WL110e V1.1 Duplex

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WL 110-SERIES Heat Exchanger with

Service Unit

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Experiment Instructions

Dipl.-Ing. (FH) Klaus Schröder

This manual must be kept by the unit. Before operating the unit:

- Read this manual.

- All participants must be instructed on

WL 110.04

WL 110.03 WL 110.01

WL 110

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For experiments with the WL 110.04 Jacketed Vessel with Stirrer and Coil the electrical con-nection for the stirrer is provided by the connect-ing socket (36). The stirrer can be turned on and off using the switch (34). The speed of the stirrer is set using the adjusting knob (32) in the range 0...100%.

The connecting sockets (37) and (38) are used to connect additional temperature sensors for the heat exchangers to the service unit.

The digital displays show the following measured variables (example of hot water circuit): • Hot water feed temperature T₁ (27).

• Hot water return temperature T₃ (26). • Hot water temperature, centre T₂ (25). • Hot water flow rate V·1 (24).

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Controllers, operation and function:

A hardware controller with display (28) is installed for the temperature control loop.

Fig. 3.13 shows this controller TIC7 (28).

It displays the actual value and setpoint. The desired setpoint can be set using the two arrow buttons.

The controller is enabled using the switch (30).

The controller TIC7 (28) regulates the hot water temperature T₇. It operates as a step controller. If the hot water temperature is too low, the heater (H) is activated. When the setpoint for T₇ is reached, the heater is deactivated.

The parameters for the TIC7 controller are preset during production (for values see Chapter 6.1, Page 79).

The warning lamp (29) indicates low water in the hot water tank (after tripping of the level switch LSL1).

If the water is low, operation of the heater is inter-rupted. The aim is to prevent overheating and unacceptably high loads on the heater.

Fig. 3.13 Controller TIC7 (28)

Actual value

Setpoint

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3.5 Data acquisition program

The data acquisition program supplied is used to record and evaluate the current measured values.

The data acquisition program provides the follow-ing options for displayfollow-ing the current measured values and calculated values:

• System diagram

• Time response for measured values

• Current temperature curve, with display of cal-culated values

• The available measured and calculated values are recorded at definable intervals in meas-ured value files. These measmeas-ured value files can be imported into spreadsheet programs (such as MS Excel®) and processed.

The program’s Help function explains how to use the data acquisition program (see also Chapter 3.5.2, Page 24).

3.5.1 Installing the program

The following is needed for the installation:

• A fully operational PC, laptop or notebook with USB port (for minimum requirements see Chapter 6.1, Page 79 onwards).

• G.U.N.T. CD-ROM.

Notice! All components necessary to install and run the program are contained on the CD-ROM supplied by G.U.N.T. along with the WL 110. No other items are necessary!

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After starting, the installation runs automatically. During the course of the installation, various pro-gram components are loaded onto the PC:

• LabVIEW® runtime program for PC data acquisition.

• Driver routines for the “LabJack®” USB converter.

Installation routine

Notice! The trainer must not be connected to the PC's USB port during the installation of the program. Only after the software has been installed can the trainer be connected.

• Start the PC.

• Insert the G.U.N.T. CD-ROM for the WL 110. • From the “Installer” folder, launch the

“Setup.exe” installation program.

• Follow the installation procedure on-screen. • Reboot the PC after the installation is finished.

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3.5.2 Operating the program

The program for the WL 110 is selected and started using:

Start / All Programs / G.U.N.T. / WL 110.

When the software is run for the first time after installation, the language to be used for the pro-gram is requested (once only).

Notice! The language selected can subsequently be changed at any time in the “Language” menu.

The system diagram for the WL 110 series then appears on the screen.

Various pull-down menus are provided for addi-tional functions.

For detailed instructions on use of the program refer to its Help function. This Help function is accessed by opening the „?“ pull-down menu and selecting „Help“.

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3.6 WL 110.01 Tubular Heat Exchanger unit description 3.6.1 Layout and function

The adjacent photo shows the WL 110.01 Tubu-lar Heat Exchanger with base plate.

The tubular heat exchanger consists of two double tubes. In the double tubes, the transpar-ent outer tube allows the stainless steel inner tube to be seen. Two separate areas are created, the tube area (inside the inner tube) and the shell (between the inner tube and the outer tube). Both the tube areas and the shells of the two dou-ble tubes are connected in series.

The split into two double tubes reduces the overall length and enables temperature measurement for cold and hot water in the centre of the overall heat exchanger.

Fig. 3.16 illustrates the flow. As determined by the two different coupling designs (7), hot water (red) flows through the tube area and cold water (shown in blue) through the shell.

Cold and hot water flow along the inner tubes either in the same direction (parallel flow) or in opposite directions (counter flow).

