1 Prospect Street, Cross HillsKeighley, BD20 7RH

United Kingdom

Tel: +44 (0)1535 287032E-mail: info@wave-refrigeration.com

Pipe Network Development System

For Refrigeration Plants

MicroPiPe

INDEX

03

03

05

07

10

10

10

10

12

14

16

18

25

29

32

39

Introduction

Customer Feedback

System Operating Modes

Design and System Data Entries

Index Circuit Mode

Network System Mode

Dual Circuit

Header Circuit

Autocad Text Files

Suction Risers

Bias Settings

Toolbar Controls

Worked Example

Designing for Efficient Oil Return

Design Guide

Example Inputs

2

INTRODUCTION&

CUSTOMER FEEDBACK

3

MicroPipe is a fully featured and comprehensive software system for the analysis and development of refrigeration system pipe-networks – with the ability to operate in SI or Imperial units.

Calculations can be carried out on systems with pipe sizes up to 6-1/8” copper and 20-inch steel tube.

Dry and wet suction, high-pressure liquid, discharge and pumped low temperature liquid pipe networks can all be calculated and analysed.

The selection of the correct gauge of copper tube is carried out automatically as is the summation of pipe lengths, system volume and refrigerant contents.

Full details of BS EN378 system design pressure requirements are built-in, with a number of tools available to check the available pipe gauges and maximum working pressures of all copper pipe sizes.

Condenser drain lines can be easily selected using the condenser drain function Window.

These are just a few of the many features available, which we are sure you will find of great value.

MicroPipe is intended for refrigeration design engineers who have a full appreciation of refrigeration system design and the implications that different operating parameters have on the pipe network design and, consequently, on system performance.

We are always pleased to give advice on the use of MicroPipe or on matters related to pipe network development – please do not hesitate to get in touch with us!

We greatly value your feedback on this product and encourage you to comment freely on any aspect of the software. Many enhancements have been implemented over the years as a result of this feedback process. Even the simplest of suggestions can result in a significant improvement or the addition of a useful function.

Please respond to this request by any means available to you. Without exception, we will reply to all communications.

INTRODUCTION

CUSTOMER FEEDBACK

4

SYSTEM OPERATING MODES

5

For the design and analysis of pumped wet suction, pipe networks between the evaporators and low-pressure receiver.

For the design and analysis of dry suction, pipe networks between the evaporators and compressors.

For the design and analysis of liquid, pipe networks between the liquid receiver and evaporators.

For the design and analysis of discharge, pipe networks between the compressors and condenser system.

For selecting the drain line sizes between the condenser outlets and the liquid receiver.

For the design and analysis of the pumped liquid, pipe networks between the low-pressure receiver and the evaporators.

SYSTEM OPERATING MODES

Dry Suction

Wet Suction

Liquid

Discharge

Condenser Liquid Drain

Pumped Liquid

6

DESIGN AND SYSTEM DATA ENTRIES

7

The refrigerant temperature in the evaporator (not saturated suction temperature at the compressor).

The suction line temperature rise, above saturation temperature, between the evaporator and compressor, expressed in Kelvin (K).

The highest refrigerant liquid temperature likely to be encountered during maximum design ambient temperature conditions.

The ‘Maximum-liquid-temperature’ data entry is used only for suction networks to differentiate between minimum and maximum liquid temperature, for the selection of risers.

The minimum liquid temperature likely to be encountered during minimum design ambient temperature conditions.

Many systems are now designed to permit the condensing temperature to reduce following prevailing ambient temperatures. This significantly increases the efficiency of the plant but can cause problems with oil return due to the reduction in system refrigerant mass-flow. It is, therefore, essential that a realistic minimum liquid temperature is entered to ensure MicroPipe will select the correct suction risers for these operating conditions.

In addition to operating with low liquid temperatures, careful consideration must be given to the possibility of the system also running at low load conditions for prolonged periods of time during winter. Again, oil return during these periods is greatly reduced due to the lower refrigerant mass flow rates. It is, therefore, advisable to enter a realistic estimate of minimum riser capacity to ensure that MicroPipe will select suction risers that are sized for the lowest capacity and liquid temperature likely to be experienced.

MicroPipe will select suction risers based on the minimum liquid temperature for oil return and the maximum liquid temperature for penalty.

The maximum pressure drop or temperature penalty that can be tolerated commensurate with good practice, system operating efficiency and economy of installation.

A value representing the height difference between the liquid refrigerant receiver and the evaporator inlet valve.

Entering a value causes MicroPipe calculate the pressure loss, due to the effects of gravity, and the amount of liquid subcooling necessary to prevent flash-gas formation in the liquid lines.

DESIGN AND SYSTEM DATA ENTRIES

Evaporating Temperature

Suction Superheat

Maximum Liquid Temperature or Liquid Temperature

Minimum Liquid Temperature

Maximum Permitted Penalty or Pressure Drop

Liquid Rise Above Liquid Receiver

8

If this field is omitted, MicroPipe will automatically calculate the discharge temperature based on the system design parameters and built-in isentropic efficiency.

If a temperature entry is made, MicroPipe will calculate the refrigerant properties based on the entered value.

The default value for isentropic efficiency is 75% and can be changed via the options menu.

The quantity of liquid-overfeed leaving the evaporator expressed as a percentage of the capacity.

