Double Pipe Heat Exchanger 121i5l

PARALLEL FLOW / COUNTERFLOW HEAT EXCHANGER AIM: (a) Calculate Heat -Transfer rate by using Parallel flow & Counter flow heat exchanger. (b) Compare the values of ∆ Tm & q in parallel and counter flow runs. THEORY: Heat exchangers are devices in which heat is transferred from one fluid to another. The process of exchanging heat between two .different fluid s is one of the most important and frequently used process found in many industries. The necessity for doing this, arises in a magnitude of industrial applications. Common examples of heat exchangers are the radiator of a car, the condenser at the back of a refrigerator and the steam boiler of a thermal power plant. Heat exchangers are classified into three categories: 1. Transfer type. 2. Storage type. 3. Direct type. A transfer type of heat exchanger is one on which both fluids simultaneously through the device and heat is transferred through separating walls. In practice most of heat exchangers used are transfer type .Transfer type heat exchanger are further classified according to flow arrangement as: Parallel flow in which fluids flow in the same direction. Counter flow in which fluids flow in opposite direction. Cross flow in which fluids flow at right angles to each other.

Hot fluid out

Hot fluid in

Cold fluid in

Cold fluid out Parallel flow Heat Exchanger

Hot fluid in

Hot fluid out

Cold fluid out

Cold fluid in Counter flow Heat Exchanger

Thi

Tco Tco

Tho Cross Flow Heat Exchanger

Temperature profiles: CASE-l When a fluid at a constant temp. gives heat to a colder fluid whose temp gradually increases. CASE-2 When a fluid at a constant temp. getting heated from a warmer fluid whose temp. gradually decreases. CASE-3 When a fluid gradually loosing heat and another fluid gradually gaining heat such system like water heater.

∆T2

∆ Tm = (∆ T1 ─∆ T2) / (ln ∆ T1 /∆ T2)

Temp. Thi

∆T1 Tco

Are

A simple Example of transfer type heat exchanger can be in the form of tube type arrangement in which one of the fluids is flowing through the inner tube and the other through the annulus surroundings it . The heat transfer takes place across the walls of the inner tube. PROCEDURE: 1. Place the thermometers in position and note down their readings when they are at room temp. and no water is flowing at either side. This is required to correct the temp. 2. Start the flow on the hot water side. 3. Start the flow through annulus and the run the exchanger either as parallel flow unit. 4. Put on the geyser.

5. Adjust the flow rate on hot water side ,between the rate of 1.5 to 4 litre per minute. 6. Adjust the flow rate on cold water side ,between the rate of 3 to 8 litre per minute. 7. Keeping the flow rates same, wait till the steady state conditions are reached. 8. Record the temps. on hot water and cold side and also the flow rates accurately. 9. Take the readings with different flow rates. 10. Repeat the experiment with a counter flow under identical flow conditions. OBSERVATIONS Inner tube material = copper – ID = 9.5 mm, OD = 12.7 mm Length of Heat Exchanger = 1.61 meter. (a) PARALLEL FLOW RUN S.N. HOT WATER SIDE COLD WATER SIDE Flow rate Thi 0C Tho 0C Flow rate Tci 0C Mh Kg/hr Mh Kg/hr

(b) COUNTER FLOW RUN S.N. HOT WATER SIDE Flow rate Thi 0C Mh Kg/hr

0

Tho C

COLD WATER SIDE Flow rate Tci 0C Mh Kg/hr

Tco 0C

Tco 0C

CALCULATIONS: i) Heat transfer rate is calculated as: qh = Heat transfer rate from hot water = mh h( T hi ─ Tho)Kcal/ hr. qc = Heat transfer rate from cold water = mc c( T co ─ Tci)Kcal/ hr. q = (qh + qc) / 2 Kcal/hr. Assume h= c = 1Kcal/kg-°c ii)Logarithmic mean temperature difference (LMTD) is calculated as follows: LMTD = ∆ Tm = (∆ Ti ─∆ To) / ln (∆ Ti / ∆ To) iii) Over all heat transfer coefficient can be calculated by using. q = U A Tm U = q / A ∆ Tm Kcal / hr-m2 - 0C Calculate Uri based on Ai = π di L

Uri based on Ao = π do L Iv) Compare the values of ∆ Tm & q in the parallel flow and counter flow runs. Note that if experiment is conducted very carefully then the superiority of counter flow Arrangement in of higher value of Tm and excess value of q for same flow rates conditions can be revealed. PRECAUTIONS: 1. During the complete experimentation open the valves of geyser before starting it. 2. For parallel flow open the valves V1& V3,and close . valves V2 & V4. 3. For counter flow open the valves V2&V4and close valves VI & V3.