FRESH.

FRESHER.
LEYBOLD

Vacuum Cooling:
Theoretical background, experiences
and trends
Pierre Lantheaume
Global Business Development Manager Industrial Vacuum
Leybold
Coventry, 10th October 2018
Vacuum Cooling: Theoretical background, experiences and trends
Agenda

1 Introduction and theoretical background

2 Conventional cooling vs. vacuum cooling

3 Vacuum system sizing for vacuum cooling

5 Vacuum pump principles – advantages and challenges for vacuum cooling

4 Practical vacuum cooling examples:

- Vegetable cooling
- Grass cooling
- Bread and pastries cooling
5 Summary and trends

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Leybold Introduction

 Founded in 1850 - 168 years vacuum application experience

 Global presence in production sales and service – 1600
employees worldwide
 Headquartered in Cologne – Germany.
 Full liner in vacuum pumps and solutions from rough to ultra
high vacuum applications
 Long lasting experience in food applications

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Vacuum Cooling: Theoretical background, experiences and trends
Agenda

1 Introduction and theoretical background

2 Conventional cooling vs. vacuum cooling

3 Vacuum system sizing for vacuum cooling

5 Vacuum pump principles – advantages and challenges for vacuum cooling

4 Practical vacuum cooling examples:

- Vegetable cooling
- Grass cooling
- Bread and pastries cooling
5 Summary and trends

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Introduction and theoretical background

 Vacuum cooling is a based on “evaporation cooling”:

Evaporation (boiling) process requires energy, and this
energy is taken from the product which cools down.

 Some simple and very concrete examples of evaporation

cooling are present in our daily life!

 Vacuum cooling is a specific form of evaporation cooling.

The chamber pressure is reduced until the boiling point at
the specific product temperature is reached. The water
will start to evaporate, taking energy out of the product
and by this reducing its temperature. By control of the
pressure, the final temperature can be adjusted.

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 Introduction and theoretical background

 Cooling is a side effect of evaporation

 Consequently:
− You can’t cool a dry product!
− During drying process, product will loose a bit of weight.
− A very important parameter is the surface exposed to vacuum.

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What is required to make a vacuum cooler?

To build a vacuum cooler, you need:

 An airtight vacuum chamber, able to resist to 1

bar Δ P, to place the food products
 A vacuum pump or vacuum pumps systems to
evacuate the chamber
 A condenser to condense the water which
evaporates during the process
 A chiller to prepare the coolant for the condenser
 A system to control the whole process (PLC or
Electro-Mechanical) …….

That is all!
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Vacuum cooling principle video

With courtesy of Weber Cooling NL

Vacuum Cooling: Theoretical background, experiences and trends
Agenda

1 Introduction and theoretical background

2 Conventional cooling vs. vacuum cooling

3 Vacuum system sizing for vacuum cooling

5 Vacuum pump principles – advantages and challenges for vacuum cooling

4 Practical vacuum cooling examples:

- Vegetable cooling
- Grass cooling
- Bread and pastries cooling
5 Summary and trends

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Advantages of vacuum cooling

Product Quality Advantages

 Improved food safety:

No circulation of possibly contaminated air around the food product.
Gas stream is from inside to outside
Quick temperature reduction: Bacteria have no time to grow!

 Uniform temperature :
As the evaporation happens on all surfaces at the same, the cooling is
homogeneous (for leafy products)
 Longer shelf life: Consequence of the previous statements

 Low weight loss:

Vacuum cooling typically removes only 2 - 3% of water from the product being
cooled, way less than the moisture loss with normal cooling or forced air cooling

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Advantages of vacuum cooling

Process Advantages

 Faster process:
Reduction of the cooling time from several hours to few minutes, up to 30x faster
compared to traditional refrigeration cooling.
20 – 30° down to 3- 5°C can be done within 20 – 30 minutes for leafy vegetables
 Less energy consumption:
Vacuum cooling is the most energy efficient cooling method
 10 kWh per 1.000 kg to cool down from 23°C to 3 °C
 Forced air cooling requires 4 times more energy!

 Minimal space requirements:

Fast cooling enables repeated usage of equipment and smaller installations
Cold room cooling requires much bigger installations

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Vacuum Cooling: Theoretical background, experiences and trends
Agenda

1 Introduction and theoretical background

2 Conventional cooling vs. vacuum cooling

3 Vacuum system sizing for vacuum cooling

5 Vacuum pump principles – advantages and challenges for vacuum cooling

4 Practical vacuum cooling examples:

- Vegetable cooling
- Grass cooling
- Bread and pastries cooling
5 Summary and trends

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Vacuum System Sizing for Vacuum Cooling

Thermodynamic Basics

Energy is never lost (law of conservation of energy)

Qreleased = Qtaken

In Evaporation Cooling the heat energy released from the object is taken by evaporating water

1st step 2nd step

Can easily be calculated Will allow to calculate the mass of
knowing product mass water to extract and therefore the
and starting and desired vacuum flow required!
temperatures!

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Vacuum System Sizing for Vacuum Cooling
How much heat must be removed to cool food?

