Double Pipe Heat Exchanger

ABSTRACTThe objective of this experiment was to find the length of the heat exchanger pipe. The apparatus we used is double pipe heat exchanger and from the basic knowledge of fluid dynamics and heat transfer we find length. Actually there is a lot of use of heat exchanger in industry (In nuclear industry as well) , so we are interested in that how exchange of heat from hot water to cold water take place and what should be the size of equipment. This experiment is arranged to understand this concept.INTRODUCTIONWe know that heat is the energy that flow from hot body to cold body due to their temperature difference. So we have heat exchanger which exchange heat from hot fluid to cold fluid while keeping both fluids separate. We also know that energy can be transferred from hot to cold body by mean of conduction, convection or radiation. In case of heat exchanger we have encounter two mechanism, convection and conduction.

In case of double pipe heat exchanger we know that it consist of two concentric pipe of different diameter. The hot fluid is allowed to flow from inner pipe and cold fluid is allowed to flow from outer pipe. So heat from center of inner pipe to wall of inner pipe is transfer due to convection and then by mean of conduction it flow from wall of inner pipe and then again by convection it reach the boundary of outer pipe. In this apparatus we have option to have both the flows (hot fluid flow and cold fluid flow) clockwise or counter clockwise. The conditions such as inlet temperatures and outlet temperatures and flow rates determine that how much heat will flow across the fluids. In this experiment we fix the flow rate of cold fluid (water) and then by changing the flow rate of hot water, we measure the inlet and outlet temperatures and using different formulas regarding Reynold number, friction factor, hs, prandlt number , after very cumbersome calculation we find length. Parallel and antiparallel flow heat exchanger may be viewed as

Parallel Flow Heat Exchanger

Counter Flow Heat ExchangeIn this exchanger hot water flow from inner pipe and cold fluid flow from annular region between concentric pipes. We know something about apparatus as

i) Inner pipe ID = 13 mmii) Inner pipe OD = 16 mmiii) Outer pipe ID = 26 mmiv) Outer pipe OD=29 mmv) Full heat exchanger length = 2 metersThe experiment is shown in Figure 1. The controls for the experiment are described below.Fig. 3 The experiment setup

We have valves, Rota meters, temperature gauges in this apparatus.

CALCULATIONSHeat flow from hot body to cold body by conduction obeying Fouriers law

Qh=hCph ThQc=cCpc Tc

While average heat can be calculated as

By writing the conservation of energy equations between the inlet and outlet of the two fluids, and the differential energy transfer (dq) between the two fluids on a differential distance dx , the overall heat transfer between the two fluids is

LMTD stands for Log Mean Temperature Difference. Where U is the overall effective heat transfer coefficient, and A is the contact surface area between the two fluids on either the cold or the hot side.

Log mean temperature difference: The log mean temperature difference (LMTD) is used to determine the temperature driving force for heat transfer in flow systems. The LMTD is a logarithmic average of the temperature difference between the hot and cold streams at each end of the exchanger. The use of the LMTD arises straightforwardly from the analysis of a heat exchanger with constant flow rate and fluid thermal properties.

For the parallel flow heat exchanger,T1=Thi-TciT2=Tho-Tco

For counter flow heat exchanger,T1=Thi-TcoT2=Tho-Tci

Surface area can be calculated asA=DhiLFrom this formula we can calculate length.To determine the area of the heat exchangers we need to calculate overall heat transfer coefficient U. whereas

Where h is

Nu is Nusselt number, k is thermal conductivity and HD is hydraulic diameter.Nusselt number is

Prantdl number is

We took values of and Cp from table. f can be calculated as

Here Re represents Reynolds number. There are two types of Reynolds number one is wetted Reynolds number and other is heated Reynolds number. The formula of the Reynolds number is given as

HD is hydraulic diameter. For wetted Reynolds number

For heated Reynolds number

While cross sectional area of cold and hot water pipes are given as

Length can be calculated as

Procedure1) Valves were set for parallel flow. 2) Hot water circulation was started. 3) After a few moments when steady state was achieved, cold water circulation was started. 4) After the steady state was achieved for the second time readings were noted for the inlet and outlet temperatures and flow rates of cold and hot waters.5) For a fixed flow rate for cold water three different flow rates for hot water were used to get observations.6) Then this whole procedure was repeated for the counter flow arrangement.7) During this arrangement flow rate for hot water is kept constant and three different flow rates for cold water were used to get observations.GraphsFor Parallel Flow

For Counter Flow

Discussion:Differences in the experimental values from each other and from the true value could be due to the fact that it is assumed a perfectly insulated system with no heat loss to surroundings, an inability for the system to properly recover a new steady state starting temperature from the previous experiment trial (improper heating/cooling time), and also there could have been heat exchange taking place before and after the thermocouples along with the possibility of a fouling factor caused by deposits and corrosion.With the decrease in temperature the length required for heat exchanger decreases. The theoretical length of heat exchanger is always less than actual length of heat exchanger. Probably the net heat transfer between the two fluids is not the same as actual because of our assumption that there is no conduction and radiation. Therefore some heat will be lost to the atmosphere. Hence practically we need a heat exchanger of greater length to compensate these losses.

Conclusion:In the parallel flow configuration, the exit temperature of the hot fluid must be higher than the exit temperature of the cold fluid. This is supported by the data taken. In the counter flow configuration, the exit temperature of the hot fluid must be higher than the entrance temperature of the cold fluid, but it does not necessarily need to be higher than the exit temperature of the cold fluid. From the calculations resulting in heat transfer, it is shown that the counter flow heat exchanger is more effective than the parallel flow heat exchanger. This supports generally held knowledge and experimental data concerning the two types of heat exchanger. In this case, the energy is transferred from hot to cold fluids with constant mass flow rates. Therefore the ratio between temperature differences does not change even though the numerical values of the temperature differences may change.The ratio between temperature difference in the hot fluid and temperature difference in the cold fluid changes with respect to the flow rates. It means the energy removed from the hot fluid is the energy added to the cold fluid. The higher the flow rate of a fluid, the lower the temperature change in that fluid will be. The opposite is also true, the lower the flow rate of the fluid, the higher the temperature change in the fluid will be.With the decrease in temperature the length required for heat exchanger decreases. The theoretical length of heat exchanger is always less than actual length of heat exchanger. Probably the net heat transfer between the two fluids is not the same as actual because of our assumption that there is no conduction and radiation. Therefore some heat will be lost to the atmosphere. Hence practically we need a heat exchanger of greater length to compensate these losses.