Retrofit Tool
Calculation of performance change of system
components
Norbert Blatz
2019-09-07
Version 1.0
Calculation of the change of performance comparing two refrigerants.
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Contents
Introduction .............................................................................................................................. 2
Frame of content ...................................................................................................................... 2
The Diagram ............................................................................................................................. 2
3.1 System conditions ............................................................................................................ 3
Component calculation ............................................................................................................ 3
4.1 Specification of the system .............................................................................................. 3
4.2 Compressor ...................................................................................................................... 3
4.3 Heat exchanger ................................................................................................................. 3
4.4 Pipes ................................................................................................................................. 3
4.4.1 Suction and discharge line ............................................................................................ 3
4.4.2 Liquid line ..................................................................................................................... 4
4.5 Control Valves ................................................................................................................... 4
4.6 Expansion valve ................................................................................................................ 4
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Introduction
Designing a new refrigeration system or just retrofitting the existing system is the first question
to be answered.
Due to the huge number of new refrigerants this can be challenging and time consuming.
The Retrofit Tool will support with a fast calculation of the change in performance and
temperatures. This will help in evaluating possible impacts and fact-based decisions.
Frame of content
Since refrigerants have limited range of application temperature there is a limited number of
refrigerants and limits for inputs.
The Diagram
A circuit with its main components only will give a focused view and keep the number of
information to the most relevant.
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3.1 System conditions
A set of design conditions are pre-defined with typical values:
Application
T evaporation
t sh
T condensation
t sub
Air-conditioning
+5°C / 41°F
8 K / 14,4 F
45°C / 113°F
2 K / 3,6 F
Refrigeration
-10°C / 14°F
8 K / 14,4 F
40°C / 104°F
2 K / 3,6 F
Freezer
-30°C / -22°F
8 K / 14,4 F
40°C / 104°F
2 K / 3,6 F
Beside of this a Custom application allows to enter individual conditions.
Component calculation
All calculations are individual and therefore independent of each other. No system balancing is
done. All calculations assume that the existing system is correctly sized.
4.1 Specification of the system
By selecting the application and Refrigerant 1 the reference system has been specified. The
operating conditions will also be used for Refrigerant 2.
The same for “Customer” system conditions.
4.2 Compressor
The compressor calculation is based on a theoretical model based on reciprocating compressor
that includes efficiency calculation based on losses.
A real compressor optimized for refrigerant and application range can therefore give different
results.
The model shall be good enough to give a trend of impact comparing two refrigerants.
4.3 Heat exchanger
Different heat exchanger requires different models to calculate / simulate correctly. Since in the
current version of Coolselector®2 no heat exchanger model is available the only value calculated
for evaporator and condenser is the change in mean temperature difference.
This change of mean temperature difference is based only on the impact of glide with the
selected refrigerant.
4.4 Pipes
The pipes are compared assuming the same diameter and allowed pressure drop per length.
Based on this the change in capacity / mass flow is calculated.
How to read the results?
4.4.1 Suction and discharge line
If the possible capacity is higher the velocity will become lower. This can be difficult for oil
transport.
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A reduction of capacity will increase the velocity and can create to high pressure drop and by
that losses with impacts directly the compressor.
Suction pressure = lower; discharge pressure = higher.
4.4.2 Liquid line
Increasing capacity will result in lowering velocity like the gas lines. But lowering velocity has no
critical limit, it only means for new pipe work the liquid line can be smaller.
Lowering capacity stands for increasing velocity. A too high velocity should be avoided because
of dynamic effects like accelerated liquid e.g. when switching solenoid valves. But also flash gas
can happen if losses due to velocity are too high.
4.5 Control Valves
Each pipe segment has a valve which reflects typical valves used. A valve has a port which can be
a direct or servo operated one. Both types are critical against too high flow rate but only servo
operated are critical against too low flow rate. If the flow rate is too low the valve may not open
safely anymore.
4.6 Expansion valve
An expansion valve consists of the port and the control element. The capacity depends in
principle on the port size only. If the port size is not suitable for the new capacity, either the
orifice can be replaced by a new one or, if this is not possible, the valve needs to be replaced.
The control element:
An electronic expansion device has an electrical actuator which is not depended on the
refrigerant type. Only the controller settings may need to be updated.
A thermostatic element has been designed for a specific refrigerant. The element can be used
for alternative refrigerants only within a small band of pressure/temperature change.
Via the setting screw a TXV can be adopted to the new refrigerant if the required re-setting
value is not higher than maybe 3 K. Else it is not recommended, and the valve need to be
replaced by another one having the correct element.
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