Fig. 3.15 WL 110.01, with base plate

Fig. 3.16 WL 110.01, schematic

Parallel flow

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3.6.2 Connection to the service unit

Fig. 3.17 explains the individual steps required to connect the WL 110.01 to the service unit: 1. Secure the base plate of the tubular heat

exchanger on the base plate (1) of the service unit using the star grip bolts.

2. Connect the plug for the hot water temperature, centre (TI2) measuring lead to the appropriate socket (item 37 in Fig. 3.12, Page 19).

3. Connect the plug for the cold water tempera-ture, centre (TI5) measuring lead to the appro-priate socket (item 38 in Fig. 3.12, Page 19). 4. Plug the couplings (7) for hot water into the

cor-responding connections on the tubular heat exchanger.

5. Plug the couplings (7) for cold water into the corresponding connections on the tubular heat exchanger. Ensure that the required flow is produced (parallel flow or counter flow).

Fig. 3.17 Connection for WL 110.01 TI5

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3.6.3 General information for tubular heat exchanger

Advantages of tubular heat exchangers: • Simple construction.

• Connecting together several double tubes enables the heat transfer area to be varied by changing the number of double tubes.

• Because it is possible to have large flow cross-sections, the unit is also suitable for high vis-cosity fluids and for products containing solid pieces or fibres.

• There is a hygienic advantage as the tube area is free of flow dead zones (important in the food industry, for example).

Tubular heat exchangers are used for applica-tions including the food industry, with an example shown in the adjacent photo.

It shows a module consisting of a large number of tubular heat exchangers connected in series. Here, the individual double tubes are arranged in several rows in a frame.

The individual double tubes are connected using double tube bends.

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3.7 WL 110.02 Plate Heat Exchanger unit description 3.7.1 Layout and function

The adjacent photos show the WL 110.02 Plate Heat Exchanger.

Fig. 3.19 shows the plate heat exchanger with the base plate. Fig. 3.20 shows an enlarged view of the plate heat exchanger screwed onto the base plate.

This plate heat exchanger is essentially made up of six plates soldered together, which form two separate flow channels. The solder points seal the plates against one another.

Fig. 3.21 illustrates the principle (illustrated with four plates). Cold (blue) and hot spaces (red) alternate in the arrangement.

Openings in the plates allow the media to flow. The surface of the plates is not smooth but has a characteristic profile (embossing). This causes narrow flow cross-sections to be established in the spaces, in which significant turbulences occur. The turbulent flow facilitates efficient heat transfer and also has a self-cleaning effect. The wall thicknesses of the heat transfer areas are generally smaller than in tubular heat exchangers.

Fig. 3.20 WL 110.02, enlarged

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Fig. 3.22 explains the individual steps required to connect the WL 110.02 to the service unit: 1. Secure the base plate of the plate heat

exchanger on the base plate (1) of the service unit using the star grip bolts (8).

2. Plug the couplings (7) for hot water into the cor-responding connections on the plate heat exchanger to give a flow in the direction of the arrow (see Fig. 3.23).

3. Plug the couplings (7) for cold water into the corresponding connections on the plate heat exchanger. Ensure that the required flow is produced (parallel flow or counter flow).

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3.7.3 General information for plate heat exchanger

Advantages of plate heat exchangers: • Outstanding heat transfer.

• Compact design.

• Little space required relative to the heat trans-fer area.

Soldered plate heat exchangers are wide-spread in refrigeration engineering and building services.

The disadvantage of the soldered design is that it cannot be opened. If this kind of plate heat exchanger is blocked by deposits, residue or foreign bodies, it has to be replaced.

The soldered design is a version developed from the widely used sealed plate heat exchangers. On the sealed design, the plate package, con-sisting of the plates and seals, is pressed together with clamp bolts. The adjacent figures show this kind of plate heat exchanger, as a photo and schematically.

Key advantages of sealed plate heat exchangers: • Opening and cleaning possible.

• Large heat transfer areas can be achieved (several 1000m² per unit).

• The heat transfer area can be varied by chang-ing the number of plates.

Fig. 3.24 Sealed plate heat exchanger

Fig. 3.25 Plate with seal

Fig. 3.26 Sealed plate heat exchanger, schematic

Platten

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Example applications of plate heat exchangers include:

• Chemical plants • Petrochemicals • Food industry

• HVAC (heating, ventilation and air condition-ing)

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3.8 WL 110.03 Shell & Tube Heat Exchanger unit description 3.8.1 Layout and function

The adjacent photos show the WL110.03 Shell & Tube Heat Exchanger.

Fig. 3.27 shows the shell and tube heat exchanger with the base plate. Fig. 3.28 shows an enlarged view.

The transparent shell allows the tube bundle to be seen. The tube bundle (shown in Fig. 3.29) is an assembly consisting of parallel tubes (seven tubes in this case). These seven tubes are sol-dered to the tube plates on both sides. This cre-ates two separate areas, the tube area (inside the tubes) and the shell area (between the tubes and the outer shell).

