Double Pipe Heat Exchanger Report x555d

Abstract: In this experiment we are going to measure the effectiveness, NTU and the overall heat transfer coefficient in a double pipe heat exchanger in two arrangements (counter & parallel flow). We start the test and took the readings of the water entering and leaving the pipes for both hot and cold flow. To find the rate of heat transfer we applied energy balance on the pipes. Then we used the Kay and London law to know the performance of the heat exchanger (effectives & NTU).

Introduction: A heat exchanger is a device used to transfer heat between a solid object and a fluid, or between two or more fluids. The fluids may be separated by a solid wall to prevent mixing or they may be in direct . They are widely used in space heating, refrigeration, air conditioning, power stations. Examples of heat exchanger: 1. 2. 3. 4.

Double pipe Shell and tube Coil Plate

Theory 𝑚̇ = V̇ ∗ ρ 𝐶𝑟 =

(𝑚̇ ∗ 𝐶𝑝)𝑚𝑖𝑛 (𝑚̇ ∗ 𝐶𝑝)𝑚𝑎𝑥

ϵ=

𝑄 𝑎𝑐𝑡𝑢𝑎𝑙 𝑄 𝑚𝑎𝑥

𝑄 𝑎𝑐𝑡𝑢𝑎𝑙 = (𝑚̇ ∗ 𝐶𝑝)𝑐 ∗ (𝑇𝑜𝑢𝑡 − 𝑇𝑖𝑛𝑙𝑒𝑡)𝑐

𝑄 𝑎𝑐𝑡𝑢𝑎𝑙 = (𝑚̇ ∗ 𝐶𝑝)ℎ ∗ (𝑇𝑖𝑛𝑙𝑒𝑡 − 𝑇𝑜𝑢𝑡)ℎ

𝑄 𝑚𝑎𝑥 = (𝑚̇ ∗ 𝐶𝑝)𝑚𝑖𝑛 ∗ [(𝑇𝑖𝑛𝑙𝑒𝑡)ℎ − (𝑇𝑖𝑛𝑙𝑒𝑡)𝑐] 𝚫𝑇𝑚 =

{(𝑇𝑖𝑛𝑙𝑒𝑡)ℎ − (𝑇𝑜𝑢𝑡)𝑐} − {(𝑇𝑜𝑢𝑡)ℎ − (𝑇𝑖𝑛𝑙𝑒𝑡)𝑐} (𝑇𝑖𝑛𝑙𝑒𝑡)ℎ − (𝑇𝑜𝑢𝑡)𝑐 ln (𝑇𝑜𝑢𝑡)ℎ − (𝑇𝑖𝑛𝑙𝑒𝑡)𝑐 𝑄 = 𝑈 ∗ 𝐴𝑠 ∗ 𝚫𝑇𝑚 𝑁𝑇𝑈 =

𝑈 ∗ 𝐴𝑠 (𝑚̇ ∗ 𝐶𝑝)𝑚𝑖𝑛

𝐴𝑠 = π ∗ L ∗ Diameter

Where 𝑚̇ 𝑚𝑎𝑠𝑠 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒 𝑖𝑛

𝐾𝑔 𝑠

V̇ 𝑣𝑜𝑙𝑢𝑚𝑒𝑡𝑟𝑖𝑐 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒 𝑖𝑛

𝑚3 𝑠

ρ density in Kg/m3

𝐶𝑟 𝑖𝑠 𝑡ℎ𝑒 ℎ𝑒𝑎𝑡𝑖𝑛𝑔 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦 (𝑑𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑙𝑒𝑠𝑠)

ϵ 𝑖𝑠 𝑡ℎ𝑒 𝑒𝑓𝑓𝑒𝑐𝑡𝑖𝑣𝑛𝑒𝑠𝑠 (𝑑𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑙𝑒𝑠𝑠)

𝑄 𝑟𝑎𝑡𝑒 ℎ𝑒𝑎𝑡 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟 𝑖𝑛 𝐾𝑤

𝚫𝑇𝑚 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 𝑙𝑜𝑔𝑎𝑟𝑖𝑡ℎ𝑚𝑖𝑐 𝑚𝑒𝑎𝑛

𝑁𝑇𝑈 𝑡ℎ𝑒 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟 𝑢𝑛𝑖𝑡𝑠 (𝑑𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛𝑙𝑒𝑠𝑠) 𝑈 𝑓𝑜𝑢𝑙𝑖𝑛𝑔 𝑓𝑎𝑐𝑡𝑜𝑟