EG: 300% liquid overfeed means that each pipeline will be required to carry 100% for the system capacity plus a further 300% for liquid over-feed.

In pumped liquid lines all of the refrigerant mass-flow rate would be liquid whereas in wet suction lines 25% of the mass-flow rate would be vapour and 75% liquid.

The vertical distance between the refrigerant pump and the evaporator inlet.

From this data entry, MicroPipe will calculate the additional pumping head due to the effects of gravity.

Each circuit can be uniquely identified with a label of up to 8 alpha/numeric characters.

The circuit label is completely independent of the MicroPipe numbering system.

Giving each circuit a unique identification label allows revisions to the system to be carried out easily. The addition and subtraction of circuits will not cause any changes to the system overall as MicroPipe always uses the circuit labels when sorting the flow-paths. The only changes required after removing or adding circuits are the ‘next flow-path circuit label’ entries.

To enable MicroPipe to sort the refrigerant flow-paths and circuiting arrangements of the pipe network; it requires details of the system interconnections and junction points. By entering the ‘next flow-path circuit label’ MicroPipe is able to establish the flow-paths to/from each terminal and, from this information, calculate the duties of the interconnecting pipe network.

Occasionally, a special valve or fitting will be required to be fitted in the system. If the component pressure-loss is already known, the temperature-penalty can be found by using the pressure-loss to temperature-penalty conversion function. If only the pressure-loss-factor is known (not Kv value), it can be entered into MicroPipe where the temperature-penalty and pressure loss will be calculated automatically.

Discharge Temperature (Optional Entry)

Liquid Overfeed Percentage

Pumped Liquid Lift

Circuit Label

Next Net-Path Circuit Label

Pressure Loss Factors

9

INDEX CIRCUIT MODE, NETWORK SYSTEM MODE, DUAL CIRCUIT &

HEADER CIRCUIT

10

Index operating mode is used for a single circuit system or an index flow-path circuit consisting of a number of circuits, in series.

The circuit label and next-circuit label data input field is not used in the Index mode as MicroPipe assumes that the system of circuits starts at 1 of 1 as the smallest capacity with the next circuit, in the flow path, being larger or equal in capacity.

An error message will be given in the event that a circuit is smaller in capacity than a preceding circuit.

The Index system is limited to 50 circuits.

Network operating mode is for the design and analysis of entire pipe networks starting, in all instances, from the evaporators or compressors (terminals) to a single end-of-line circuit via the pipe network. (See example diagram)

Circuits can be entered in any order required. However, the circuit-labels and next-circuit-labels must be correctly entered to enable MicroPipe to establish the flow-paths.

Errors will be reported where the flow-path inputs are found to be incorrect.

In Network mode the Suction, Liquid and Pumped Liquid systems are limited to 200 circuits with any single flow-path up to 100 circuits in length. The Discharge system is limited to 50 circuits with any single flow-path up to 20 circuits in length.

This option instructs MicroPipe to select two pipes for parallel duty instead of one single pipe. Each would be sized for 50% of the required capacity.

This facility can be useful where the maximum available pipe size is not large enough for the required capacity or where the maximum working pressure of the single larger pipe is lower than the system design code requirement.

This function informs MicroPipe that you intend to make the selected circuit a header.

MicroPipe will permit multiple, identical next-circuit-labels to a nominated header circuit. Normally, only 2 identical next-circuit-labels are allowed for each circuit as it is not possible to have more than 3 connections on a TEE junction.The MicroPipe fitting selection and costing program requires accurate entry information regarding pipe junctions. Inappropriate fittings will be selected if the entry information is inaccurate.All of the junctions connected to the header will be automatically set to 90°.

INDEX CIRCUIT MODE

NETWORK SYSTEM MODE

DUAL CIRCUIT

HEADER CIRCUIT

11

AUTOCAD TEXT FILES

12

During the ‘Save File’ procedure, MicroPipe will create and save (if required) a text file containing a list of the circuit labels and pipe sizes. The system must be fully validated for this facility to be activated.

Using Microsoft Word, the text file can be loaded, copied and pasted into AUTOCAD where it will can be formed into a table of pipe sizes to match the relevant isometric drawing. (See the example drawing in the manual).

The text-file name-extensions for each mode are:

Dry Suction: FILENAME.STX

Wet Suction: FILENAME.WTX

Liquid: FILENAME.LTX

Discharge: FILENAME.DTX

Pumped Liquid: FILENAME.PTX

AUTOCAD TEXT FILES

13

SUCTION RISERS

14

15

Suction risers can be placed anywhere in the system by clicking the appropriate button on the toolbar.

SUCTION RISERS

Single suction risers can be used where the riser minimum capacity, for oil return, does not go below approximately 70% or where higher suction riser and system penalties can be tolerated.

The use of single risers, with high penalties, can prove to be expensive as this will cause MicroPipe to compensate for the higher penalties by increasing the pipe sizes of the rest of the system.

Double risers should always be used where the minimum capacity, for oil return, could be lower than 50% or where high system penalties, associated with single risers, cannot be tolerated.

Notes:

In addition to the minimum capacity requirement, it is also important for you to remember that the refrigerant minimum liquid temperature must be considered as this can, because of significant reductions in refrigerant mass-flow, seriously affect the oil return characteristics of the suction risers.

Systems that operate with low liquid temperatures, due to permitting condensing pressures to reduce following prevailing ambient conditions and/or low system loads, will have reduced oil return up risers which can lead to oil starvation of the compressor system.

