Kylteknik (”KYL”)

Refrigeration course # 424503.0

4.

v. 2014

Refrigeration process comparison; process equipment Ron Zevenhoven Åbo Akademi University Themal and Flow Engineering Laboratory / Värme- och strömningsteknik tel. 3223 ; [email protected]

Åbo Akademi Univ - Thermal and Flow Engineering

Piispankatu 8, 20500 Turku

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ÅA 424503 Refrigeration / Kylteknik

4.1 Refrigeration process comparison

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Process comparison /1  The four most important refrigeration processes are (1) vapour compression (v-c), (2) absorption, (3) gas expansion and (4) thermo-electric (T-E) processes  The processes can almost all be separated in a high pressure part and a low pressure part (or high voltage and low voltage part for T-E) see ---- boundaries in the figures below

 The energy input occurs at the point where the pressure of the refrigerant gas is increased Pictures: S90

Vapour-compression

Absorption

Åbo Akademi Univ - Thermal and Flow Engineering

Absorption + heat exchange Piispankatu 8, 20500 Turku

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 In the compressor, work is converted into enthalpy, in an absorption process heat is converted into enthalpy  Compressor (or absorber + regenerator) efficiency is very important, as is also the expansion device  For a given refigeration situation (capacity, temperatures) a final choice for a given system is based on costs, related to – investment and space, – energy use (efficiency !!), – cooling water needs, – maintenance needs, reliability, ease of operation 12.11.2014

Åbo Akademi Univ - Thermal and Flow Engineering Piispankatu 8, 20500 Turku

Picture: http://138.100.110.21/bk3/c18/Refrigeration.htm#_Toc101323318

Process comparison /2

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Vapour-compression systems ←Vaporiser Temperature°C

Capacity → 100 W

1 MW

10 kW

10°C 1-stage open reciprocal

(Semi-)hermetic reciprocating -20

2-stage open reciprocal

-50

2-stage cascade open reciprocal

1-stage open screw 2-stage open screw

2-stage cascade open screw

Centrifugal / Turbo compressors

-80

Typical equipment and compressor type ranges (source: S90) 12.11.2014

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Absorption systems ←Vaporiser Temperature°C

Capacity → 100 W

1 MW

10 kW

10°C

LiBr / H2O NH3 / H2O + H2 NH3 / H2O 1-stage

-20

-50

NH3 / H2O 2-stage

-80

Typical ranges for absorption processes (source: S90) 12.11.2014

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Process comparison /3  Vapour-compression and (2-stage) NH3/H2O absorption refrigerators can cool down to - 60 ~ - 80°C  For lower temperatures - 80 ~ - 200°C: – gas expansion devices based on reversed gas cycles (regenerative cooling) – Linde-type processes (Joule-Thomson effect) Pictures: S90

Gas-expansion

Gas expansion + heat exchange

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Linde process

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 Thermo-electric cooling gives relatively small capacity, low efficiency, high investment costs (special materials!), no moving parts, small space requirements: used in special applications  Typical ranges -35 ~ +5°C; cooling capacity < 100 W

Thermo-electric refrigerator

Picture: http://www.williamson-labs.com/images/tec-plot.gif

Process comparison /4

Picture: S90 Åbo Akademi Univ - Thermal and Flow Engineering

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ÅA 424503 Refrigeration / Kylteknik

4.2 Compressors displacement compressors, dynamic compressors

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Compressor types /1  The tasks of the compressor are to (1) pump the refrigerant vapor from the evaporator and (2) increase the pressure (and temperature) Compressors can be divided into two main categories: 1. Displacement compressors - Reciprocating - Rotary vane, wankel - Screw, Scroll 2. Dynamic compressors - Turbine - Centrifugal Picture: D03 Åbo Akademi Univ - Thermal and Flow Engineering

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Compressor types /2  In general, displacement compressors are used for larger pressure increases, while dynamic compressors are used for larger flow rates  In refrigeration applications: – Reciprocating compressors (sv: kolvkompressor) and screw compressors (sv: skruvkompressor) are used for 0.1 - 300 kW; – For the range up to 10 kW also spiral or scroll compressors (sv: spiralkompressor) are used. – Centrifugal compressors (sv: centrifugal kompressor) are used for large capacities (300 kW – 20 MW), including turbo compressors (sv: turbokompressor) for air conditioning and water cooling applications – In the range 10 kW – 500 kW, screw compressors are replacing reciprocating compressors: less moving parts, more reliable, less maintenance, lower risk for refrigerant loss Åbo Akademi Univ - Thermal and Flow Engineering

