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NEW_ EFFECTIVE_ LOW-COST THROTTLING TECHNOLOGY

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NEW_ EFFECTIVE_ LOW-COST THROTTLING TECHNOLOGY Powered By Docstoc
					                                 LOW-COST, ENERGY-SAVING,

                        FLOODED-EVAPORATOR-TECHNOLOGY

                                           Lars Zimmermann M. Sc.
                                              Hoejgaardsparken 18,
                                              8380 Trige, Denmark
                                     E-mail: zimmermann@throttling.eu

                                                  ABSTRACT

Thermostatic Flow Controller (TFC) is a new, flooded-evaporator-technology, usable for all sizes of plants and
all refrigerant, except those with big temperature glide. The flow to the evaporator is controlled by the receiver
pressure, which is controlled by a heat exchanger – without any moveable parts.
Compared to a Thermostatic Expansion Valve the other components, especially the evaporator, are exploited
better, improving the Cost-Benefit relation.
TFC is robust, reliable and don’t need service, and can be placed at inaccessible places.
For small cooling and frost application the heat exchanger is just a small tube, which also acts as receiver. The
amount of circulating refrigerant automatic adjusts to current condition and the application can be moved
freely between cold and warm location.
A flooded evaporator increases cooling effect and COP, and for most refrigerants the heat exchanger does the
same, in all the COP increase 8-17%, depending on the type of refrigerant.
The improvement comes from a more efficient cooling circuit and adds to all other energy saving measures.
According to IIR the wold use 15% of all produced electrical energy for cooling and air-conditioning.
Reducing this by 8-17% equals 1-2% of all electrical energy – corresponding to several big power plants – and
that’s without extra use of other resources.
TFC is tested thorough for several years, in polar and tropical territories, for cooling and frost applications, and
for small and big cooling effect (100W–40kW).

                                                 1. HISTORY

For use in an environment-friendly heat pump, I decided to look for a throttling device with a simple
construction, but nevertheless able to keep the evaporator flooded and the suction gas superheated for all
conditions. A number of known throttling-methods were combined in new ways and tested by
computer-simulation. Combination of 2-step throttling separated by a suction gas heat exchanger proved to
comply with all predetermined claims, and search in patent literature tells that it’s a complete new technology.
Two implementation techniques are patent-protected1+2.

                                                2. TFC-CYCLE


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TFC has 2-step-throttling separated by a receiver with integrated suction gas heat exchanger. The pressure
drop between condenser and evaporator is shared between the two steps, and the receiver pressure is controlled
by the difference between gas supplied and gas removed. If the heat exchanger removes more gas than
supplied by the first throttling step, then the receiver pressure goes down – otherwise the receiver pressure goes
up.
Drawing the TFC-cycle in a h-log(P)-diagram, as figure 1, it’s obvious that the evaporator becomes full
flooded and the suction gas superheated.




Figure Error! Unknown switch argument. : Basic TFC-cycle. The pressure-drop is parted in
2-steps, BC from condenser to receiver and DE from receiver to evaporator. The suction gas
heat exchanger, CD, removes al gas supplied and the condense-heat is transferred to the
evaporator outlet FG. The suction gas is super-heated to receiver-temperature, the dashed line
CG. The evaporator, EF, is drawn the distance DC into the 2-phase-region and becomes full
flooded.


From the h-log(P)-diagram it’s simple to calculate the necessary heat transfer area of the suction gas heat
exchanger, and it’s also easy to calculate the dimension of the restriction between condenser and receiver,
otherwise with the restriction between receiver and evaporator. It’s an indispensable assumption that the flow
of refrigerant from the receiver to the evaporator increases/decreases when the temperature in the receiver
increases/decreases, but the assumption is violated because the refrigerant leaving the receiver is at the boiling
point and flashes in the restriction. If the refrigerant is sub-cooled before entering the capillary tube, flashing is
avoided, and now the assumption is fulfilled.

                                         3. LIQUID SUB-COOLING

To avoid flashing, liquid leaving the receiver had to be sub-cooling before entering into the restriction. The
heat from the sub-cooling can be transferred - either to the evaporator inlet - or to the evaporator outlet and the
difference between the two methods are described in the following sections.

3.1. Heat transferred to evaporator inlet
When heat is transferred to the evaporator inlet, the cooling circuit looks like shown at figure 2.