Entering the correct minimum liquid temperature will ensure that MicroPipe will select suction risers that will return oil efficiently under all operating conditions.

For efficient oil return, MicroPipe will select suction risers sizes based on the minimum liquid temperatures and capacity but will calculate the penalty based on the maximum liquid temperature.

Frequently, system configuration and/or operating conditions can lead to the majority of the penalty being used up on the risers. When this occurs, MicroPipe will show a warning message. The solution is to permit a higher penalty.

All pipe-size selections are checked for their oil return capabilities relative to vertical riser capacities. Percentages are displayed in black for lines that will return oil or red if they will not. This feature is to remind users that some circuits should be treated as risers and not as horizontal circuits. It also serves to highlight circuits that require oil return slopes.

Single Risers

Double Risers

BIAS SETTINGS

16

17

BIAS SETTINGS

Increasing the bias value, above zero, will increase the amount of penalty that MicroPipe allocates for suction risers.

Care must be exercised in using this bias as the higher the proportion of the available penalty expended on the risers the larger will be the pipe-sizes of the rest of the system.

The default value is 0.

See chapter on risers.

Allows the user, based on experience or empirical data, to adjust the level of increase in penalty that is attributed to liquid-hold-up in wet suction lines.

For example, a value of 1.5 will increase the suction line penalty from zero for a liquid-overfeed ratio of 0% to a maximum of 1.5 for a liquid-overfeed ratio of 2,000%.

The default value is 1.5.

Riser Bias

Liquid-Hold-Up Maximum Penalty Bias

TOOLBAR CONTROLS

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19

TOOLBAR CONTROLS

Erases all of the pipe system data, but leaves the system design data.

For loading MicroPipe project files.

Select ‘Open DOS version file…’ from the menu for files created with the DOS version of MicroPipe.

For loading a complete project consisting of Suction, Liquid, Discharge and Pumped Liquid files. These files are assumed to have the same name EG: HT1.SUC, HT1.LIQ etc.

Loads and merges the selected project file starting immediately after the currently displayed circuit.

For saving MicroPipe project files.

Inserts a blank circuit immediately before the currently displayed circuit.

Inserts a blank circuit immediately after the currently displayed circuit.

Copies the currently displayed circuit and inserts it immediately before the current circuit.

Copies the currently displayed circuit and inserts it immediately after the current circuit.

Pipe-size selection menu for the calculation of the minimum oil-return carrying capacity of the selected pipe-size, at the entered system design conditions.

Pipe-size selection menu for manually checking the penalties and operating conditions of the selected pipe-size. Pipe-sizes can also be fixed when using this function.

This function also has a scroll bar to enable you to step through the system. There is also a ‘Window Lock’ control button to prevent the Window from automatically closing whenever a pipe-size is fixed.

New Project

Load Project File

Load Complete Project

Merge Project File

Save Project File

Insert Circuit

Insert Circuit

Copy and Insert Circuit

Copy and Insert Circuit

Oil Return Menu

Manual Calculations

TOOLBAR CONTROLS

Displays a summary of the entire system volume, pipe lengths, refrigerant charge and pipe-wall thickness gauges.

Maximum working pressures, for copper pipe, are displayed in green if the pressure rating of the pipe-wall thickness gauge is within the system design pressure rating or red if they are not.

The system must be run and validated for this facility to function.

Data entered here will be saved or printed with the project file.

Clicking the ‘Globalise References’ button will set the Suction, Liquid, Discharge and Pumped Liquid system references to those displayed in the current references Window.

This function is available with all systems.

Sets the labels of the pipe network using the entered increment value.

EG: For an increment value of 10, circuit 1 will be labeled 10, circuit 2 will be 20 and so on.

Circuit labels can be any combination of characters except trailing and leading spaces, which will be removed automatically.

Switches the currently displayed circuit from/to a dual pipe circuit.

See chapter on dual circuits for more information.

Switches the currently displayed circuit from/to a header circuit.

See chapter on header circuits for more information.

Unconditionally un-fixes the pipe-sizes of all fixed circuits throughout the entire system.

Switches the currently displayed circuit to the selected riser type.

Clicking the button again switches the selection off.

See chapter on suction risers for more information.

Pipe Network Summary

Project References

Label Increment Setting

Dual Circuit

Header Circuit

Unfix all Circuits

Double and Single Suction Risers

20

TOOLBAR CONTROLS

If the system has been run and validated, this function will fix the pipe-size to the size selected by MicroPipe.

If the system has not been validated a tone will sound and the function will not engage.

If the pipe-size is already fixed, the function will unfix the pipe-size.

If the system has been run and validated this function will fix the pipe-sizes of the entire system to the sizes selected by MicroPipe.

Individual circuit pipe-sizes can be selected and fixed using this function by pointing and double clicking the mouse button or by positioning the focus on the required pipe-size and clicking the “Accept” button or pressing the Enter key.

This function also has a scrollbar to enable you to step through the system. There is also a ‘Window Lock’ control button. This prevents the window from automatically closing whenever a pipe-size is fixed.

Checks and validates all data entries.

This function is also automatically engaged when the system calculations are started.

Checks and validates all data entries and starts the automatic pipe-sizing and system analysis calculations.

Sets the system to Network mode.

See chapter on Network mode.

Sets the system to Index mode.