The shell area is divided by four baffle plates. They deflect the fluid in the shell area, thus improving the heat exchange. The flow in the shell area is essentially perpendicular to the tubes, i.e. the directions of flow cross (cross current flow). Fig. 3.30 illustrates the principle (illustrated with more than seven tubes). As determined by the two different coupling designs (7), hot water (red) flows through the tube area and cold water (shown in blue) through the shell area.

Viewed axially, the flows can run in the same direction or in opposite directions. We therefore differentiate between cross parallel flow and cross counter flow.

Fig. 3.28 WL 110.03, enlarged

Fig. 3.29 WL 110.03, tube bundle

Fig. 3.30 WL 110.03, schematic, cross counter flow

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Fig. 3.31 explains the individual steps required to connect the WL 110.03 to the service unit: 1. Secure the base plate of the shell and tube

heat exchanger on the base plate (1) of the service unit using the star grip bolts (8). 2. Plug the couplings (7) for hot water into the

cor-responding connections on the shell and tube heat exchanger. Connect the hot water feed to the lower connection to support bleeding. 3. Plug the couplings (7) for cold water into the

corresponding connections on the shell and tube heat exchanger. Ensure that the required flow is produced (cross parallel flow or cross counter flow).

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3.8.3 General information for shell and tube heat exchanger

Advantages of shell and tube heat exchangers: • Excellent heat transfer.

• Compact design.

• Comparatively little space required relative to the heat transfer area.

• On many designs, the tube bundle can be removed from the shell for cleaning and main-tenance.

Fig. 3.32 shows two tube bundles with tubes, baffle plates and tube plates.

Shell and tube heat exchangers are widely used. The wide range of designs means that: • Wide permitted temperature ranges can be

achieved, if almost unrestricted expansion of the tube bundle is possible.

• Adaptations are possible for processes with a change of phase (evaporation, conden-sation).

• Wide variety of material combinations can be used, depending on temperatures, pressures and fluid properties (corrosion etc.).

Example applications of shell and tube heat exchangers include:

• Chemical plants • Petrochemicals • Food industry • Power stations

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3.9 WL 110.04 Jacketed Vessel with Stirrer and Coil unit description 3.9.1 Layout and function

In many process engineering applications, several basic operations are combined, for example a fluid is heated by another fluid while being stirred, with a chemical reaction taking place at the same time. Such processes frequently take place in tanks.

Depending on the specific perspective, the corre-sponding tanks can have various designations, including agitating vessels, chemical reactors or heated reaction tanks. The process can generally be carried out in batches or continuously. The WL 110.04 Jacketed Vessel with Stirrer and Coil is a model of this type of tank. It focuses on investigation of heat transfer.

On the WL 110.04, heat transfer can occur through the wall of the tank. To allow this, the tank has a double jacket and the outer jacket is insu-lated. As an alternative to the double jacket, an internal heating coil can be used to transfer the heat.

The installed stirrer improves the heat transfer. Fig. 3.33 shows the jacketed heat exchanger with base plate. After loosening the three knurled screws (41), the transparent cover can be removed. Fig. 3.34 shows the cover with the stir-rer attached to it. Next to the stirstir-rer is the immer-sion sleeve with temperature sensor for measur-ing the water temperature in the tank.

Fig. 3.34 WL 110.04, cover

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Fig. 3.35, Page 35 shows an enlarged view of the propeller of the stirrer.

The top view of the open jacketed heat exchanger is shown in Fig. 3.36. As well as the heating coil (52) the flow breakers (51) attached to the tank wall can clearly be seen. They ensure a good stirring effect by preventing rotation of the liquid in the tank.

The tank is filled with cold water before an exper-iment. This cold water in the tank is then heated by hot water.

The schematic view in Fig. 3.37 illustrates the experiment options:

• Heating with hot water flowing through the heating jacket (a)

• Heating with hot water flowing through the heating coil (b)

As determined by the two different coupling designs (7), only hot water (red) can be connected to the jacket and heating coil.

The WL 110.04 is mainly suitable for batch experiments.

With practice, continuous operation can also be achieved. In this case, heated water flows out of the tank while fresh cold water is simultaneously fed in.

However, the level in the tank can only be roughly adjusted when doing this. Therefore, reproducibil-ity of the experiments is limited with continuous operation.

Fig. 3.36 WL 110.04, top view, cover removed

Fig. 3.37 WL 110.04, flow, schematic

52 51 51

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Fig. 3.38 explains the water connections for the WL 110.04. The cold water return (E) can be iden-tified by the ball valve (V4).

The tank can be drained by opening the ball valve V4.

The tank can be filled with cold water using the cold water feed (B).

Fig. 3.38 WL 110.04, water connections A Hot water heating jacket B Cold water feed

C Hot water heating coil D Hot water heating coil E Cold water return V4 Ball valve for draining F Hot water heating jacket

F C D B E V4 A

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The figures below explain the individual steps required to connect the WL 110.04 to the service unit:

1. Secure the base plate of the jacketed heat exchanger on the base plate (1) of the service unit using the star grip bolts (8).