                           International Congress of Refrigeration 2007, Beijing.
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Figure Error! Unknown switch argument.: TFC-cycle with heat transfer to the evaporator inlet.
The cycle is composed by evaporator (1), compressor (2), condenser (3), receiver (4) with suction
gas heat exchanger (5) placed in the receiver, two restrictions (6,8) and a heat exchanger (7) between
receiver outlet and evaporator inlet.


In figure 3 the cycle is shown in a h-log(P)-diagram, and from here it’s clear, that liquid sub-cooling
doesn’t change the characteristic of the h-log(P)-diagram, shown in figure 1. The difference is local,
the liquid is sub-cooled before passing the restriction and then the heat is added to the liquid again,
and there is no difference at the evaporator inlet from figure 1 to figure 3.



                                                                       AB     Condenser              (3)
                                                                       BC     1.throttling step      (6)
                                                                       CD     condensing gas         (5)
                                                                       DX     sub-cooling liquid (7)
                                                                       XY     2.throttling step      (8)
                                                                       YE     heat from XD           (7)
                                                                       EF     evaporator             (1)
                                                                       FG     heat from CD           (5)
                                                                       Numbers in parentheses refer to figure 2.

Figure 3: The cycle in figure 2 shown in a h-log(P)-diagram. Heat is transferred from receiver
outlet, DX, to evaporator inlet, YE. The heat transfer is local and doesn’t influence on
evaporator inlet, E.


The first throttling step adds heat to the receiver, which tend to increases temperature and pressure.
The suction gas removes heat from the receiver, which tend to decreases temperature and pressure.
Pressure and temperature in the receiver is forced towards equilibrium between heat added and heat
removed, and at the point of equilibrium, relation R1 becomes valid:
        CPliquid * ( Tcondenser - Treceiver) = CPgas * ( Treceiver - Tevaporator ) + RT * Y    (R1)
where
        CP is the heat capacity of the refrigerant. Index                  for gas or liquid form.
        RT is the heat of evaporation
        Y is the rate of liquid at the evaporator outlet.
An essential purpose of the circuit is to keep the evaporator flooded, which implies that Y is positive.
This requirement is substituted into R1 and makes R2:
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        R1  ( Y>0 ) 
        CPliquid * ( Tcondensor - Treceiver ) > CPgas * ( Treceiver - Tevaporator ) 
        ( Treceiver - Tevaporator ) < ( CPliquid / CPgas ) * ( Tcondensor - Treceiver )     (R2)

Relation R2 sets an upper limit on how much of the total pressure drop there is allowed for the
2-throttling step. The 2-throttling step also establishes the temperature difference across the heat
exchanger, and it’s essential that this pressure drop is as big as possible to ensure that the heat
exchanger area is as small as possible.

3.2. Heat transferred to evaporator outlet
Instead of transferring heat to evaporator inlet, as figure 2, the heat can be transferred to evaporator outlet, as
shown at figure 4. The only difference between the figures is the placement of heat exchanger (7).




Figure 4: TFC-cycle with heat transfer to the evaporator outlet. The cycle is composed by
evaporator (1), compressor (2), condenser (3), receiver (4) with suction gas heat exchanger (5) placed
in the receiver, two restrictions (6,8) and a heat exchanger (7) between receiver outlet and evaporator
outlet


The cycle is shown in a h-log(P)-diagram, at figure 5, and here it’s obvious that the heat transfer,
according to liquid sub-cooling, is no longer local, but instead it push the evaporator deeper into the
2-phase area, making the evaporator more flooded.


                                                                    AB     condenser              (3)
                                                                    BC     1.throttling step      (6)
                                                                    CD     condensing of gas      (5)
                                                                    DX     sub-cooling of liquid (7)
                                                                    XE     2.throttling step      (8)
                                                                    EF     evaporator             (1)
                                                                    FY     heat from DX           (7)
                                                                    YG     heat from CD           (5)
                                                                    Numbers in parentheses refer to figure 4.

