Lab7 - Energy - notes PDF

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Department of Mechanical and Industrial Engineering MECH 441 Energy Systems Lab Section: L04

[Spring 2015]

Experiment (7): Heat Exchanger

Submitted by: Group (2): 1. Name:

Abdullah Piroozi

ID: 201107592

2. Name:

ID:

3. Name: Abdalla Takrouny

ID: 201106921

Instructor: Dr. Mohamed Al Khawaja Submitted to: Eng. Pratheesh Ben

Date of experiment: 20/5/2015 Date of submission: 27/5/2015

1. Objective 

To demonstrate the differences between a plate type heat exchanger and a tubular heat exchanger operating in counter current flow configuration.

2. Method The results from the experiment which evaluates the variation of temperature efficiencies (effectiveness) with hot water flow rate of both the heat exchangers will be used to assess the performance of the heat exchangers.

3. Equipment Required HT30XC Heat Exchanger Service Unit The Service Unit is assembled on a supporting base/plinth, which is vacuum formed in robust ABS plastic and designed for bench mounting. It includes mountings and service connections for any one of the interchangeable heat exchangers designed for use with the service unit, as well as a drainage channel. The appropriate heat exchanger is attached to the plinth top by locating the holes in the support plate on the studs in the plinth top and securing it using the knurled thumbscrews provided with the exchanger.

The equipment should be installed and switched on as described in the HT30CX product manual, which also includes installation information for the Heat Exchanger accessories (HT31, HT32, HT33 and HT34). Installation requires physical access to the equipment. This online Help Text describes the remote operation of the HT30XC and Heat Exchanger accessory using the computer software.

Setting the hot water flow rate and direction: The hot water flow rate can be controlled from the computer software by varying the rotational speed of the re-circulation pump. Again this can be set from 0% to 100%, with the actual flow rate being measured by a flow meter and displayed in L/min on the computer screen. The hot water flow direction is set as a default value in the Armfield Software. If a counter-current exercise is chosen, the flow is in the direction indicated by the arrows adjacent to the two hot water connections. If a co-current exercise is chosen the direction of rotation of the pump and therefore the flow of water is reversed. Note: A change in the temperature of the water will affect the viscosity of the water resulting in a small change in flow rate. It will therefore be necessary to adjust the hot water flow control in the software if it is required to perform tests at the same flow rate but different temperatures.

Setting the hot water temperature: Two modes are available for controlling the hot water temperature, a manual (or open loop) control mode to provide constant heater power and an auto (or closed loop) temperature control mode. Both modes are accessed via the software. In manual mode, the heaters are set to be on for a fixed proportion of time, operator selectable from 0% to 100%. This mode is useful when assessing energy balances or settling times. In auto mode, the power to the heaters is modulated in accordance with a PID algorithm to achieve a stable temperature at one of the sensors (usually the hot water inlet to the heat exchanger). Advanced users may change the P, I and D parameters to perform process control investigations.

To access the heater control mode click the software ‘control’ button close to the appropriate sensor. Mimic Diagram screen of HT30XC computer software

On top of the plinth is the hot water vessel, heated by an electrical heating element which incorporates an over-temperature thermostat to prevent overheating. The operator is protected from hot water in the vessel by a screen and lid. Hot water is circulated through the system by a gear pump mounted beside the vessel, which may be used to change the flow direction through the heat exchanger (and thus to switch between countercurrent and concurrent flow). Cold water flow (the process flow) for the heat exchanger is derived from the local mains water supply, with the equipment protected by a pressure regulator and integral filter and the flow controlled by a proportioning solenoid valve. Flow meters measure the hot and cold flow rates, and thermocouple sensors measure the temperature at key points throughout the heat exchanger. Once a heat exchanger is connected, the unit can be entirely operated via a computer and all sensor outputs can be logged. A panel on the front of the Service Unit contains the mains switch and the Emergency Stop switch. The panel also includes the connector for the USB interface to the computer, and two USB status indicators. A red ‘power’ LED lights when the unit is connected to the PC and a green ‘active’ LED lights when the unit has been recognized by the PC. As the unit is software controlled, it is fitted with a ‘watchdog’ circuit which switches off the heaters, pump and cold water control valve in the event of a software ‘crash’ or breakdown in communication between the software and the HT30XC.

HT31 Tubular Heat Exchanger

Technical Details of the HT31

Technical details of the heat exchanger construction are as follows: 

Each inner tube is constructed from stainless steel tube, 9.5mm OD, 0.6mm wall thickness.



Each outer tube is constructed from clear acrylic tube, 12mm ID, 3.0mm wall thickness.