See chapter on Index mode

Sets the minimum oil-return duty data entries to be a percentage of the circuit duty.

Sets the minimum oil-return duty data entries to kW.

Fix/Unfix the Current Circuit

Fix all Pipe-Sizes

Fix Pipe-Size, Selection Menu

Validate Data Entries

Start Analysis

Network Mode

Index Mode

Percentage Mode

kW Mode

21

TOOLBAR CONTROLS

In the Design Data Window this function will globalise the common design data entries of the other systems.

In the Pipe-System Window the function will globalise the pipe junction types (90° or 180°) to that of the currently displayed circuit.

Fixes or un-fixes the Mouse Pointer over data entry fields and pipe-size menus.

Globalise Design Data Entries or Junction Types

Mouse Pointer

Shows or Hides the Data Entry Windows

Displays the System Design Data Window

Displays the Suction System Design Data Entry Window

Transfers Circuit Data between Suction and Liquid

Displays the Liquid System between Design Data Window

Transfers Circuit Data between Suction and Pump Liquid

Displays the Pumped Liquid System Design Data Window

Displays the Discharge System Design Data Window

Displays the Condenser Drain Selection Window

Pressure Relief Valve and Pipework Calculations

22

TOOLBAR CONTROLS

Allows the user to change the system design temperature from the default settings

Function to obtain the temperature related to pressure or pressure related to temperature.

Function to obtain the temperature penalty equivalent from pressure drop or pressure drop equivalent from temperature penalty.

Allows the user to step through the flow-path from the terminal to the end-of-system circuit.This function can only be used if the system has been run and validated.

The flow path always starts at a terminal circuit. The toolbar and Scroll Buttons will be enabled (turn white) as a terminal circuit is displayed.

Once turned on, using the highlighted Scroll Buttons or PgUp and PgDn keys you can step through the flow-path one circuit at a time.

When this function is switched on, MicroPipe will automatically move to the next circuit when the Enter, Tab of down arrow key is pressed when the focus is at the last data-entry at the bottom of the Pipe-System Details Window.

This facility cuts down the time required for data entry by reducing the number of key operations required.

This function will automatically switch off when last circuit is reached.

Excludes 1/4” pipe from the pipe-size selection process.

Excludes 1/4” and 3/8” pipe from the pipe-size selection process.

System Design Temperature

Temperature to Pressure Relationship

Temperature Penalty to Pressure Drop Equivalent

Flow-Path-Circuit Trace

Automatic Circuit Increment

Omit 1/4” Pipe

Omit 1/4” and 3/8” Pipe

In Copper Pipe mode, this displays BS EN378 system design pressures and a pipe-size selection menu. Also displays weights of services and pipe support details.

Selecting a pipe-size from the menu automatically displays the maximum working pressure of each available gauge of pipe-wall thickness.

Maximum working pressures, for copper pipe, are displayed in green if the pressure rating of the pipe gauge is within the system design pressure rating or red if they are not.

Copper Pipe Gauges and System Design Pressures and Tables

23

TOOLBAR CONTROLS

Excludes 3-5/8” pipe from the pipe-size selection process.

Subtracts up to 10 circuits (depending on number of circuits) starting at the current circuit upwards or from the end of the system down, if the current display is less than 10 circuits from the last circuit number (not label).

You can add up to 10 lines of notes.

Each page or screen of the program can now be printed, if required. This is useful for the pressure relief valve and vent line calculations etc.

Each circuit can be included/excluded from the pipe-length summary by clicking on the button to the left of the ‘Circuit included in pipe-length summary’ field.

Omit 3-5/8” Pipe

Subtracts up to 10 Circuits

Deletes the current Circuit and Data

Add Notes

Print Current Window

Include or Exclude Circuit Data from Summary

Screen Refresh

Toggles the Background Graphic On/Off

Adds 10 Circuits to the end of the System

Adds 1 Circuit to the end of the System

Pipe circuit data can be copied and pasted to anywhere in the system using the Copy and Paste buttons.

Copy and Paste

24

WORKED EXAMPLE

25

WORKED EXAMPLE

The example has built-in error to make the session a little more educational as you seek to find it.

• File name: EXAMPLE1.SUC• Click on the MicroPipe Icon in the Windows programs menu to start MicroPipe.

On start-up, MicroPipe will display the System Design window and be set for Network mode, R404A and Copper Tube. The settings are displayed on the Status Bar at the bottom of the screen.

Except where the meaning or intention is obvious, MicroPipe will display a context sensitive message whenever the mouse pointer is positioned over a control.

26

• Click on the Suction System icon to set MicroPipe to Suction System mode.

• Click on the Suction System button to display the suction-pipe-system-details window.

• Set the number of circuits to 28 by pressing the +10 and +1 buttons until the required number of circuits is reached. Use the –10 and –1 buttons to delete circuits, if required. Circuits can also be added by pressing the Ctrl + PgUp keys together.

• Return to the Design Data window by clicking on the Design Data Icon.

• Click on the ‘Show Example System’ button to display a typical circuit diagram.

• Click on the Refrigerant and Pipe menu options on the Menu Bar to ensure the system is set for the desired HFC refrigerant and copper tube. Point and click on the item name to select. A tick will be displayed on the left-hand side of the name when it has been selected.