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 Smaller compressors (up to a few kW) are usually of the hermetic type, with compressor and motor inside a sealed housing  The refrigerant from the evaporator and lubricant oil contact the mechanical components  Electic resistance heating reduces efficiency  Used in household refrigerators, freezers and air-conditioning  Typical refrigerant: R-134a, R600a and other hydrocarbons

Picture: http://138.100.110.21/bk3/c18/Refrigeration.htm#_Toc101323318

Hermetic compressors

Air-cooled condensing unit using a hermetic compressor Picture: D03

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Semi-hermetic, open compressors  Larger than a few kW, refrigeration compressors are of the semi-hermetic (up to ~ 300 kW) or open (up to a few MW) type  In the semi-hermetic type the motor and compressor are two sections in the same housing but the refrigerant does not flow over the compressor rotor. No refrigerant leakage problem.  In the open type motor and compressor are separated, the crankshaft extends into the compressor; special seals prevent refrigerant (and air) leakage.  Typical refrigerant: all kinds, especially the open types are flexible

Single-stage semi-hermetic reciprocating compressor

Pictures: D03

Open type reciprocating compressor Åbo Akademi Univ - Thermal and Flow Engineering Piispankatu 8, 20500 Turku

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 Typical range up to ~ 300 kW  Often built for a certain (type of) refrigerant  Limitation ranges for Tmin ↔ Tmax, pmin ↔ pmax  Contact between lubrication oil and refrigerant can give foaming problems  Often with several cylinders; for z cylinders with displacement volume (sv: slagvolym) 3 Vdispl (m ) and rotation speed n (1/s) the total displacement volume stream equals n·z·Vdisp (m3/s) 12.11.2014

Åbo Akademi Univ - Thermal and Flow Engineering Piispankatu 8, 20500 Turku

picture: http://www.mmrefrigeration.com/mmsite/images/reciprocating/recip3.gif

Reciprocating compressors /1

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Picture: http://www.aircompressor.org/img/technology/recip_compressor.gif

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Reciprocating compressors /2  Important for compressor efficiency is the volumetric efficiency ηc,vol, which is based on the displacement volume Vdispl, and the specific volume of the refrigerant on entrance (suction) and discharge vin and vout: η c ,vol

V V  R    clear Vdispl Vdispl

 v in      v out 

Compressio n ratio CR 

Vclear  Vdispl

for refrigerant suction volume VR and clearance volume Vclear (sv: skadligt rum) (= cylinder volume minus displacement  volume)  After discharge, Vclear is filled with (hot) gas at p = pdischarge, taking up a larger volume when p = psuction → less ”new” vapour can be taken in

Vclear

For motor power input P and isentropic compression (sin=sout) the compressor efficiency ηc is c isentrop 

  (hout (sin  s out )  hin ) m P Picture: Ö96

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 Screw compressors were earlier used for lower temperatures (down to -40 .. -50°C) than reciprocating compressors but are now replacing reciprocating compressors  Benefits are – Efficiency and reliability – Costs – Less moving parts – Less vibration – Less refrigerant loss !!

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Picture: D03

Large capacity double-screw compressor

Åbo Akademi Univ - Thermal and Flow Engineering Piispankatu 8, 20500 Turku

Hermetic screw compressor

Picture: http://www.hartfordcompressors.com/vertical_screw_compressors.htm

Screw compressors

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Rotary vane, scroll compressors  Small systems make use of rotary vane compressors (compression ratios up to ~7)  Simple, relatively low speeds, high compression ratio, low maintenance  Scroll compressors are recent developments, that have good part-load performance and operation characteristics, and high efficiency

↑ Rotary vane compressor ↓ Hermetic scroll compressor

Scroll compressor: http://www.youtube.com/watch?v=f_6xolDoqs0 12.11.2014

Pictures: D03

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 Centrifugal compressors are used for larger refrigeration systems, typically 300 kW → 20 MW cooling  Usually 2 or more stages  Pressures typically up to 40 bar  Evaporator temperatures as low as 100°C possible  Adiabatic efficiencies up to 80%  Typical refrigerants R-123, R-134a, R-22

Centrifugal compressor for CO2, NH3 (1920s)

 Special type: turbo compressor, when suction pressures and volumes are high, compression ratios and TH-TL not too high (otherwise multi-stage!)

