Energy Effiency Refrigeration Airconditioning Systems Flooded Ejector Circulation of refrigerant

1. ID: 913
Flooded evaporator in small refrigeration- and heat pump systems
Ericsson S
Bubble Expansion Valve Corporation, Lonnrunan 54, 42346 Torslanda,Sweden
info@bxv.se fax +4631563709
ABSTRACT
This paper describes tests of a system that obtain flooded evaporation in small refrigeration and heat
pump systems. With flooded evaporation the heat transfer in the evaporator can be improved, which
increases the evaporation temperature as well as the cooling and heating coefficients of the system.
(Prof Dr.-Ing. Michael Kauffeld 30%) (Rizzvi Z and Dr Peter Heggs 16-48%)
The system is based on three main newly developed system components:
• An automatic self-acting bubble expansion valve to secure a bubble free condensate.
• New technique to recirculate compressor oil from evaporator/accumulator,
• New strategy to recover the pressure drop losses by circulating the cold refrigerant several times
between the condenser and the evaporator.
Energy savings with this method has been evaluated and an improvement of 10% compared with a
standard system using superheated gas strategy has been verified.
Keywords
Flooded evaporator, ejector pump circulation, bubble expansion valve, super heat.
State of the art
The mechanisms when the refrigerant boils and evaporates, forms the basis for the design and development
of a new method of heat transfer solution which is presented in this paper.
Standard cooling system efficiency is dependent on a thermostatic expansion valve to achieve a superheated
gas. The temperature of the incoming coolant or air heats the evaporated gas to a preset overheating
temperature level that is higher than the evaporating temperature.
If overheating occurs in the vital evaporator, this results in reduced heat absorption.
For example, if the expansion valve superheat is adjusted to 6K overheating to make a stable running of the
system, the temperature difference between the incoming coolant to heat the superheated gas from the
evaporating temperature must in most cases be above 6.5 K. Each degree change of the evaporation
temperature level affects compressor efficiency by 3.5 to 4%.
Project goals
• No gas overheating in the evaporator.
• More efficient mixing ratio between gas and liquid in the evaporator.
• Ensure an effective refrigerant circulation with an even distribution of coolant inside the evaporator.
• The oil return from evaporator to the compressor is made with heated oil and without interference of
refrigerant liquid.
• Design of a system capable of operating at low condensation temperature.
• Design of a refrigerant control for flooded evaporating system and condensate sub-cooler without
need of condensate-tank (normally used on high pressure float valve level system)

2. Figure 1: Overview of system with flooded evaporation.
Briefly the system works in the following manner.
The evaporator is fed from an accumulator
mixture of liquid and gas return back to the same
top of the receiver. The refrigerant is circulated
pressure difference between condenser
The refrigerant liquid flow to the evaporator is controlled via a
valve is created by the flow through choke 1 and is controlled by
If the condensate after the condenser contains gas bubbles, the mass flow through the restriction decreases.
The flow through choke 2 is not affected by the state before
pressure decreases between choke 1 and 2, the pressure signal to the valve drops and the valv
ejector effect drives the flow from the
Compressor oil is pumped by ejector pump press
condensate sub cooler.
Overview of system with flooded evaporation.
Technology
Briefly the system works in the following manner.
ccumulator tank, which also acts as a liquid separator. After the evaporator, a
back to the same accumulator. The compressor sucks saturated gas from the
is circulated through the evaporator by an ejector pump, driven by the
pressure difference between condenser and the evaporator.
flow to the evaporator is controlled via a bubble expansion valve. The signal to the
valve is created by the flow through choke 1 and is controlled by the refrigerant state at the condenser exit.
If the condensate after the condenser contains gas bubbles, the mass flow through the restriction decreases.
2 is not affected by the state before choke 1. If bubbles pass through
1 and 2, the pressure signal to the valve drops and the valv
flow from the accumulator back into the evaporator via the ejector pump.
by ejector pump pressure and returns back to the compressor through the
, which also acts as a liquid separator. After the evaporator, a
. The compressor sucks saturated gas from the
pump, driven by the
expansion valve. The signal to the
nt state at the condenser exit.
If the condensate after the condenser contains gas bubbles, the mass flow through the restriction decreases.
1. If bubbles pass through choke 1 the
1 and 2, the pressure signal to the valve drops and the valve closes. An
ccumulator back into the evaporator via the ejector pump.
compressor through the

3. Figure2: Overview of system with flooded evaporation
transfer.
: Overview of system with flooded evaporation liquid / Gas proportion affect heatiquid / Gas proportion affect heat