Figure 5: The cycle in figure 4 shown in a h-log(P)-diagram. Heat is transferred from receiver
outlet, DX, to evaporator outlet, FY. The heat transfer is no longer local, but push the
evaporator deeper into the 2-phase area.
The liquid in the bottom of the receiver will be sub-cooled close to the evaporator temperature and the
suction gas will be super-heated to close to the receiver temperature. At equilibrium between added
and removed heat relation R3 is valid:
                        International Congress of Refrigeration 2007, Beijing.
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       CPliquid * ( Tcondensor - Tevaporator ) = CPgas * ( Treceiver - Tevaporator ) + RT * Y (R3)
A main purpose of the circuit is to keep the evaporator flooded, which implies that Y is positive. This
requirement is substituted into R3 and makes R4:
       R3  ( Y>0 ) 
       CPliquid * ( Tcondensor - Tevaporator) > CPgas * ( Treceiver - Tevaporator ) 
       ( Treceiver - Tevaporator ) < ( CPliquid / CPgas ) * ( Tcondensor - Tevaporator ) (R4)
The heat capacity of liquid is always higher than the heat capacity of gas. This relation is substituted
into R4 making R5:
       R4  ( CPliquid / CPgas ) > 1 
       ( Treceiver - Tevaporator ) < ( Tcondensor - Tevaporator) 
       Treceiver < Tcondensor                                                 (R5)
Relation R5 is always true - and the evaporator will be full-flooded without any restrictions on
receiver temperature, in contrast to relation R2. It means that the receiver temperature can be higher,
and thereby the heat area accordingly smaller.

                                     4. THEORY CONFIRMED BY TEST

4.1 Walk-in freezers
Lillnord A/S in Denmark, expert in equipment for bakeries, has tested TFC with 159 walk-in freezers
of difference sizes and placed in difference climatic regions of Europe. The refrigerant is R404a and
the cooling effect is between 4-24kW. The most important quality of TFC, with regards to bakeries, is
the flooded evaporator, because it results in a high relative humidity in the storeroom. With
TFC-throttling dough is stored uncovered for weeks without deterioration in product quality - which
not possible with ordinary throttling-methods.

4.2 Chest freezers
Metalfrio Solutions A/S in Denmark, specialized in “plug-in” freezers, has tested TFC with chest
freezers placed in supermarkets all over the globe. The table below shows an energy-test, for two
identical freezers, with respectively ordinary capillary tube throttling and TFC.

Throttling          Compressor           Box             Run-time          Energy consumption Energy
method                                   Temp.                                                saving
Capillary tube      GP14CB               -18ºC           82,8 %            4,91 kwh/24 hours      -
TFC                 GP14CB               -18ºC           76,2 %            4,10 kwh/24 hours   16 %
TFC                 GP12CB               -18ºC           80,5 %            4,09kwh/24 hours    17 %

The upper two lines show, that TFC reduces - both energy consumption and run-time. The lower two
lines show that a 15% smaller compressor can do the same work. Smaller compressor means less
consumption of resources. The results are confirmed by large-scale field-tests and furthermore the
TFC-freezer is thoroughly tested at Bristol University.

4.3 Liquid coolers
Westfalia Surge in Denmark has tested TFC with 19 milk cooling tanks, of difference sizes and
placed in difference climatic regions of Europe. The volume varies between 400-20000 liter, and the
cooling effect between 7-40kW. The refrigerant is R404a, R134a or R22. The most important quality
                            International Congress of Refrigeration 2007, Beijing.
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of TFC, with regards to milk cooling, is the flooded evaporator, which prevent ice from building up
on the evaporator. Ice is very harmful to the milk quality.
To prove the energy saving, a milk-cooling tank is installed with both TEV and TFC. 200 liter of
water is cooled from 12ºC down to 4ºC, with either throttling-methods, and the main results appear in
the table below.

Water: 12ºC4ºC        TEV            TFC              Relative   Comments
Duration               43 minutes     37 minutes       86 %       TFC works faster
Energy consumption     1091 Wh        909 Wh           83 %       Energy saving : 17%

The first line shows that TFC speeds up the cooling process, which means that the compressor can be
smaller – and thereby reduces consumption of resources. The last line tells that TFC reduces energy
consumption with 17%.
See reference3 for details about these tests.

                                        5. CONCLUSION

The new TFC-throttling-technology causes a very efficient cooling cycle. The primary benefits are
flooded evaporator and superheated suction gas, and from these follow less consumption of energy
and other resources. These theoretical founded qualities are confirmed by long-term field-tests,
proving the technology to be general.
1
    WIPO Publication Number: WO/2001/073360

2
    WIPO Publication Number: WO/2005/028971

3
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