• Enter following parameters:

Evaporating temperature:

Suction superheat:

Maximum liquid temperature:

Minimum liquid temperature:

Maximum permitted penalty:

30K

-30K

40°C

10°C

1.5K

Note: Each relevant component in the system will have a yellow label displayed against it as the data input focus is moved. Click on the ‘Hide Example System’ button to remove the image.

Note: All of the system settings are displayed on the status bar at the bottom of the screen.

WORKED EXAMPLE

27

• Click on the Circuit Label increment button, enter 1 as the required value and then click the OK button. The circuit labels now will be arranged in the same order as the circuit numbering system using the entered increment value.

For the sake of this exercise, set all junctions to 90°. Use the globalise circuit ‘Junction Types’ function for this by clicking on the ‘Junction-Type’ button until 90° is showing and then clicking on the Globalise button on the tool bar and confirming the required action. All circuits will now be set to 90°.

Double Suction Riser. (See appropriate chapter)

Single Suction Riser. (See appropriate chapter)

Automatic Circuit Trace. (See appropriate chapter)

• Clicking on the small grey buttons to the left of the data-entry description will enable/disable the relevant data entry field. When the button shows a pressed condition, the data entry field is disabled. When entering data or moving the focus it will by-pass the disabled field and move to the next enabled field. Using these buttons, you can turn off any fields that are not required to reduce the time taken for entering data.

• Please note that the Circuit Label, Next Flow-Path Circuit Label, Circuit Duty and Minimum Oil Riser Duty cannot be disabled from the buttons. They are enabled and disabled from the relevant function buttons on the tool bar.

Use the following controls to enable various functions and entry text boxes:

Click the ‘Circuit-Type’ button or press the ‘Space bar’ to change to/from a Network to a Terminal circuit. A ‘Terminal-Circuit’ is an Evaporator in Suction, Liquid and Pumped Liquid mode and a Compressor in Discharge mode. You will not be able and do not need to enter circuit duties for a Network circuit; MicroPipe calculates these from the terminal duties and flow-path data entries.

Clicking the ‘Junction-Type’ button will set the circuit ‘Junction-Type’ to 90° or 180° as appropriate. Please remember that there can only be one 90° and one 180° or two 90° junctions on each tee junction.

• Using the Scroll-Bar control or PgUp and PgDn keys, scroll through the system and enter the data listed on the EXAMPLE1 system design sheet in the manual.

• After each data entry, pressing the Enter or Tab key will automatically advance the focus to the next enabled entry field. If you have turned on the automatic circuit increment function, MicroPipe will also increment the system counter by one circuit and advance the system to the next circuit as the last enabled entry field is completed.

You will find it easier if you quickly go through the system and set the terminal circuits first. This will automatically enable the ‘Circuit Duty’ data entry fields so that, when you are entering data, the focus will automatically by-pass any data entry fields that are not terminals.

On completion of the data entry, check through the system for errors. Although MicroPipe comprehensively checks for data and path errors it is possible to enter an incorrect flow path and be totally unaware of the error.

WORKED EXAMPLE

28

Once checked, click on the validate button on the toolbar. This will cause MicroPipe to check all data entries, flow-paths and calculate the duties of the interconnecting pipe network. Any errors found will cause a message box to be displayed informing you of the error and giving advice, where possible, on the cause. The focus will then jump to the locality of the error.

Once the system has been validated, it will then be possible to use the flow-path circuit trace and flow-path functions to check your flow-path entries. The flow-path will be displayed whenever a terminal circuit is being viewed. Validation is necessary for these functions, as MicroPipe cannot display the flow-paths until it has carried out label and next flow-path-label checks and recorded the flow-paths.

Notes: MicroPipe locates the ‘end-of-line’ circuit by looking for a circuit with the next flow-path label omitted. Once validated, the end circuit will be identified by the ‘END’ message being shown in the Junction Type display. There can only be one end circuit in the system. An error message will be displayed in the event of more than one ‘end of circuit’ indication being detected.A ‘validated input’ message will appear on the status bar once the system has been fully checked and validated.

The flow path will be displayed at the bottom of the data output window. To the left of the display is a scroll control that will enable you to step through the flow-path. The Shift + PgUp and Shift + PgDn keys can also be used to step through the flow path.

The system should now be ready to run. Click on the run button or press the F12 key. If all data has been correctly entered MicroPipe will commence the process by first re-validating all data entries and then, if no errors are found, start the pipe sizing calculations. A small window will appear to indicate which terminal circuit is currently being analysed and how many optimisation stages have taken place. MicroPipe can optimise up to a maximum of 100 iterations.

The time taken for the calculations is totally dependent on the speed of the computer processor. If the MicroPipe has completed the calculations without an error being reported the output data fields will be filled with the details of the system. In an error has been generated, a window will appear informing you of the error and the cause.

DESIGNING FOR EFFICIENT OILRETURN

29

DESIGNING FOR EFFICIENT OIL RETURN

30

To maintain maximum refrigeration system efficiency and reliable compressor oil level control it will be necessary to reduce the volume of oil circulated around the refrigeration system to a practicable minimum. This requirement makes it necessary to take positive measures to ensure that the oil will be continuously returned to the compressor system and will not be allowed to accumulate anywhere in the system.

To reduce the amount of oil carried over into the services, most multi-compressor plants are fitted with an oil level control system consisting of an oil separator, oil reservoir and oil level controls fitted to each compressor. Single compressor plants are normally only fitted with an oil separator.

