in areas subjected to below freezing temperatures is the ability to reverse the direction of the fan.  With the fans operating in reverse the air flow through the tower is reversed.  Heated moist air from the tower fill area is supplied to the lower fill area and air inlet louvres which effectively removes any ice formed in those areas.  The fans would be placed into reverse periodically for about 15 to 30 minutes.  Fans should not be operated in reverse for extended periods of time since ice will form on fan blades, fan stacks and drift eliminators causing damage to those items.

Temperature Control

The tendency for ice to form in the cooling tower will be lower the higher the operating temperature of the cooling water system.   The overall cooling water system should be operated at the highest possible temperature consistent with the efficient operation of the plant.  Temperature control can be achieve by putting the maximum possible heat into the system from the process and controlling the cooling by proper operation of the cooling tower fans.

Water temperature can also be controlled by bypassing a portion of the water around the cooling tower directly into the cooling tower basin.  This method sends warm water into the basin thus raising the temperature of the supply water.

Water Flow Control

Water flow rate is an important part of winter operation.  Low water flows to a cooling tower or cell should be avoided since this may result in areas of the tower or cell being starved of water.  These areas would then be susceptible to ice formation.  Cells should be taken out of service so that the remaining operating cells are supplied with their normal water flow.  The last cell to be taken out of service should always be the one located at the end of the water distribution header.  This will avoid freezing the water held up in a dead leg of the distribution header.

Cold Weather Startup and Shutdown

A cooling water tower is most susceptible to ice formation during the initial startup and shutdown of the cooling tower.   During these periods of operation the heat load on the system will be minimal.   To successfully startup or shutdown a cooling tower in cold weather, the system must be equipped with a bypass that will allow return water to be sent directly to the cooling tower basin.  Circulating water over the tower is only permitted once the return water temperature approaches the normal operating temperature.  Cells and towers should be brought on-line only as required to achieved the desired supply temperature.  As well, fans should be left off until required.

To assist in the operation of the cooling tower, the process should be brought up as quickly as possible to increase the heat being transferred into the cooling water system.

Shutting down the cooling water system is just the reverse of starting up the system.  Fans should be turned and cells should be taken out of service off as soon as possible.  Return water should be bypassed around the tower directly to the basin at the earliest possible moment.