2. Connect the plug for the stirrer cable to the con-necting socket on the service unit (item 36 in Fig. 3.12, Page 19).

3. Connect the measuring lead for the water tem-perature (TI5) in the tank to the corresponding socket on the service unit (item 38 in Fig. 3.12, Page 19).

4. Plug the couplings (7) for cold water into the corresponding connections on the jacketed heat exchanger as shown in Fig. 3.38, Page 37.

5. Plug the couplings (7) for hot water into the corresponding connections on the jacketed heat exchanger, see also Fig. 3.38, Page 37. Fig. 3.40, Page 39 shows the WL 110.04 connected for option (a) from Fig. 3.37, Page 36, heating with hot water flowing through the heating jacket. The hot water feed is connected to the lower connection to support bleeding.

Fig. 3.41, Page 39 shows the WL 110.04 connected for option (b) from Fig. 3.37, Page 36, heating with hot water flowing through the heating coil.

Fig. 3.39 WL 110.04, attachment and electrical connection

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3.9.3 General information for jacketed heat exchanger

The adjacent photo shows a jacketed heat exchanger for production.

Advantages of jacketed heat exchangers:

• Product temperature precisely adjustable with even temperature distribution.

• Easy cleaning of the interior of the tank

Jacketed heat exchangers are widely used for batch processes

Example applications of jacketed heat exchang-ers include:

• Chemical plants • Food industry

Fig. 3.40 WL 110.04, operation with heating jacket

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3.10 Commissioning

• Observe the safety instructions (see Chapter 2, Page 5).

• Install the data acquisition program on the PC (see Chapter 3.5.1, Page 22 onwards).

• Connect the service unit to the PC using the cable supplied (USB port, see item 11 in Fig. 3.4, Page 13).

• Connect the service unit to the mains.

• Connect the hoses to the connecting block (2) (see Fig. 3.8, Page 16).

• Set the main switch (item 35 in Fig. 3.12, Page 19) to „1“.

• Fill the hot water tank (B) with water (see Fig. 3.6, Page 15).

• Connect one of the available heat exchangers (see also Chapter 3.6 to Chapter 3.9).

• Open the cold water feed at the cold water mains. Fully open the regulator valve for cold water V2 (10).

• Start the pump (P).

If the pump does not start running, stop it immediately. Continue from section 3.11 on page 41 onwards.

• Repair any leaks. • Turn on the heater (H).

• Operate the experimental unit for a few min-utes. During this time, practice using the data acquisition program.

• Turn off the heater (H). • Stop the pump (P).

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• Close the cold water feed at the cold water mains. Close the regulator valve V2 (10). • Set the main switch (item 35 in Fig. 3.12,

Page 19) to „0“.

3.11 Hot water pump does not start?

This section can be skipped if the hot water pump (P) starts up with no problems.

Experience has shown that long stoppages make it more difficult to start up the hot water pump (P). It is possible that the pump will not start up at the first attempt.

Starting the pump repeatedly does not resolve the problem. On the contrary, it can actually cause the electric fuse to trip.

The following procedure is recommended:

1. Rinse the pump with cold water.

Using the admission pressure of the cold water feed is recommended. Reversing the flow direction through the pump enables the cold water to be fed into the hot water tank (B).

This circuit is created by connecting the couplings for the cold water feed and the hot water feed (see adjacent photo).

Fig. 3.44, Page 42 shows a schematic view of this circuit, based on the process schematic in Fig.

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Method:

First ensure that the pump is not actuated (set the switch (33) in Fig. 3.12, Page 19 to "0" position). After connecting the couplings, completely open regulating valves V1 and V2. Open the cold water supply. As soon as water is flowing, allow the level in the tank (B) to rise by several litres. Then shut off the cold water supply and disconnect the couplings. Reconnect the original heat exchanger and continue with commissioning or the experi-ment.

If the pump still does not start up, continue with 2. below.

Fig. 3.44 Rinse the pump (P) with cold water, schematic circuit

TI 1 FI 1 TI 4 FI 2

H

TI 7 LSL 1 TIC

B

V2 V1

P

B

Kaltwasser / Cold Water

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2. Turn motor shaft with screwdriver.

First ensure again that the pump is not actuated (set the switch (33) in Fig. 3.12, Page 19 to "0" position). Disconnect the WL 110 service unit from the mains electricity.

The adjacent figure repeats the photo from Fig. 3.7, Page 15. It shows the pump in the right-hand half of the housing, with the rear panel removed. The front of the motor shaft has a slotted design and is intended to be accessible.

However, the carrying handle on the right-hand half of the housing next to the pump must be removed first. The motor shaft can then be turned with a screwdriver.

The series of photos below illustrates the procedure.