At their rated operating conditions, oil separators are normally capable of removing a fairly high percentage of the oil from the compressor discharge vapour. This still leaves a small volume of oil that will be carried over into the pipe network. Evaporators, unless circuited incorrectly, will not normally collect oil as they are continually being flushed with liquid refrigerant, which effectively eliminates the possibility of oil accumulation. Consequently, the oil will be moved through the evaporators into the suction pipe network.

Oil is transported in suction pipelines by two mechanisms. One of these mechanisms is known as miscibility. This is where the oil and refrigerant mix together and the oil is transported as part of the mixture. The degree of miscibility with the refrigerant can vary considerably and is dependent on the system operating conditions. In any event it is not the most significant mechanism. The second, and most important, mechanism is by the oil being transported along the tube wall by the force (velocity pressure/kinetic energy) of the refrigerant flow.

The driving force (kinetic energy) of any fluid stream is known as the ‘velocity pressure’. This is a function of the fluid-density (kg/m3) and fluid-velocity (m/s) and is calculated in Pa from the SI units equation: [(0.5 x fluid-density) x fluid-velocity2]. Naturally, as the fluid mass-flow decreases the velocity will also decrease proportionally. As the velocity-pressure of the fluid is proportional to the square of its velocity then even a small change in mass-flow will have a marked effect on the oil transport properties. For example, a 25% reduction in mass-flow will reduce the velocity pressure by 44%.

Where the velocity pressure (kinetic energy) of the refrigerant flow in a suction riser is insufficient to overcome the effects of gravity on the oil then, regardless of the height of the riser, the oil will not be transported efficiently unless is has been selected to be capable of doing so.

With most refrigeration systems, it is not possible to accurately predict the quantity of oil carry over at any given moment in time as the efficiency of the oil separator is constantly changing due to the variations in refrigerant mass-flow, density and temperature. Consequently, the only methods available to the system designer, to ensure efficient oil return, are in the design of the suction pipe network. If the suction pipe network is designed with the correct operating parameters, pipe-falls, oil traps and risers, oil will always be returned regardless of the volume of oil carry-over.

The common belief that oil return is dependent on a given refrigerant velocity, for all system operating conditions, is entirely incorrect.

As explained above, oil is transported in suction pipes by the forces of the refrigerant flow. As the refrigerant density varies considerably over the range of possible operating conditions it is, therefore, not correct to select risers solely on the basis of velocity.

DESIGNING FOR EFFICIENT OIL RETURN

31

Fluid velocities in tubes are not uniform across the radius. The friction of the fluid on the tube wall causes a lower velocity at the wall than at the centre. Because of the parabolic-like shape of the velocity profile across the tube, larger diameter tubes require higher average velocities to maintain the oil-transporting velocity pressure at the tube wall.

To avoid the problems associated with poor oil return it is important to fully understand the mechanisms of oil return. This understanding can then be usefully employed in the design of suction pipe networks that will not accumulate oil under any predictable circumstances.

Velocity Profile

DESIGN GUIDE

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DESIGN GUIDE

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Wherever possible, all horizontal pipe runs should have a minimum fall of 1 in 500, in the direction of flow to the compressor. Of course, the more pipe-fall the better the oil return.

When it is necessary to slope the pipeline upward, in the direction of flow, treat the circuit as a riser and provide an oil trap at the lowest point of the circuit and, if it is felt necessary, insert intermediate oil return traps.

Never permit a horizontal pipeline to lead into to a riser without fitting an oil trap and correctly sized suction riser. Large amounts of oil can and will collect in horizontal pipelines and the only reliable means of keeping them free of oil accumulation are the gravity oil-return pipe-falls, oil traps and suction-risers.

Never connect a branch line below the centre line of the line it is joining. Inactive or low duty branch lines can collect oil. The oil running along the bottom of the pipeline will run into the branch line and form an oil pool. If all branch lines are above the centre line of the horizontal line they are joining, oil will be prevented from accumulating, due to the pipe junction.

Never connect a branch line below the centre line of the line it is joining. Inactive or low duty branch lines can collect oil. The oil running along the bottom of the pipeline will run into the branch line and form an oil pool. If all branch lines are above the centre line of the horizontal line they are joining, oil will be prevented from accumulating, due to the pipe junction.

Horizontal Pipework

Branch Connections

Branch Connections

Recommended

Not Recommended

Satisfactory

DESIGN GUIDE

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Before deciding whether a particular circuit is to be a single or double riser, it will be necessary to establish the minimum load for the circuit and the minimum refrigerant liquid temperature at which the system could operate during low ambient temperatures. Many refrigeration systems are now designed to take advantage of the lower condensing temperatures, which are possible during the winter. During these periods, refrigeration plants consume much less energy and are able to meet their required capacity with a considerably lower refrigerant mass-flow rate. Unfortunately, the lower mass-flow rate significantly reduces the oil return capabilities of the suction pipe network. Consequently, it becomes important to ensure that reasonably accurate estimates are made of the minimum system heat load and refrigerant liquid temperatures to ensure that risers will be selected for continuous and reliable oil return.

If a particular circuit has not been selected to act as a riser, even a small rise in height (not necessarily vertically) can seriously reduce its oil carrying capability and cause it to be a potential oil accumulation point. It is, therefore, very important to consider every circuit that rises above the horizontal as a suction riser.

Single risers are probably best used in situations where the minimum pipe-network duties are high and/or where higher penalties can be tolerated.

