Determination of Effectiveness of a Heat Exchanger by Parallel Flow

Aim

To determine the overall heat transfer co –efficient in Counter flow heat exchanger

Specifications:

Length of the heat exchanger= 1800 mm

Inner Copper Tube (ID) = 12 mm

Inner Copper tube (OD) = 15 mm

Outer GI tube Id = 40 mm

Apparatus Required:

• Experimental setup of heat exchanger
• Stabilized power Supply
• Stop Clock

Diagram

Procedure:

1. Connect water supply to the pipe arrangement.
2. Adjust the water supply on hot and cold sides by opening the valves, so that cold water flows counter to the hot water.
3. Set a constant flow rate of 2 lit /min for hot water and 5 lit / min for cold water by controlling the valves.
4. Switch on the water heater and allow the water to get heated to a higher temperature and reach a steady state.
5. Observe the temperature of water at the inlet and exit of cold water & hot water after a steady state is reached.
6. Measure time taken for flow of water by collecting water in a 500 ml flask.

Observations:

Formula Used

Heat content of Hot Water

Qh = mh Cph (T_hi- T_ho) watts

Where,

m_h - Mass flow rate of hot water, kg,/sec

T_hi - Temperature of hot water inlet (°C)

T_ho - Temperature of hot water outlet (°C)

C_ph - Specific heat of hot water in kJ/kg K

Heat content of cold water:

Q_c = m_c C_pc (T_co - T_ci) watts

Where,

m_c - Mass flow rate of cold water, kg,/sec

T_ci - Temperature of inlet cold water (°C)

T_co - Temperature of outlet cold water (°C)

C_pc - Specific heat of cold water in kJ /Kg K

Q = Q _h + Q _c/2 KJ/Sec

Logarithmic Mean Temperature Difference ( LMTD):

LMTD (ΔTm) = (δT _{i-δT o)}

In (δT _{i-δT o)}

= (T_{ho-Tco) - (Thi-Tci)}

In (T_{ho-Tco/Thi-Tci)}

Where,

δT _{i = T ho-T co}

δT _{o = T hi-T ci}

Overall Heat Transfer Co-Efficient

Ui - = Q/A _iΔT_mW/m²°C

Where,

Ui - Overall heat transfer coefficient based on inner surface area in W/m²°

Ai -Inner surface area in m²

ΔTm - Logarithmic mean temperature difference

Uo - = Q/A _oΔT_mW/m²°C

Where,

Uo - Overall heat transfer coefficient based on inner surface area in W/m²°C

Ao -Outer surface area in m²

ΔTm - Logarithmic mean temperature difference

Effectiveness of Heat Exchanger = m_c C_po(T _{o = T co-T ci)}

m_h C_ph(T _{hi - T ho)}

Result:

The effectiveness of heat exchanger for Counter flow is ______ _ __

The overall heat transfer co –efficient based on inner surface area of heat exchanger is ___

The overall heat transfer co –efficient based on outer surface area of heat exchanger is ___

Viva-Voce Questions

What is heat exchanger?

Heat exchanger is equipment which transfers the energy from a hot fluid to cold fluids, with maximum rate & minimum investment

Explain LMTD?

LMTD is defined as the temperature difference which, if constant, would give the same rate of heat transfer as actually occurs under variable conditions of temperature difference.

For evaporators & condensers, what is the value of LMTD for parallel & counter flow?

For evaporators & condensers, LMTD for parallel & counter flow is equal/same.

What is the value of LMTD if heat capacity of both fluids is same?

If heat capacity of both fluids is same, then LMTD is equal to temperature difference at either ends.

What is Relative direction of motion of fluids?

Relative direction of motion of fluids: Parallel, Counter & Cross flow.

When NTU method is particularly useful in design of heat exchangers?

NTU method is necessary if outlet temperature of both fluids is not known as priority.