Ask me if you wanna know the way to Coolsville.
Well I hear you wanna go back to Coolsville.
Well come on honey, take you back to Coolsville.

("Coolsville," Rickie Lee Jones, 1978)

 

NATURALLY COOL, MAN.

The tall and sleek tower in the above featured photo is a Hyperbolic Cooling Tower.

This natural draft cooling tower does not use a fan to induct airflow up the tall chimney-like structure. Cooling is done "au naturale" by drafting the evaporated heat and entrained water droplets up and out of the tall structure.

 

HOW NATURAL DRAFT WORKS

PTOA Readers and Students have probably noticed that "heat rises" in their homes. The top floor of a house is warmer than the lower floor; on a cold winter day the heat from the radiator or sidewall heat vents can be felt as a 'natural draft' flowing up the stairs.

The flow of hot air currents with colder air currents can also be felt in a room with a radiator (or other heat source) on one end of the room and a window at the opposite end of a room.

"Heat rises" because warm air particles want more space between them and "rise above" colder air particles that stay grouped closer together at lower levels.

The draft that is caused by lower-density hot air rising above higher-density cold air is even more evident in a stack structure because the hot air naturally wants to rise and rise and rise up more!

The modified-hour-glass shape of the Hyperbolic Cooling Tower also encourages the upward hot airflow because the smaller-diameter area in the middle of the tower...the "throat"...makes the air flow velocity increase in this area.

The natural draft Hyperbolic Cooling Tower may not use a fan to expel hot, moist air into the sky but the exiting hot air still has sufficient velocity to billow upward.

The interaction between the dry, cold outside ambient air and the hot, moist exiting air creates a vertical version of the natural draft observed in the room with a radiator and a window. The cold air enters the bottom of the tower through vents and pushes the hotter air upward.

 

TYPICAL HYPERBOLIC COOLING TOWER FEATURES

The typical Hyperbolic Cooling Tower is as tall as an American football field is long (100 yards) and the diameter of its base would span from the end zone to the 70 yard line.

A Hyperbolic Cooling Tower is usually associated with nuclear and coal power plants. The tree-like metal structures next to the Hyperbolic Cooling Tower in the photo to the left are transformers that help transmit and distribute electricity.

PTOA Readers and Students can infer from the above paragraph that there's something about making electricity that generates a lot of heat that must be removed pronto!

Power Plants use Hyperbolic Cooling Towers because:

• Power Plants need a lot of cooling water to remove heat...like 38,500 kitchen trashcanfuls of water per minute!  How would you like to take that much trash out per minute?
• Although they cost more to build, the cost to operate a Hyperbolic Cooling Tower is much less than the cost to operate a cooling tower that requires a fan to induce or force air through the structure.

 

COMPARE AND CONTRAST COOLING TOWERS

PTOA Readers and Students have already learned about Induced-Draft industrial Cooling Towers like the one shown above.

To review, Induced-Draft Cooling Towers have these features:

• Side louvers admit air flow into the structure.
• A fan at the top of the structure inducts cool, dry air to flow cross-wise through the downward-trickling hot water and thence draws the warm, moist air upward and eventually blows it into the sky.
• Heat is removed from the hot water primary by evaporation.

Natural draft Hyperbolic Cooling Towers also remove heat from water primarily via evaporation.

When 1% of the cooling tower water inventory is evaporated the temperature of the remaining water decreases by 10 °F (5.6°C).

Basically, the structure of a Hyperbolic Cooling Tower is very similar to the Induced-Draft Cooling Tower...with the major exception that a really tall stack substitutes for the fan and a fan deck.

Otherwise, the below enumerated similarities between the two Cooling Towers are evident:

1. A hot water return header (labelled "Hot Water" in the above picture) brings hot water to the Hyperbolic Cooling Tower.

2. The hot water is sprayed out over fill (the criss-cross section labelled "Exchange Surface" in the above graphic). The fill breaks up the water and enhances contact with upward-flowing cooler air.

3. The now-colder water collects in a basin and is pumped back to the plant area in the Cold-Water Header (labelled "Cooled Water" in the graphic). The pump is not shown in the graphic.

4. The air enters the structure through vents and louvers that are located on the bottom of the structure.

5. Drift Eliminators are located above the hot water spray area which help to knock hot water droplets back to the fill area and thus reduce water losses.

6. Hyperbolic Cooling Tower Process Operators must also be vigilant for heebie-jeebie build up. A Blowdown valve will be included in the cooling water system.

The sources of heebie-jeebie build up in cooling water systems will be featured in future PTOA segments that focus on stationary equipment operations.

7. Although not shown in any of the graphics, Make-Up Water would be needed to replenish water losses due to evaporation, drift, and blowdown.

The one major difference between the Hyperbolic and Induced Draft Cooling tower is the air flow pattern.

Air flow and water flow in the Induced Draft Cooling Tower are designed to cross-flow. The air enters the side louvers and the water trickles vertically downward.

Air flow and water flow in the Hyperbolic Cooling Tower are designed to counter-flow. The air flows upward and the water trickles downward.

Hyperbolic Cooling Towers can be designed to cross-flow the cold air and hot water; however a separate box outside of the structure is required for this feature.

 

CLASSIFICATIONS OF COOLING TOWERS

This segment is getting long and wordy but let's go ahead and cover the classifications of Cooling Towers.

How is the hot air removed?

The primary classification is about the air flow mechanism.

Cooling towers that don't use fans are classified as "Natural Draft" Cooling Towers.

Cooling Towers that use fans are classified as "Mechanical Draft" Cooling Towers.

A subcategory of Mechanical Draft Cooling Towers identifies where the fan is situated in the tower.

A fan on top of the tower draws cool, dry air in from the side louvers and ejects the warm moist air into the atmosphere. This design is called an "Induced Mechanical Draft."

A cooling tower with fan(s) placed at the side of the tower for the purpose of forcing air into the structure identify a  "Forced Mechanical Draft" design. Forced Mechanical Draft designs are so inefficient that Your Mentor has never seen one.

How do the air and water streams interface?

The cool, dry air that enters the Cooling Tower can can interface with the hot, drizzling water by cross-flow or counter-flow patterns.

The Induced Mechanical Draft Cooling Tower featured in the previous PTOA segment featured a cross-flow design.

The Hyperbolic (aka Natural Draft) Cooling Tower featured in this segment used a counter-flow between the downward drizzling hot water and the upward flowing cold air.

 

TAKE HOME  MESSAGES: Hyperbolic Cooling Towers can cool very large flowrates of hot water and are usually associated with power plants.

Hyperbolic Cooling Towers use natural draft and therefore have no moving mechanical parts...like fans...to repair or pay utilities for.

Hyperbolic Cooling Towers have the same features and operating concerns as Mechanically-Induced Cooling Towers with these exceptions:

The tall hyperboloid chimney replaces the fan and fan deck.

The air is typically counter-flows upward through the drizzling, falling hot water.

There are no mechanical parts on a natural draft Cooling Tower to maintain and operate.

Both Hyperbolic and Mechanical-Draft Cooling Towers remove most heat by evaporation. A 1% loss of water to evaporation cools the remaining water by 10 deg F.

Cooling towers are classified by the draft type which can be natural or mechanically induced.

Mechanically induced drafts use fans and are identified as either induced draft (most common) or forced draft.

The air/water flow pattern is a second classification of cooling towers. Some towers use a cross-flow of air and water and others use a counter-flow (opposite flow) of air and water.

 

©2015 PTOA Segment 00041
Process Industry Temperature Changing Equipment

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