Fig. 3.45 Right housing section, rear panel removed

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3.12 Shutting down

• Observe the safety instructions (see Chapter 2, Page 5).

• Disconnect the service unit from the mains electricity supply.

• Remove the USB cable from the USB port on the service unit.

• Drain the hot water tank (B). This is done by opening the ball valve V3.

• Open the regulator valve V2 (10).

• Uncouple the cold water feed and return hoses. • Store the trainer covered, in a clean, dry and

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The basic principles set out in the following make no claim to completeness. For further theoretical explanations, refer to the specialist literature.

4.1 Heat transfer

We differentiate between direct heat transfer and indirect heat transfer.

An example of direct heat transfer is introducing hot steam into water to rapidly heat the water stored in a tank. The hot steam condenses in the liquid water and gives up its condensation heat to the content of the tank.

Direct heat transfer can only be used if the heat carrier introduced does not interfere with the com-position and concentration of the tank filling. In indirect heat transfer the heat is transferred from one fluid to another through a partition in a heat exchanger.

The fluid flows on the two sides of the partition do not mix.

In terms of the flow directions of the fluids on both sides of the partition, we differentiate between parallel flow, counter flow and cross flow. In other words, the fluids either flow in the same direction, in opposing directions or perpen-dicular to one another.

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4.2 Indirect heat transfer

Whilst the hot fluid is flowing along the partition, it is cooled and gives up heat to the partition. In turn, the heated partition gives up heat to the cold medium flowing along the other side of the parti-tion. This heats the cold fluid.

Heat transfer at the partition can be sub-divided into three separate processes.

1. The hot fluid gives up heat to the partition. 2. The partition conducts heat from the hot

surface to the cold surface.

3. The partition gives up the heat to the cold fluid. The temperature curve at the partition is shown schematically in Fig. 4.1. Each of the three heat transfer processes is assigned a temperature difference ( , and ).

Note: Below, the variables for the hot side are indicated by the suffix h and those for the cold side with the suffix c. The suffix p represents the partition, while the suffixes in and out indicate the inlet and outlet.

The efficiency of a heat exchanger is defined by the quality of the transfer of heat during the three heat transfer processes.

Fig. 4.1 Temperature curve at the partition T_h T_p,h T_p,c T_c ΔT_h s ΔT_p ΔT_c ΔT

Hot fluid Partition Cold fluid

Travel

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4.2.1 Heat transfer from fluid-partition

The ability to transfer heat from one fluid to the partition or vice versa is described by the coeffi-cient of heat transfer .

(4.1)

The formula defines the amount of heat Q trans-ferred in the time t. As well as the coefficient of heat transfer and the partition area A, the tem-perature difference between the fluid and par-tition temperatures is a crucial factor in the heat transfer.

In general the heat flow is of interest, i.e. the amount of heat per unit time that a heat exchanger transfers. The heat flow is specified using a power unit, e.g. kW or kJ/s.

For heat flow the equation is generally:

(4.2)

In the specific case of the hot side of the partition with hot fluid (suffix h) or the cold side with cold fluid (suffix c): (4.3) where (4.4) and (4.5) α Q = _α⋅ ⋅A _ΔT t⋅ α ΔT Q· Q· = _α_{⋅ ⋅}A _ΔT Q· = _α_h⋅ ⋅A _ΔT_h ΔT_h = T_h–T_{p h}_, Q· = _α_c⋅ ⋅A _ΔT_c

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4.2.2 Thermal conduction in the partition

Within the partition the heat is transferred from the hot side to the cold side by thermal conduction. Here the following relationship applies:

(4.7)

where (4.8)

Here is the thermal conductivity of the parti-tion material and s is the wall thickness of the partition.

4.2.3 Heat transmission

Because the three heat flows are of equal magni-tude in a steady state:

(4.9)

or, summarised at the mean coefficient of heat transfer k_m of the heat exchanger:

(4.10)

With the heat flow

(4.11) Q· λ s --- A⋅ ⋅_ΔT_p = ΔT_p = T_{p h}_, –T_{p c}_, λ Q· = _α_h⋅ ⋅A _ΔT_h = λ--- A_s⋅ ⋅_ΔT_p = αc⋅ ⋅A ΔTc k_m 1 1 αh --- s λ --- 1 αc ---+ ---+ ---= Q· Q· = k_m⋅A_m⋅_ΔT_lm

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1. As the temperatures along the partition are not constant, a mean temperature difference must be used for calculations. The temperature curve is non-linear, which means that rather than the arithmetic mean, the logarithmic mean temperature difference must be used.

(4.12)

and relate to the temperature difference between the fluids, each at one point in the heat exchanger. Further information follows in Chapter 4.4, Page 53.

2. The surfaces on the cold and hot sides are not generally of equal size. For example, in a tubu-lar heat exchanger, the inner surface of the tube is smaller than the outer surface, which means that in this case a mean area A_m should be used.

