Heat Exchangers and Condensers

Question 1

Explain the difference between surface and direct contact heat exchangers.

Answer 1

In surface heat exchangers, the two fluids are separated by a solid surface such as a tube or metal plate. In direct contact heat exchangers, the two fluids come in contact with each other.

Question 2

Classify shell and tube heat exchangers by flow paths.

Answer 2

(1) counter flow, (2) parallel flow, and (3) cross flow.

Question 3

Describe basic heat transfer in a heat exchanger.

Answer 3

Primarily by conduction and convection. Conduction is the process of heat transfer between interacting adjacent molecules of the material through which the heat is being transferred. Convection is the transfer of heat by a process of bulk motion and mixing of macroscopic portions of a fluid.

Question 4

Identify and describe major components of a heat exchanger.

Answer 4

Typical shell and tube heat exchangers consist of a shell, a tube bundle, tube sheets, two water boxes, and two fluids.

Question 5

Explain the procedure for filling a shell and tube heat exchanger.

Answer 5

Gradually introduce fluids into the heat exchanger by

throttling the inlet valves with the vent valves open

Question 6

State the definition of thermal shock.

Answer 6

Thermal shock occurs when the temperature of the fluid in a system suddenly increases or decreases, causing a differential expansion or contraction of the metal. If the
internal pressure of the system is high or suddenly becomes high, it is possible for brittle fracture of the system to occur due to the additional stresses.

Question 7

Describe fluid hammer and how it is prevented.

Answer 7

Occurs when the velocity of a fluid flowing in a system suddenly changes. Water hammer is minimized by slowly opening and closing valves and keeping fluid systems filled and vented.

Question 8

Explain the procedure for startup/shutdown of a heat exchanger.

Answer 8

During startup, the cold fluid is circulated first for and the hot fluid is valved in last to prevent thermal shock. For shutdown the hot fluid is valved out first.

Question 9

Describe temperature control in heat exchangers.

Answer 9

Flow of one or both fluids in a heat exchanger is the primary means used to regulate temperature. Cooling systems more commonly control the cooling water flow rate to regulate temperature in the system of concern.

Question 10

Identify the relationship between flow rates and temperature.

Answer 10

The convective heat transfer coefficient is higher for

higher mass flow rates,

Question 11

Explain the effect of heat exchanger flow rates that are too low.

Answer 11

At low flows, the flow is in the laminar region, heat
transfer is by conduction from the metal surface
and through the fluid. This can lead to thermal
stratification of the fluid. This reduces the performance of the heat exchanger because part of the fluid is not actively participating in heat transfer. The low fluid velocity also allows more time for scale and sediment deposits to form.

Question 12

Explain the effects of tube fouling on heat exchanger operation.

Answer 12

It restricts the flow through the heat exchanger and increases the pressure drop across the exchanger. Heat transfer is decreased as fouling and solid material build up on the heat transfer surfaces.

Question 13

Explain the effects of scaling on heat exchanger operation.

Answer 13

Refers to deposits occurring due to inorganic salts coming out of solution on the heat transfer surfaces. Affects conductive heat transfer through the tube walls.

Question 14

List the consequences of heat exchanger tube failure.

Answer 14

Inventory loss, introduction of fouling chemicals, loss of radioactive material control.

Question 15

List the reasons for noncondensable gas removal from a condenser or heat exchanger.

Answer 15

If not removed, it greatly reduces condenser efficiency because the steam must diffuse through a film of air and noncondensables before reaching the condensing
surface

Question 16

Explain the effect of heat exchanger flow rates that are too high.

Answer 16

The higher fluid velocity leads to higher tube erosion

and vibration.

Question 17

Explain the relationship between condenser vacuum and backpressure.

Answer 17

Vacuum is often expressed in terms of condenser backpressure. Condenser backpressure is measured in inches of mercury absolute and is equal to atmospheric
pressure (29.92 in Hg abs/14.7 psia) minus condenser vacuum.

Question 18

Identify and describe major components of a condenser.

Answer 18

All condensers have a shell, tubes, tube sheets, water

boxes and a hotwell.

Question 19

Explain the principle of operation of condensers.

Answer 19

As water circulates through the tubes, steam exhausted from a turbine enters the top of the condenser. The steam flows down and around the tubes. Heat is transferred from the steam through the tubes and into the circulating water. The temperature of the circulating water increases and the temperature of the steam remains the same. However, a phase change does take place. The steam condenses on the tubes and its volume decreases, creating a vacuum in the condenser shell.

Question 20

Describe the basic operation of a U-tube type steam generator.

Answer 20

The reactor coolant flows through the primary side, or inverted U-tubes, entering and leaving through the hot and cold leg nozzles. The head is divided into inlet and outlet chambers by a vertical partition plate extending from the head to the tube sheet. The steam-water mixture is generated on the secondary, or shell side, and flows upward through the moisture separators to the outlet nozzle at the top of the vessel providing an essentially dry and saturated steam.

Question 21

Describe the basic operation of a once through steam generator.

Answer 21

Reactor coolant enters the top of the steam
generator through one inlet nozzle. It gives up heat to the secondary fluid as it flows through the tubes. The coolant flows into the lower head where it is directed back to the reactor through two outlet nozzles.

Question 22

Describe the benefits of the once through steam generator compared to the U-tube steam generator design.

Answer 22

1. No steam drum is required.
2. Superheated steam improves steam quality.
3. Superheated steam also minimizes turbine
   size and reduces steam flow requirements for
   a given megawatt output.
4. Tube stresses are less in an OTSG than a U-tube design. This results in fewer stress related tube failures.

Question 23

Describe the benefits of the U-tube steam generator compared to the once through steam generator design.

Answer 23

The volume of water maintained in a U-tube SG at 100% power is much larger than in an OTSG. During a loss of feedwater transient, this provides a greater margin of time for operators to respond before reactor core cooling is lost.