It is very important to remember that having high penalties on single risers can cause the remainder of the pipe-network to be increased in size to compensate for the smaller amount of penalty available, for the remainder of the network, after selecting the risers. In these circumstances the system could prove to be more costly than necessary and it will probably prove more cost effective to use double risers.

Double risers should always be used where there the pipe network has to operate at low mass flow rates for extended periods and/or where penalties must be kept to a minimum.

Suction Risers

Single Pipe Risers

Double Pipe Risers

Riser Configurations

Double Riser Single RiserIntermediate,

Sloping Riser Trap

DESIGN GUIDE

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There are frequent occasions where the formation of a deep double riser trap is not possible. To avoid an oil accumulation point, a combination of single and double riser can be employed.

A typical example is a Supermarket cabinet, which has a very limited space between the floor and the underside of the case. During re-fits, it is common practice to form a well in the floor to accommodate the oil traps. This can create problems with noise and dust, and is also an extra cost and complication to what is normally a very tight programme of works.

As the sketch indicates, the lower horizontal suction line should be pitched down to form a very shallow trap. This creates a collection point where the oil level will reach the bottom of the single riser pipe without flooding the main horizontal line with oil. Once over the bottom of the single riser tube the oil, because of the higher velocity, will be entrained and carried into the double riser.

The single riser ‘A’ is sized for the minimum load. Because it is very short, the pressure loss is kept to a minimum. The double riser ‘B’ should be sized in the normal manner.

Because this is a special case, MicroPipe will detect the combination and automatically calculate and add the additional pressure drop penalty created by the arrangement. (See note below)

It is recommended that several calculations be tried to avoid unnecessary pressure drop in the riser combination.

Note:

The facility for MicroPipe to automatically detect and calculate the additional pressure drop created by the single/double riser arrangement was made available from 1/12/2000.

Special Case Suction Risers

Double Riser

Single Riser

Shallow trap arrangement

Compressor system suction headers should be designed to prevent oil accumulation and to be capable of returning oil only to those compressors that are running. Oil return, to individual compressors, should be metered in a manner that positively prevents over-feeding the compressor, with oil, due to the suction header being incorrectly balanced.

Compressor System Suction Headers

DESIGN GUIDE

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Minimum loads of riser circuits must be considered very carefully. The type and number of refrigerated fixtures, defrost intervals, and minimum heat load of each fixture must all be taken into consideration.

Generally, supermarket cabinets range from 50% to 100% of their maximum rated heat load as the surrounding ambient air temperature and humidity varies between winter/summer and night/day operation. Add this to the number and different types of fixtures that could be on/off at any given moment in time and it appears to be a complex estimating problem. This is not the case. If, for example, we take a typical supermarket run of frozen food well cabinets, we can apply simple rationale for estimating the minimum load, as follows:

• Cabinets are usually designed for ISO 3 conditions of 25°C, 60% RH.

• The minimum store temperature is normally permitted to fall to 12°C at night. The RH during this period will be about 70%.

• Variations in cabinet radiant heat gain are proportional to the fourth power of the temperature difference. This will reduce the cabinet radiation heat-gain to approximately 84% of its maximum at 25°C. For the example, take 25% of the maximum total heat gain to be from radiation.

• Take the heat gain through the cabinet structure to be 10% of the maximum total heat gain. This will be reduced to 73% of its maximum value.

• Take the fan heat gains as 10% of the maximum total heat gain. These gains will be unchanged.

• Take the heat gain from the air ingress to be 55% of the maximum total heat gain. This will be reduced to 62% of its maximum value. (See table 1)

Riser Minimum Load

Table 1

Heat gain calculation

ISO 3 (25°C, 60% RH) air enthalpy:

Radiation 25% x 0.84 21%

12°C, 70% RH air enthalpy:

Fabric 10% x 0.73 7.3%10%34.1%

10% x 155% x 0.62

-23°C frozen food cabinet air enthalpy:

FansAir ingress

Total 100% 72.4%

55.95kJ/kg

27.55kJ/kg

-19kJ/kg

DESIGN GUIDE

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For the example, if the cabinet run has 10 sections and each has a minimum heat load of 72%, this would mean that the average minimum load for the entire run would also be 72%. Because of the random nature of the load caused by the variations in control cycles and controls, the number of cabinet sections actually on at any given time could range from 0% to 100%. However, the large load variations are not important as, on average, there will be sufficient load to ensure oil return, providing the network and risers have been correctly designed and selected. If the low load cycles are of a relatively short duration, the small amount of oil accumulated during these periods will be flushed through when the load increases. Because of this, it is not always necessary to select the risers for a very low load.

In this example, selecting a single riser for 72% minimum capacity and 5°C minimum liquid temperature would probably lead to a riser temperature penalty that is unacceptable at full load. A more acceptable solution would be to select a double riser for a minimum oil return capacity of 50%.

To ensure the integrity of the pipe network design, regarding oil return, a minimum heat gain estimate must be carried out for each type of fixture taking into consideration the design ambient conditions (ISO conditions), operating temperature and the minimum ambient air temperature and humidity around the fixture.