(4.13)

3. ln defines the natural logarithm for the base e = 2,71828.

The mean coefficient of heat transfer k_m char-acterises the heat exchanger. It can be used to compare different heat exchangers with one

ΔT_lm ΔT_lm ΔTmax–ΔTmin ΔT_max ΔT_min ---    ln ---= ΔT_max _ΔT_min A_m Ah–Ac A_h A_c ---    ln ---=

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When comparing coefficients of heat transfer from different sources, we recommend paying atten-tion to the reference. In many cases, for heat exchange tubes they do not refer to the mean area A_m but, due to the reduced calculation required, to either the inner or outer surface of the tube.

However, all figures for the WL 110 series refer to

A_m to allow a fair comparison between the WL 110.02 plate heat exchanger on the one hand and the WL 110.01 tubular heat exchanger and WL 110.03 shell & tube heat exchanger on the other.

4.2.4 Analogy to fluid dynamics and electrics

The inverse of the mean coefficient of heat transfer k_m is known as the heat transition coef-ficient.

Rearranging Formula (4.10), Page 48 gives the heat transition coefficient:

(4.14)

The term s / is known as the thermal conduc-tivity resistance. The quotients 1/ and 1/ are also known as the heat transfer resistance. The heat transmission can thus be understood as series connection of the three individual resistances. As in fluid mechanics and electrics, the total resistance here is given by the sum of the individual resistance values.

1 k_m --- 1 αh --- s λ --- 1 αc ---+ ---+ = λ αh αc

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4.3 Heat flow through the heat exchanger

Fig. 4.2 shows a schematic view of the energy and heat flow in a heat exchanger (losses are not indicated).

The transferred heat flow is calculated from the difference between the input and output heat flows ( - ). In an ideal heat exchanger with-out losses it is not relevant whether the hot or cold medium is used for the calculation (see Fig. 4.2). Generally the heat flow is determined from the mass flow rate , the specific heat capacity c_p and the absolute temperature T :

Fig. 4.2 Energy flow in heat exchanger (loss free) Hot fluid

= Transferred heat flow

Q·c,out Q·c,in Q·h,out Q· Cold fluid Q·h,in Q· Q·in Q·out m·

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The transferred heat flow is thus:

(4.16)

for the hot fluid, and

(4.17)

for the cold fluid.

With no exchange of heat with the surroundings:

(4.18)

If the heat flow figures differ, the mean value is calculated.

(4.19)

This enables the mean coefficient of heat trans-fer k_m for the heat exchanger to be calculated:

(4.20)

(4.21)

where (4.22)

and (4.23)

Q·h = Q·h,out–Q·h,in = c_p,h⋅m·h⋅(Th,out–Th,in)

Q·c = Q·c,out–Q·c,in = cp,c⋅m·c⋅(Tc,out–Tc,in)

Q· = –Q·h = Q·c Q·m Q·m –Q·h + Q·c 2 --- Q·c–Q·h 2 ---= = k_m Q·m A_m⋅_ΔT_lm ---=

k_m cp,c⋅m·c⋅(Tc,out–Tc,in)–cp,h⋅m·h⋅(Th,out–Th,in)

2⋅A_m⋅ΔT_lm

---=

m·_h = _ρ_h⋅V·h m·_c = _ρ_c⋅V·c

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If we plot the fluid temperatures in the heat exchanger in a combined graph against the travel

x we obtain the temperature curve. The travel x

runs along the heat transfer surface from the fluid inlet to the outlet.

The adjacent two figures show example tempera-ture curves for a tubular heat exchanger with parallel flow (Fig. 4.3) and counter flow (Fig. 4.4). The temperatures are normally exponential rather than linear.

This is clearly illustrated by the parallel flow exam-ple (see Fig. 4.3). The temperature difference is at its maximum when the fluids enter the heat exchanger (x =0) and at its minimum at the outlet. With the maximum temperature difference, a large heat flow can be transferred, i.e. the temper-atures change quickly. As the temperature differ-ence is reduced, the temperatures change more slowly.

With parallel flow, the outlet temperature always remains lower than .

By contrast, with counter flow the outlet tempera-ture of the heated fluid can be higher than the outlet temperature of the cooled fluid.

Fig. 4.3 Temperature curve for parallel flow

Fig. 4.4 Temperature curve for counter flow Hot fluid T_c,out T_c,in T_h,in T_h,out Hot Cold fluid Cold Travel x Hot fluid T_c,in T_c,out T_h,in T_h,out Hot Cold fluid Cold Travel x T_c,out T_h,out T_c,out T_h,out

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In Chapter 4.2.3, Page 48 onwards, the

logarith-mic mean temperature difference was

used to calculate the temperature differences and . The following equations explain these temperature differences:

For parallel flow:

(4.24) (4.25)

For counter flow, as shown by the example in Fig. 4.4, Page 53, the equations are:

(4.26) (4.27)

At this point, it is important to mention that for counter flow there are also temperature curves in which the difference increases along the travel x.