Picture: http://www.friotherm.com/about_us_histo_7828.htm

Centrifugal compressors

Picture: D03 12.11.2014

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Compressor efficiency /1 (Ö96)  For an open compressor the efficiency ηc is the product of (1) mechanical efficiency, (2) motor-to-compressor efficiency and (3) power-to-motor efficiency. .  For power input P, mass stream m:

 c real

  (h out  h in ) m  P Pictures: Ö96

 The fraction (1-ηc real)·P gives heat loss to the surroundings, but this is also part of the high-temperature heat QH = QL + P  For a isentropic compression

c isentrop 

  (hout (sin  s out )  hin ) m P

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Compressor efficiency /2 (Ö96)  The total process efficiency ηt is given by P = Pcarnot / ηt  This gives:

Picture: Ö96

QL TL  COP  t  COPcarnot  t  P TH  TL t 

c real c isentropic



where

COPprocess with isentropic compression COPcarnot

• The diagram gives some efficiency data from commercial vapourcompression refrigerators (T1 = TH, T2 = TL, data from 1976) 12.11.2014

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ÅA 424503 Refrigeration / Kylteknik

4.3 Condensers water-cooled, air-cooled, evaporative

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Water-cooled condensers  Most common water-cooled condensers are shell and tube type condensers with water (coolant) through the tubes and refrigerant through the shell  Condensed refrigerant accumulates in the lower part of the shell: horizontal construction is important !  Cooling water must be available, a water treatment plant will be needed  Freeze protection may be necessary.

Picture: D03 Åbo Akademi Univ - Thermal and Flow Engineering

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Picture: http://www.ref.or.kr/upload/image_BUSUNG_WaterCooledRemoteCondenser_BCW_BCWD.gif

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Air-cooled condenser  Air-cooled condensers are based on convective heat transfer caused by an outside air flow that is drawn past the refrigerant tube  Cheap, no water handling problems  Low investment and maintenance costs  High condensation temperature  Long tubing (requires much refrigerant!)  Much fan power needed per kW cooling  Noise  Large units require multiple systems Åbo Akademi Univ - Thermal and Flow Engineering

Picture: http://www.gea.co.za/acc.htm

Picture: D03

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 Evaporative coolers are based on water cooling, allowing for some of the cooling water to be evaporated and transported away with moving air  Efficiency depends on – Air humidity (wet bulb temperature)

– Air flow – Air/water contacting  Water treatment is necessary  Small compared to air-cooling; some pump power needed  High use and consumption of water (to avoid problems with dissolved solids)

↑ Evaporative cooler ↑ Cooling tower Pictures: D03

Picture: http://www.trj-inc.com/content/indusRefrig.html

Evaporative condenser

To be continued for cooling towers Åbo Akademi Univ - Thermal and Flow Engineering

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Condenser heat transfer (Ö96)  Typical values for the overall heat transfer coefficient refrigerant ↔ air or water coolant; for NH3, R-12, R-22 : U = 15-30 W/(m2·K) with air, forced convection U = 500-1200 W/(m2·K) with water, shell and plate heat exchanger U = 100-200 W/(m2·K) with evaporating water  For the heat transfer coefficient α (W/m2·K) at the side of the condensating refrigerant the following can be used for tube outside diameter D (m), inside diameter d (m), tube length L (m) and heat flux q (W/m2): α = α0·(D·q)-0.33 for condensation on the outside of horizontal tubes, and also vertical surfaces with height D α0 ≈ 13 W/m2·K for R-12, R-22; ≈ 80 for NH3 and ≈ 180 for H2O α = α0·(q)-0.50 ·(L·d)-0.14 for condensation inside horizontal tubes α0 ≈ 9 W/m2·K for R-12, R-22; ≈ 70 for NH3 and ≈ 150 for H2O Åbo Akademi Univ - Thermal and Flow Engineering