4. Figure3: Overview of bubble control
Two identical heat pumps were used to evaluate ejector
The first phase of the project verified
performance.
One of the heat pump units was then
describes. Finally comparative measurements were made
Both of the heat pumps had a nominal heat
pump units were fitted with:
• Evaporator SWEP B25x24=
expansion system.
• Condenser SWEP B25x40.
• Copeland Compressor Scroll type ZB21KCE.
• Thermostatic expansion-valve
temperature/ out radiator water
percent of ethylene glycol.
bubble control system with flooded evaporation
Tests
pumps were used to evaluate ejector pump circulation and precooled
first phase of the project verified by accurate measurements that the two heat pump
then equipped with new developed components as the a
. Finally comparative measurements were made on the modified heat pump and the
Tested units
a nominal heat capacity of 8 kW and used R404a as refrigerant. The two heat
= 1, 39 m2 with a long thermal length which is adapted to use for
Copeland Compressor Scroll type ZB21KCE.
valve Danfoss TUAE carefully set to 6K work superheat value
water 0 / 35 ° C. The brine for the evaporator was water with
cooled liquid.
pumps had the same
with new developed components as the above principle sketch
the modified heat pump and the unmodified one.
efrigerant. The two heat
ch is adapted to use for direct
6K work superheat value, at brine in
the evaporator was water with 35 weight

5. A new system with bubble expansion valve
condensate, has been tested and verified and offers 3.5K less temperature differences i
heat exchanger, than in a conventional system
Figure 3: Evaporating improvement
Figure 4: improvement of logarithmic
-14
-12
-10
-8
-6
-4
-2
0
-5/45 *
Bubble -9,3
DX system -12,43
Evaporatingtemperature
Improvement 3,5K evaporating temperature
0
1
2
3
4
5
6
7
8
-5/45 *
Bubble 2,84
DX system 6,05
EvaporatingLMTDK
Logarithmic mean temperature difference
* C0
Glycol in to evaporator / Water out from condenser
Test results
expansion valve, ejector pump circulation, oil return system,
has been tested and verified and offers 3.5K less temperature differences i
heat exchanger, than in a conventional system. This resulted in 10% lower power consumption.
Evaporating improvement is 3,5K higher temperature
logarithmic mean temperature difference
0/35 * +5/35 *
-4,95 -0,44
-8,41 -3,9
Improvement 3,5K evaporating temperature
0/35 * +5/35 *
3,14 3,44
6,82 7,07
Logarithmic mean temperature difference
Water out from condenser
return system, subcool-
has been tested and verified and offers 3.5K less temperature differences in the evaporator
consumption.

6. Operating levels -5/45 0/35 5/35 unit
System DX BXV DX BXV DX BXV
WW To radiator 45,1 45,1 34,4 34,8 35,0 35,1 °C
WWDt radiator 7,83 8,05 10,15 9,84 11,63 11,69 K
Warmwaterflow 697 755 686 777 694 769 l/h
Brine to evaporator -5,3 -5,3 0,0 0,0 5,1 5,2 °C
Dt brine 2,0 2,1 3,0 3,1 3,5 3,8 K
Brineflow 1835 1844 1892 2010 1941 2065 l/h
Evaporating pressure 3,96 4,41 4,55 5,11 5,29 5,92 bar
Evaporating temperature -12,4 -9,3 -8,4 -5,0 -3,9 -0,4 °C
Returngas temperature -6,0 -3,1 -1,1 -0,1 3,9 3,6 °C
Hotgas temperature 68,5 66,2 53,5 52,0 53,4 50,0 °C
Liquid before expvalve 36,8 16,9 24,9 11,0 24,7 11,5 °C
Logarithmic mean
temperatur of Evaporator
6,05 2,84 6,82 3,14 7,07 3,44 K
Condensereffect 6394 7122 8241 9021 9530 10627 W
Increase condenser effect 728 780 1097 W
Cond increase in % 11,4 9,5 11,5 %
Table 1: Test results. Direct expansion system (DX) and Bubble expansion valve system (BXV)
CO CLUTIO S
A bubble expansion valve system has been shown to reduce the temperature difference by more than 50% in
comparison with a conventional system with a direct expansion valve.
At the same time the heat transfer effect in the evaporator has improved, which means that the
efficiency of the heat transfer, expressed as average heat transfer rate increased from 616Wm2
K to
1579Wm2
K or even to 245 % after the modification to a flooded evaporator system. The improved use
of the evaporator surface may be attributed to one or more of the following factors:
• The need for overheating has been eliminated, resulting in a significantly higher heat transfer rate
and smaller temperature differences in the evaporator. (Prof Dr.-Ing. Michael Kauffeld)
• The need of superheating is eliminated which may reduce the sensitivity of the uneven distribution
of refrigerant flow between the parallel refrigerant channels of the plate heat exchanger. Distortion is
considered to be a common cause of loss of performance for this type of evaporator.
(Stefan S. Jensen)
• The proportions between the refrigerant gas and liquid in the evaporator, is a higher amount of gas in
the direct expansion than in the flooded evaporator system. Results reported in the scientific
literature show that the heat transfer rate at flooded evaporator in many cases has a maximum for
gas/liquid proportion around 70/30. (T.N.Tran , M.W.Wambganss)
(Raja Balakrishnan,Mohan Lal Dhasan) (C.K Rice Ph.D)

