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 to 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 Background

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)

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

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.

, 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 oil is pumped by ejector pump pressure

condensate sub cooler.

Figure2: Overview of system with flooded evaporation

transfer.

by ejector pump pressure and returns back to the compressor through the

: Overview of system with flooded evaporation liquid / Gas proportion affect heat

compressor through the

iquid / Gas proportion affect heat

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

equipped with new developed components as the above principle sketch

the modified heat pump and the unmodified one.

R404a as refrigerant. The two heat

ch is adapted to use for direct

6K work superheat value, at brine in

the evaporator was water with 35 weight

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

Ev

ap

ora

tin

g t

em

pe

ratu

re

Improvement 3,5K evaporating temperature

0

1

2

3

4

5

6

7

8

-5/45 *

Bubble 2,84

DX system 6,05

Ev

ap

ora

tin

g L

MT

D K

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

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

10% lower power consumption.

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)

• 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 parallel

flow couple achieves higher heat transfer at the refrigerant inlet, which affects the heat transfer by

stronger bubble formation at the evaporator inlet area. 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, Australia Circuiting 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 (Forced convective boiling mass quality and mass flux)

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.

“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

Aim of the congress

To bring together the most important and prominent industrial and academic stakeholders from

all refrigeration fields

To present and review the principal technology innovations and research findings accomplished

over the last 4 years since the previous congress in Beijing, China, in 2007

To debate current hot topics, issues and future trends

ANSWERE

Dear Sirs

Thank you very much for your reviews about “flooded strategy” paper.

I have tried to do improvements by:

i) Explanation about difference from use of hp float valve system / subcool the condensate for

ejectorpump and liquid / gas optimise.

ii) I have also done some changes for Abstract “tests of” and for References and explanation about

Conclution see parallellflow text.

Question to the author without effect on paper acceptance: the flooded system with

ejector could also work with ohter expansion valves, e.g. with high pressure float valve. Has the bubble expansion valve any other benefits?

--------------------------------------------------

See

Project goals, motive is now described in paper.

Answere

“It is not so easy to subcool the condensate with float valve without a H.P receiver”.

paper version 1 - reviewer 765: Author did not prepare paper as required by the organizer. No reference is included

in the work. This does not concern only the introduction (state of the art or as author call it background), but also results or statements. It is not clear how

certain values were obtained (experimentally, calculation or from certain literature). Otherwise from the technical point of view paper shows interesting

work

Comments

Author did not prepare paper as required by the organizer. No reference is included in the work.

This does not concern only the introduction (state of the art or as author call it background), but

also results or statements. It is not clear how certain values were obtained (experimentally,

calculation or from certain literature). Otherwise from the technical point of view paper shows

interesting work

I hope that this will explain the technology better and straightens out some questions, please inform me

if you think that i should do something more / else

Thank you for yours very

competent examinations.

Sincerely

Bubble Expansion Valve AB

Svenning Ericsson M.D

BBXXVV ®®

WWW.BXV.SE