Each heat transfer section is 330mm long giving a combined heat transfer area of approximately 20000mm² which is equivalent to that of the HT33 Shell and Tube Heat Exchanger for direct comparison.



Temperatures are measured using type K thermocouples, each with a miniature plug for direct connection to the electrical console on the service unit. Thermocouples are installed at the following 6 locations (when operated countercurrent): o Hot fluid inlet T1 o Hot fluid mid-position T2 o Hot fluid outlet T3 o Cold fluid inlet T4 o Cold fluid mid-position T5 o Cold fluid outlet T6

HT32 Plate Heat Exchanger

Technical details of the heat exchanger construction are as follows: 

Pack of seven plates and gaskets arranged for multi-pass operation with passes in series (pattern of holes in the plates and shape of the gaskets determine the direction of flow through the exchanger).



High quality 316 stainless steel plates incorporate a locating groove for the gasket and pressed chevron pattern to promote turbulence and provide multiple support points.



Silicone rubber gasket on each plate ensures that the adjacent flow channels are sealed from each other.



Frame to support the plates with tightening bolts to compress the pack. Frame incorporates fixed and moving end plates with connections for hot and cold fluids.



Exchanger easily dismantled for inspection of the heat transfer surfaces: o Number of active plates: 5 o Plate overall dimensions: 75mm x 115mm o Effective diameter: 3.0mm o Plate thickness: 0.5mm o Wetted perimeter: 153.0mm o Projected heat transmission area 0.008 m² per plate



Temperatures are measured using type K thermocouples, each with a miniature plug for direct connection to the electrical console on the service unit. Thermocouples are installed at the following 4 locations (when operated countercurrent): o Hot fluid inlet T1 o Hot fluid outlet T2 o Cold fluid inlet T3 o Cold fluid outlet T4

4. Equipment set-up Connect the hot and cold water supplies to provide countercurrent operation (flows in opposite directions).

Nomenclature

5. Theory/Background For finding the efficiency (effectiveness) of the heat exchanger under Countercurrent operation When the heat exchanger is connected for countercurrent operation the hot and cold fluid streams flow in opposite directions across the heat transfer surface (the two fluid streams enter the heat exchanger at opposite ends). For tubular heat exchanger the hot fluid passes through the inner tube, the cold fluid passes over the hot fluid through the outer shell. For

the plate type heat exchanger it flow is opposite directions in alternate flow channels between the plates.

Countercurrent Temperature Profile Tubular Heat Exchanger The reduction in hot fluid temperature: ∆ T hot =T 1−T 3(℃) And the increase in cold fluid temperature: ∆ T cold =T 6−T 4(℃)

A useful measure of the heat exchanger performance is the temperature efficiency of each fluid stream. The temperature change in each fluid stream is compared with the maximum temperature difference between the two fluid streams giving a comparison with an exchange of infinite size. Temperature efficiency for hot fluid: ηh =

T 1−T 3 ∙ 100 % T 1 −T 4

Temperature efficiency for cold fluid: ηc =

T 6−T 4 ∙ 100 % T 1− T 4

Plate type heat exchanger: Consider Temperature inlet and outlet as specified in the equipment description and use similar equations.

6. Procedure: Operational Procedures: 

Load the HT31 software and select Countercurrent Operation. For best results ensure that the filter mode (Options - IFD Sampling Parameters) is set to custom.



Enter the temperature controller screen and set the set point to 30°C and mode to automatic.



Adjust the cold water flow control (not the pressure regulator) to give 1.5 liter/min and the hot water flow control to give maximum flow rate in liter/min.



Operate the Refrigerating/Heating circulator to provide water at 5°C and start the circulator. This water should be connected to the cooling water side of the heat exchanger.



Allow the heat exchanger to stabilize. (Use the IFD Channel History screen to monitor the temperatures).



When the temperatures are stable, take a sample.

Vary the hot fluid flow rate and set it to the next 0.5 l/min rate and take a sample reading when the temperatures become stable. Repeat the experiment until a flow of 0.5 l/min is reached. Repeat the experiment for the next heat exchanger.

7. Results and Calculations Each set of readings is presented in the table you obtain from the software. The result table is taken from the data acquisition software.

1. Calculate the temperature efficiency for all data sets. ( done in Excel ) 2. Plot a relevant curve to find the variation of temp efficiencies w.r.t the hot water flow rate. Please note that all curves should be in a single graph sheet. ( done in Excel ) 3. Find the flow rates of both heat exchangers at which the hot and cold fluid temperature efficiencies are the same.

8. Conclusions 1. Your results from this exercise should indicate clearly the basic differences between the performance of both the heat exchangers. 2. Comment on the differences between the hot and cold fluid temperature efficiency

of both heat exchangers....