ΔT_lm

ΔT_max _ΔT_min

ΔT_max = T_h,in–T_c,in

ΔT_min = T_h,out–T_c,out

ΔT_max = T_h,in–T_c,out

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The selection of experiments makes no claims of completeness but is intended to be used as a stimulus for your own experiments.

The results shown are intended as a guide only. Depending on the construction of the individual components, experimental skills and environmen-tal conditions, deviations may occur in the experi-ments. Nevertheless, the laws can be clearly demonstrated.

Note: In the experiments performed by G.U.N.T., the temperature of the incoming cold water T₄ was around 15°C. Different temperatures T₄ lead to changed measured values.

Note on the water quality of the water used: Cold water from the cold water mains is required for cooling. The hot water tank is normally filled with tap water.

We recommend avoiding the use of water with a high water hardness (>5°dH). The lower the water hardness, the fewer deposits form in the experi-mental units.

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This experiment chapter is split into two sections because the WL 110.04 Shell & Tube Heat Exchanger has a different design to the other three heat exchangers.

As explained in Chapter 3.9, Page 35 onwards, the WL 110.04 is primarily intended for batch operation, in contrast to the other three heat exchangers. In addition, only the WL 110.04 has a stirrer. Therefore, there are also different exper-iment aims, methods of performing experexper-iments, measured values and evaluations.

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5.1 Experiments with WL 110.01, WL 110.02 and WL 110.03

The experiments described here relate to the fol-lowing three heat exchangers from the WL 110 series:

• WL 110.03 Shell and Tube Heat Exchanger

5.1.1 Experiment aims

1. Comparison of parallel flow and counter flow operation. Heat transmission and representa-tion of temperature curves.

2. Investigation of heat transmission when chang-ing the cold water and hot water flow rates. 3. Investigation of heat transmission when

chang-ing the hot water temperature.

4. Comparison of heat transmission for the differ-ent heat exchanger types.

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5.1.2 Experiment series, general conditions

The WL 110 series offers a wide range of options for carrying out experiments under widely varying conditions.

A series of experiments is set out below, which can be used to investigate the experiment aims described in Chapter 5.1.1.

This series of experiments includes experiments V1-01 to V10-03. The first number in this designa-tion indicates the experiments, while the second number indicates the heat exchanger used.

Tab. 5.1 summarises the selected general condi-tions. Changed general conditions are shown with yellow shading.

The designation SP(T₇) indicates the setpoint (SP) for the temperature T₇ of the hot water in the tank (B).

Experiment HE Flow direction

SP(T₇)

Key: ltr/min °C

HE: Heat exchanger V1-01 01 PF 0,7 70

PF: Parallel flow V2-01 01 PF 1,4 70

CF: Counter flow V3-01 01 PF 2,1 70

: Cold water flow rate V4-01 01 CF 1,4 70

: Hot water flow rate V5-01 01 CF 1,4 45

SP(T₇): Setpoint for T₇ V6-01 01 CF 1,4 20

V7-02 02 PF 1,4 70

V8-02 02 CF 1,4 70

V9-03 03 PF 1,4 70

V10-03 03 CF 1,4 70

Tab. 5.1 Parameters for experiments V1-01 to V10-03

V·_c,V·_h

V·_c V·_h

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Connected WL 110 Heat Exchanger Service Unit, commissioning carried out as described in Chapter 3.10, Page 40, in conjunction with the associated heat exchangers.

5.1.4 Performing the experiment

The parameters to be set are set out in Tab. 5.1, Page 58.

1. Observe the safety instructions (see Chapter 2, Page 5).

2. Secure the selected heat exchanger on the base plate of the service unit as described in Chapter 3.6, Page 25 to Chapter 3.8 and connect.

3. Set the main switch (item 35 in Fig. 3.12, Page 19) to „1“.

4. Check the water level in the hot water tank (B) (see Fig. 3.6, Page 15).

– If the hot water tank (B) is empty: Add water until the low level is reached (level switch LSL1 trips and the low water

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warn-– If the hot water tank (B) is filled but with an unknown volume above the low level: Partially drain the hot water tank (B) (see Fig. 3.8, Page 16) until the low level is reached (level switch LSL1 trips and the low water warning lamp lights up). Then add 0,5ltr of water with a beaker.

5. Start the PC. Start the data acquisition pro-gram.

6. Open the cold water feed at the cold water mains.

7. Open the regulator valve for cold water V2 (10).

8. Open the regulator valve for hot water V1 (9). 9. Start the pump (P).

10. If required, bleed the heat exchanger (Detach the heat exchanger base plate. Turn the heat exchanger with a through flow such that the air can escape upwards. Re-attach the base plate).