Fixtures such as cold rooms can, when turned on, be considered to be at 100% capacity as the cooler load is a function of the fixture air temperature only and is not directly affected by the surrounding ambient air temperature. The percentage of load reduction on the pipe network will be reflected in the ratio of the cooler refrigeration on/off cycle as dictated by the heat gains from various sources. The suction risers from the individual evaporators can, therefore, always be considered to be at 100% load. Where multiple, individually controlled room coolers or fixture evaporators are fed into a common pipe network, careful consideration must be given to the minimum load (as described above) when sizing the risers.

Systems with a high proportion of cold rooms require extra consideration as the minimum load can be very low for extended periods of time. Normally, the room coolers are selected for 16-20 hour running with the pipe system sized to match the cooler load. When the rooms are not in use during the night or at weekends the room heat loads, due to the reduction in product throughput, door openings and lighting etc., will be considerably lower. If the system and risers are not designed to accommodate the load reductions oil logging of the network will occur.

DESIGN GUIDE

38

Where the system condensing temperature is allowed to reduce, during low ambient temperatures, it is possible for it to reach to within 5-10K of the condenser ‘air on’ temperature. For operational reasons, condensing temperatures are normally limited to a minimum of 5° to 15°C for high temperature systems and 0° to 10°C for low temperature systems.

Because of the reduced mass-flow through the condenser during these periods, the refrigerant in the condenser will have a longer dwell-time, which will significantly increase the volume of liquid refrigerant in the condenser. The longer dwell time and increased flooding of the condenser tubes has a marked beneficial effect on the efficiency of the system as the liquid is able to achieve a greater degree of sub-cooling which increases the Refrigeration Effect of the refrigerant in circulation for no additional energy consumption. Liquid sub-cooling to within 3 to 5K of condenser ‘air on’ temperature is quite normal during these periods.

Because of the increase in Refrigeration Effect, the system will be able to meet the heat load with a much lower refrigerant mass-flow rate, which, in turn, will reduce the oil return capabilities of the pipe network. (See previous comments). It is, therefore, important that risers are selected to be capable of returning oil even when operating at the lower mass-flow rates caused by the reduced liquid temperatures and low fixture heat loads.

For example, if we have a system operating with a liquid temperature of 5°C and a heat load of 72%, the refrigerant mass-flow rate would be reduced to 44% of its value at 100% load and 40°C liquid temperature. At these operating conditions the pipe network velocity pressures would be 20% of their full load values.

Minimum Liquid Temperature

EXAMPLE INPUTS

39

40

Project: ACME Supermarkets PLC Evaporating Temperature -35 °C Suction Superheat30 K Liquid Overfeed N/A %

Reference: Project 3206. Drawing 3206/3A Maximum Liquid Temperature 38 °C Discharge Temperature N/A °C Maximum Pressure Drop N/A kPa

Date: 1st January 2000 Minimum liquid Temperature 10 °C Liquid Head Difference N/A +/- M Maximum Penalty 1.5 K

System: LT Suction, Circuit No 1 Condensing Temperature 40 °C Pumped Liquid Lift N/A M Sheet 1 of 1

Circuit Number

Circuit Details

Duty (Kw)

Pipe Length (m)

Long Radius Bends

Short Radius Bends

45° Bends

Machine Bends

P Traps U Bends Ball Valves

Globe Valves

Circuit Label

Next Circuit Label

Pressure Loss Factors

Minimum Oil Return % or

kW1 T 1.5 2 2 1 1 9

2 T 2.5 1 1 1 2 9

3 T 2.5 1 1 1 3 11

4 T 2.5 1 1 1 4 13

5 T 2.5 1 1 1 5 12

6 T 2.5 1 1 1 6 12

7 T 2.5 1 1 1 7 14

8 T 1.5 2 2 1 8 10

9 N 3 9 10

10 N 0.5 10 11

11 N 1.5 11 15

12 N 3 12 13

13 N 0.5 13 14

14 N 1.5 14 15

15 N 20 2 15 16

16 DR, N 4 6 16 17 35%

17 N 5 1 17 26

18 SR, T 4.5 2 7 1 18 20 100%

19 SR, T 4.5 1 6 1 19 20 100%

20 N 10 1 20 25

21 SR, T 3.2 1 6 1 21 22 100%

22 N 4 22 24

23 SR, T 3.2 1 6 1 23 24 100%

24 N 6 24 25

25 N 3 1 25 26

26 N 28 1 26 27

27 DR, N 4 6 27 28 35%

28 N 2 28

DR: Double Riser Circuit. SR: Single Riser Circuit. T: Terminal N: Network Circuit. H: Header Circuit. D: Dual Pipe Circuit

41

EXAMPLE INPUTS

EXAMPLE INPUTS

42

Single Circuit Example

Condensing Unit

Evaporator

Circuit 4

Circuit 3

Circuit 2

Circuit 1

Design Parameters

Circuit duty:

Evaporating temperatureMaximum suction superheat

Circuit 1

Circuit 2 pipe length

Circuit 3

Circuit 4 pipe length

Maximum Liquid temperature

Circuit 1

Circuit 2 long radius bends

Circuit 3 pipe length

Circuit 4 long radius bends

Circuit 3 long radius bends

Minimum liquid temperature

Circuit 1 pipe length

Maximum liquid temperature

Circuit 1 long radius bends

Maximum permitted penalty

30°C

12.5kW

20K

Terminal

15m

Single riser. Set 100% minimum capacity

25m

35°C

Single riser. Set 100% minimum capacity

1

5

3

6

25°C

1.5m

35°C

6

1.5K