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4.4 Evaporators

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Evaporators

 The evaporator gives the cooling effect based on vaporisation of the refrigerant  Can be based on full evaporation (a) in Figure → or with partial evaporation with siphon (b) or pump (c) driven circulation, evaporating 20-50% of the liquid  Usually shell and tube heat exchangers where refigerant vaporises in the tubes (”dry”), otherwise the refrigerant can vaporise on the outside of the tubes ”(flooded”)  Contact with air will require defrosting arrangements

Åbo Akademi Univ - Thermal and Flow Engineering

Picture: http://www.vahterus.com/refrig_case1.htm

Picture: Ö96

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Evaporator heat transfer (Ö96)  Typical values for the overall heat transfer coefficient refrigerant ↔ air or water coolant ; for NH3, R-12, R-22 U = 3-8 W/(m2· K) with air, natural convection U = 12-25 W/(m2· K) with air, forced convection 2 U = 150-500 W/(m · K) with water, shell and plate heat exchanger  For the heat transfer coefficient α (W/m2· K) at the side of the evaporating refrigerant the following can be used for tube outside diameter D (m), inside diameter d (m), tube length L (m) and heat flux q (W/m2): α = α0· (q)0.5 for evaporation on outside of horizontal tubes / plates α0 ≈ 15 W/m2· K for R-12, R-22; ≈ 30 for NH3 and ≈ 30 for H2O α = α0· (q)0.8 · (L)0.4· (d)-1.0 for evaporation inside horizontal tubes α0 ≈ 0.01 W/m2· K for R-12, R-22; ≈ 0.02 for NH3 ; ≈ 0.02 for H2O Åbo Akademi Univ - Thermal and Flow Engineering

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ÅA 424503 Refrigeration / Kylteknik

4.5 Throttling devices, additional devices, control

Åbo Akademi Univ - Thermal and Flow Engineering

Piispankatu 8, 20500 Turku

12.11.2014

Picture http://www.n-c.com/Images/ImageManager/heat.jpg Picture: http://courses.washington.edu/cheme260/images/homework/hw5-p5.gif

Throttling devices  A controlable isenthalpic pressure reduction of a saturated liquid is accomplished by – Thermostatic expansion valves (controlled by the temperature of super-heated vapour from the evaporator) – Constant pressure expansion valves (keeping pressure constant at the outlet) – Float valves (controlling the liquid flow to the evaporator, on the high pressure (”high-side float”) or low pressure (”low-side float”) side of the throttling device) – Capillary tubes, (used in small < 30 kW hermetic compressor-based systems) reduce pressure in a narrow (0.4-3 mm), long (1.5-5 m) copper tube) Åbo Akademi Univ - Thermal and Flow Engineering

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Additional necessary devices         

Tubing Insulation material Fans Defrosting / purging Accumulator (avoiding

liquid into compressor) Receiver (preventing condenser overflow) Oil separators (remove oil from refrigerant) Strainers (removes dirt from refrigerant) Driers (remove moisture from refrigerant)

↑ A v-c refrigerator with control devices ↓ Air cooler evaporators (a) room type (b) industrial

 Control system  Power or heat Åbo Akademi Univ - Thermal and Flow Engineering

Pictures: D03 Piispankatu 8, 20500 Turku

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     

A11: R. C. Arora ”Refrigeration and air conditioning”, 2nd. Ed. PHI Learning Private Limited , New Delhi (2011) D03: İ. Dinçer “Refrigeration systems and applications” Wiley (2003) S90: A.L. Stolk ”Koudetechniek A1”, Delft University of Technology (1990) TW00: A.R. Trott, T.C. Welsh ”Refrigeration and Air-Conditioning” 3rd Ed. Butterworths-Heineman (2000) Ö96: G. Öhman ”Kylteknik”, Åbo Akademi University (1996) See also: Martinez, I. ”Lectures on Thermodynamics” – lecture 18 (English or Spanish) http://webserver.dmt.upm.es/~isidoro/bk3/index.html updated and based on “Termodinámica básica y aplicada", Ed. Dossat, Madrid (1992) ISBN 84-237-0810-1

Åbo Akademi Univ - Thermal and Flow Engineering

Piispankatu 8, 20500 Turku

12.11.2014

Pictures: http://www.ftrefrigeration.co.uk/process-cooling.php

Sources #4

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