Safety Hazards

Thermal Systems Laboratory Rooms 106 & 110

HAZARD: Rotating Equipment

Be aware of pinch points and possible entanglement

Personal Protective Equipment: Safety Goggles; Standing Shields,

Sturdy Shoes

No: Loose clothing; Neck Ties/Scarves; Jewelry (remove);

Long Hair (tie back)

HAZARD: Projectiles / Ejected Parts

Articles in motion may dislodge and become airborne

Personal Protective Equipment: Safety Goggles; Standing Shields

HAZARD: Heating - Burn

Be aware of hot surfaces Personal Protective Equipment: Safety Goggles; High Temperature Gloves;

HAZARD: Electrical - Burn / Shock

Care with electrical connections, particularly with grounding and not

using frayed electrical cords, can reduce hazard. Use GFCI receptacles near

water.

HAZARD: High Pressure Air-Fluid / Gas Cylinders / Vacuum

Inspect system integrity before operating any pressure / vacuum equipment.

Gas cylinders must be secured at all times. Use appropriate equipment guards.

Personal Protective Equipment: Safety Goggles

HAZARD: Water / Slip Hazard

Clean any spills immediately.

HAZARD: Noise

Personal Protective Equipment: Use Rated Ear Plugs

14

ME 406: Experiment 3

HEAT TRANSFER BY FREE AND FORCED CONVECTION

I. Objective:

To experimentally determine the overall coefficient of heat transfer (based on tube outside area) for the flow of heat through a condenser tube wall in both free and forced convection, compare it to that obtained from empirical correlations available in the literature, and estimate the heat loss from the equipment from combined convection and radiation.

II. Background:

Heat transfer in condensation, because of the involvement of phase change, requires a complex analysis. When a vapor is cooled to the extent that condensation takes place, heat is transferred in a fundamentally different manner than when heat is added (or taken away) from a fluid without a change of phase. The condensation of vapor liberates a considerable amount of energy, but at the same time a barrier is set up in the form of a liquid film which either partially or completely covers the cooler surface. The thickness of this film is influenced both by its viscosity and by the position and height of the surface (its geometry). Drainage from a vertical or an inclined surface is naturally more rapid than from a horizontal one and the film is accordingly thinner; but if the vertical height is great, the accumulation of condensation at the lower portion of the surface will thicken the film and make the lower portion less effective than the upper in transmitting heat. Other factors that may influence the conductance of films formed by condensation are:

a) velocity of vapor over surface of the film b) the formation of the film as a collection of liquid drops instead of a

continuous film (dropwise condensation). c) non condensable gases forming a “blanket” on the cold surface.

Nusselt developed the mathematical treatment of this phenomenon. Most heat transfer texts address this subject.

III. Equipment:

Steam is produced by an electric boiler and fed into a condensing tower. This tower is comprised of a closed jacket and a central single aluminum tube. Cooling water passes upward through the inside of this condenser tube, causing the steam to condense on the outside surface. Steam also condenses on the inside surface of the jacket as heat escapes out into the room. A boiler supply tank is used to provide and maintain a constant level in the boiler. This insures that the mass within the system remains constant during the experiment.

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Cooling water is provided by a head control system that allows the experiment to be performed with (1) free convection or (2) forced convection. All the copper-constantan (type T) thermocouples are monitored using a high impedance millivoltmeter. Tube wall and shell wall condensates are collected separately from drain tubes provided, and cooling water flow through the condenser tube is collected in the weigh tank mounted on the scale.

IV. Procedure:

1. Free Convection

a) Adjust valve (B) slowly until the water level in the condenser water

level control tank is at the free convection level. Maintain this level throughout Part 1 of the experiment.

b) Fill the boiler supply tank with water to the fill mark. Do not allow water in this tank to go below the low-level mark.

c) Open petcock (A) permitting the water to flow from the boiler supply tank into the boiler. Fill only to the fill mark. Close petcock (A).

d) All thermocouples are permanently connected to a multiple position switch mounted on the table. Connect a millivoltmeter to this switch. Provide for an ice point reference junction.

e) Turn on the power to the electric boiler. Set the variac so that the ampmeter reads 6 amps. When the steam temperature entering the condenser ( )1t reaches 220-240oF, refill the boiler by careful adjustment of the petcock (A) and thereafter maintain the water level at the red mark. Maintain input to boiler at 6 amps and never permit the boiler to run dry while connected to the power supply.

f) Allow adequate time for the system to reach equilibrium. This is established by monitoring all temperatures until they are stabilized (This should take about 15 minutes). The tube condensate, shell condensate and cooling water outlet containers should now be initialized. Record these and the time, and record all data every 5 minutes for a period of 30 minutes. Use the data sheet provided for this purpose.

2. Forced Convection

Repeat the procedure of part 1 after adjusting and maintaining the water level in the condenser water level control tank at the level marked “Forced”.

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V. Analysis 1. Complete the computation sheet provided and determine the value of U0,

the overall heat transfer coefficient through the condenser tube wall based on the tube outside surface area. Compare this value with that obtained from empirical correlations available in the literature.

2. Estimate the rate of heat loss from the equipment.

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