11. Set the desired hot water setpoint SP(T₇) on the TIC7 controller (28) (see also Fig. 3.13, Page 21).

12. If the temperature T₇ of the hot water in the tank (B) is higher than the setpoint SP(T₇): Cool the hot water circuit until T₇ < SP(T₇). 13. Set the desired cold water flow rate using

the regulator valve V2 (10).

14. Set the desired hot water flow rate using the regulator valve V1 (9).

V·c V·h

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15. Turn on the heater (H).

16. Make settings for the measured value file. Start automatic measured value recording. 17. Observe the measured values. Wait until a

steady state is reached, i.e.:

– The water temperature T₇ is no longer ris-ing.

– The parts in contact with the product have taken on the water temperatures.

– The measured values only change slightly.

– The heat flow values and are sim-ilar.

18. Save screenshots for the time response of the measured values and the current tem-perature curve in a file.

Give the file a name that will allow you to identify the values in the measured value file later.

19. When the experiment is complete, first turn off the heater (H).

20. Then stop the pump (P).

21. Close the regulator valves V1 (9) and V2 (10).

22. If a further experiment is to be performed with a different heat exchanger, continue from step „2“.

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23. If a further experiment is to be performed with the same heat exchanger, compare the current water temperature T₇ with the new setpoint SP(T₇).

– If the hot water has a significantly higher temperature level, drain the hot water tank (B). Continue the experiment from step „2“.

– If the hot water temperature level is simi-lar or lower, continue the experiment from step „7“.

24. When the last experiment is complete, stop recording and save the measured value file. 25. Close the cold water feed at the cold water

mains.

26. Set the main switch (35 in Fig. 3.12, Page 19) to “0”.

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The data acquisition program saves the ured values in measured value files. This meas-ured value file contains a chronological sequence of measured data records. A measured data record is a snapshot of all the measured values at a given point in time.

An interval of 1s was selected (representing the time between recording of two measured data records). A recording duration of 60min results in 61200 measured values (each measured data record contains 17 measured values).

A tabular representation of the complete meas-ured value file, with all measmeas-ured data records, would be too extensive to set out at this point. Therefore, just a selection will be presented.

The table below supplements the data from Tab. 5.1, Page 58. In addition to the parameters for the experiment series, the following measured values and calculated values are set out here: • Hot water feed temperature T₁

• Hot water return temperature T₃ • Cold water feed temperature T₄ • Cold water return temperature T₆ • Mean coefficient of heat transfer k_m • Mean heat flow Q·m

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The hot water feed temperature T₁ is included here so that the actual hot water temperature can be incorporated into the evaluation.

The cold water feed temperature T₄ helps in com-paring the results from your own experiments. For experiment aim 3, the temperatures T₃ and T₆ are also required.

Experi-ment HE Flow direction SP(T₇ T₁ T₃ T₄ T₆ k_m ltr/min °C °C °C °C kW/(m²K) kW V1-01 01 PF 0,7 70 67,0 51,1 15,6 31,5 0,93 0,76 V2-01 01 PF 1,4 70 67,8 54,7 15,6 29,5 1,43 1,32 V3-01 01 PF 2,1 70 67,0 55,6 15,1 27,9 1,83 1,75 V4-01 01 CF 1,4 70 67,1 54,4 15,3 29,5 1,37 1,31 V5-01 01 CF 1,4 45 43,7 37,4 15,1 22,9 1,30 0,69 V6-01 01 CF 1,4 20 20,6 19,8 15,0 17,5 1,68 0,16 V7-02 02 PF 1,4 70 65,2 43,1 14,3 37,5 2,25 2,22 V8-02 02 CF 1,4 70 61,2 36,2 15,2 41,7 2,58 2,50 V9-03 03 PF 1,4 70 67,3 57,4 15,2 26,5 1,27 1,03 V10-03 03 CF 1,4 70 67,8 57,1 15,0 26,7 1,30 1,08

Tab. 5.2 Parameters for experiments V1-01 to V10-03, with measured and calculated values added

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5.1.6 Analysis, comments and evaluation

Experiment aim 1,

comparison of parallel flow and counter flow operation. Heat transmission and representa-tion of temperature curves.

The comparison is made using the example of the WL 110.02 Plate Heat Exchanger. Experiments V7-02 (parallel flow) and V8-02 (counter flow) are included.

The following temperature curves are produced using the data acquisition program. Due to the limited number of measuring points, the links between the feed and return temperatures are shown as simplified straight lines here.

In the figures, the designation of the water tem-peratures is enlarged.

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Fig. 5.1 Temperature curve for experiment V7-02, WL 110.02, parallel flow

Fig. 5.2 Temperature curve for experiment V8-02, WL 110.02, counter flow

T₁ T₄ T₃ T₆ T₁ T₆ T₃ T₄

WL110e V1.1 Duplex

WL 110-SERIES Heat Exchanger with

Service Unit

Experiment Instructions

Table of Contents

W

H

B

P

H

B

P

B