𝑊 ∗𝐾

𝑚2

𝐴𝑠 𝑖𝑠 𝑡ℎ𝑒 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑎𝑟𝑒 𝑖𝑛 𝑚2

Apparatus & procedure: 1. After setting up a given configuration (parallel or counter, switch the pump on (hot stream flow). 2. Open the tap cold water valve to initiate the cold stream 3. Set the heater control to the required temperature. Do not exceed 80°C. 4. Allow the water in the tank to stabilize at the set temperature. Monitor the stability of the system through watching the inlet and outlet hot water temperature. 5. Set the mass flow rate of the hot and cold streams, allow the system to stabilize. 6. Record the readings of the temperatures profiles for the hot and cold streams especially the terminal temperature (in and out for both flows) 7. Record the mass flow rates of the two streams (me, mh). The cold water flow rate is calculated as the volume of water es (read from the water flow meter on the tap water piping per certain period of time (use stop watch). The same thing is valid for the hot stream flow.

Inner Pipe

Outer Pipe

Outlet Cold Water valves

Inlet Cold Water

Take readings of temperature from this knob that is attached to different thermocouples

Outlet Hot Water Inlet Hot Water pump

P

Water Tank

We control the flow if parallel or counter by the valves. In our setup we only control cold flow

Discussion: As the area increase in the heat exchanger as the rate of heat transfer increase , this exist in shell and tube heat exchangers because we can add more pipes without taking much space, so they are used in industries. Coil heat exchangers used in refrigeration cycles Plate heat exchanger used in automotive vehicles Heat exchangers wall usually very thin, so the resistance of the conductivity decrease. Our goal is to achieve the highest temperature in the cold flow at exist and the lowest temperature in the hot flow exit. (Max heat transfer wanted) In counter flow 𝑇𝑐𝑜 > 𝑇ℎ𝑜 In parallel flow 𝑇𝑐𝑜 𝑐𝑎𝑛 𝑛𝑒𝑣𝑒𝑟 𝑏𝑒 𝑔𝑟𝑒𝑎𝑡𝑒𝑟 𝑡ℎ𝑎𝑛 𝑇ℎ𝑜 That is because the temperature difference in parallel flow at the entrance is very high and will decrease with length, but in counter flow the temperature difference is uniform. This means that thermal stress may occur in parallel flow heat exchanger So why we want parallel flow? It is used in any application which we want to maintain specific temperature.

We usually put the hot stream inside so that is losses the heat to the cold fluid surrounded by it, That means we don’t have to use insulators.(saving money and space)

(𝑚̇ ∗ 𝐶𝑝)𝑐 = 𝐶𝑐 , (𝑚̇ ∗ 𝐶𝑝)ℎ = 𝐶ℎ = The heat capacity ratio If Ch = Cc the cold flow temperature eventually will reach the hot flow temperature If Ch is much bigger than Cc, the hot flow will experience low temp difference due to large mass flow rate

If Ch is much smaller than Cc, the cold flow will experience of low temp difference due to large mass flow rate

Since we have non-linear relationship we used logarithmic mean temperature difference and because we have 4 temperature and we need the average.

Conclusion: As the area increase in the heat exchanger as the rate of heat transfer increase. Heat exchangers wall should be very thin. In counter flow 𝑇𝑐𝑜 > 𝑇ℎ𝑜 In parallel flow 𝑇𝑐𝑜 𝑐𝑎𝑛 𝑛𝑒𝑣𝑒𝑟 𝑏𝑒 𝑔𝑟𝑒𝑎𝑡𝑒𝑟 𝑡ℎ𝑎𝑛 𝑇ℎ𝑜 The maximum heat transfer happen at the lower heat capacity ratio If Cr = 0, it is a special case of heat exchanger which is a condenser or evaporator And 𝜖 = 1 − 𝑒 −𝑁𝑇𝑈 What the cold water gain, the hot water losses so both Q should be equal but due to human and machines errors.