7. • Mass flow rate of refrigerant in the flooded evaporator is higher because of the recirculation of the
refrigerant. Higher mass flow generally increases the heat transfer rate with flooded evaporator.
(T.N.Tran , M.W.Wambganss)
• At parallel flow couple of the evaporator with a small glide medium and with significant pressure
drop, a more even temperature profile through the evaporator is obtained. Moreover, the refrigerant
is evaporated more effective with increasing temperature-differences at the evaporators inlet and
liquid / gas proportion decreases faster (motive see curve fig 2 )
• Positively surface concentration of oil in the evaporator will be changed when switching from a
direct expansion system to a flooded evaporation system. This change affects the thermal transition
positively, not only in the superheating area of the evaporator. (J. Fei, W. Y. Hu, Z. M. Wang)
Heat exchangers and compressor had original configuration during the tests .The flooded system
showed better cooling effect (+16 %) and heating effect (+10%) than the original model.
A completely fair comparison had demanded a smaller electric motor and compressor model for the
bubble expansion valve system. This had increased the energy efficiency ratio difference even more
in favour of the bubble expansion valve system.
(Ericsson Svenning Palm Björn (measured data) Bilaga 1 ”korrigerad kyleffekt”= cooling effect)
References
Prof Dr.-Ing. Michael Kauffeld Trends and Perspectives August 2007, Beijing, China 22nd IIR
International Congress of Refrigeration (ICR2007)
Supermarket refrigeration systems have an energy savings potential
Flooded evaporator more at moderate costs 30% Energy savings.
Rizzvi Z and Dr Peter Heggs 2003 Defrosting refrigerators 21th International Congress of
Refrigeration, Washington, USA, IIR/IIF: paper ICR0660.
“Flooded Circuit cop + 16-48 %”
Stefan S. Jensen, B.Sc.Eng. MIEAust Scantec Refrigeration Technologies Pty. Ltd.
Brisbane, Queensland, AustraliaCircuiting errors are most forgiving in liquid
6 © IIAR 2009 Technical Paper #7 2009 IIAR Industrial Refrigeration Conference & Exhibition,
Dallas, Texas overfeed applications and potentially most disastrous in dry expansion feed cooling
coils.
T.N.Tran , M.W.Wambganss, Boiling Heat Transfer in VCompact Heat Exchangers D.M. France 1994 San
Fransisco
The heat transfer coefficient is dependent of mass flux and increased with quality.
J. Fei, W. Y. Hu, Z. M. Wang, et al. Proc. 22nd int. Congr. Refrig., Beijing/C. R.
Experimental study on pool boiling heat transfer of R-134a/oil mixtures
The heat transfer coefficients with 100 ppm increase to 103.7-107%
Raja Balakrishnan, Mohan Lal Dhasan 2008 Heat transfer Thermal Science Year 2009
Refrigerant composition on the heat transfer coefficient are investigated experimentally.
C.K Rice Ph.D Oak Ridge National Laboratory The effect of void fraction 1987 Ashrae Winter meeting NY.

8. “Design consideration of flow control and charge inventory interactions “mass quality”
Ericsson Svenning Palm Björn Flödande förångare i små värmepumpssystem. EFFSY2 (In Swedish)
http://www.effsys2.se/Publicerade%20dokument/P18/Ejektor_rapport_v13.pdf
Ericsson Svenning
Bubble Expansion Valve AB (BXV AB)
Lonnrunan 54
42346 Torslanda, Sweden
Mail: info@bxv.se
Phone: +4631563709