Qreleased = mfood x cp x ΔT
cp [kJ/(kg*K)] Moisture [%]
mfood = total mass of food to be cooled [kg]
Water 4.2 100
cp = specific heat capacity of the food [kJ/(kg x K)] Leafy Vegetables 3.9 90
Cooked Meats 3.5 74
ΔT = Temperature difference before/after cooling Baked Food 2.6 35
Source:
Rapid cooling of porous and moisture foods by using vacuum cooling technology
Trends in Food Science & Technology 12 (2001) 174-184

Example: Cooling of 1000 kg salad from 25°C to 5°C

Qreleased = 1000 kg x 3.9 kJ/(kg*K) x 20K = 78000 kJ

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Vacuum System Sizing for Vacuum Cooling
How much water must be evaporated to take this energy?

Qtaken = mwater x Δhv

Evaporation Heat of Water
Temperature [°C] Δhv [kJ/kg]
mwater = total mass of evaporated water [kg]
5 2490
Δhv = evaporation heat of water [kJ/kg] 10 2478
Average between 5°C and 25°C 15 2466
20 2454
25 2443
30 2431
Example: How much water must be evaporated to take the heat released from the salad?
mwater = 78000 kJ / 2466 kJ/kg = 31.6 kg

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Vacuum System Sizing for Vacuum Cooling
Which mass-flow must be handled by the vacuum system?

msteam = mwater / t
msteam = steam flow [kg/h]
mwater = total mass of evaporated water [kg]
t = total cooling time [h]

Example: How high is the steam flow during the cooling time?
Total process time: 30 min (of those 5 min for pump down)
25 min to evaporate 31.6 kg water  msteam = 31.6*60/25 = 76 kg/h

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Vacuum System Sizing for Vacuum Cooling
Which volume-flow must be handled by the vacuum system?

𝑽𝒎 𝒕𝒆𝒇𝒇 𝑷𝑵
𝑽𝒆𝒇𝒇 = msteam × × ×
𝑴 𝒕𝑵 𝑷𝒆𝒇𝒇

Veff = Effective steam volume flow [m³/h]

msteam = Steam flow per time [kg/h] tN = Norm temperature [273 K] PN = Norm pressure [1013 mbar]
Vm = Molar volume [22.4 Nm³/kmol] teff = Effective temperature Peff = Effective pressure
M = Molar mass of water [18 kg/kmol]

Example: How high is the steam volume flow over the 25 min cooling time?
Start: 25°C  Veff = 3299 m³/h @ 31.7 mbar
End: 5°C  Veff = 11188 m³/h @ 8.72 mbar Vapor Pressure of Water
Temperature [°C] Pv [mbar]
5 8.72
The vacuum system must be able to remove those flows…. 25 31.7

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Vacuum System Sizing for Vacuum Cooling
How big must be the vacuum system?
Handling of the vapour flow:
 Vapour flow at final pressure is huge and would require a big and expensive
mechanical vacuum system
− Example case: 11188 m³/h @ 8.72 mbar
 Using a condenser to trap vapour flow is more economical!
 Efficient condenser cooling requires glycol/water coolant (-6 to -10°C)

Condenser sizing:
 Rule of thumb: ~1 m² surface per 10 kg/h water vapor flow
− Example case: msteam = 76 kg/h  Condenser size: ~8-10 m²

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Vacuum System Sizing for Vacuum Cooling
How big must be the vacuum system?
Which gas flow is left behind the condenser?
 Coolant temperature -6°C to -10°C
 Assumption: Condenser gas outlet temp.: ≤ 0°C

~5
Gases leaving the condenser:

1. Air Leakage: ~5 kg/h (flange sealed 10 m³ chamber)

 124 m³/h (31,7 mbar, 0°C)
 449 m³/h (8,72 mbar, 0°C)
Source: Vacuum Technology for Chemical Engineering, Leybold AG

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 Vacuum System Sizing for Vacuum Cooling
How big must be the vacuum system?
Gases leaving the condenser:

2. Not condensing water vapour

Vapor Pressure of Water
− Water vapour pressure at 0°C: 6,1 mbar
Temperature [°C] Pv [mbar]
 6,1 mbar of total pressure at condenser exhaust are water vapour 0 6,10

 Point 1 (25°C): 124 m³/h air-flow at 31,7 mbar

Water flow of 6,1 mbar at 31,7 mbar = 19%
Air-flow of 124 m³/h = 81%
 Total flow @ 31,7 mbar = ~153 m³/h

 Point 2 (5°C): 449 m³/h air-flow at 8,72 mbar

Water flow of 6,1 mbar at 8,72 mbar = 70%
Air-flow of 449 m³/h = 30%
 Total flow @ 8,72 mbar = ~1500 m³/h

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Vacuum System Sizing for Vacuum Cooling
How big must be the vacuum system?
The vacuum system must also evacuate the chamber evacuation down to the final cooling pressure
𝑽𝒄𝒉𝒂𝒎𝒃𝒆𝒓 𝑷𝒔𝒕𝒂𝒓𝒕
𝑺𝒆𝒇𝒇 = × 𝐥𝐧( )
𝒕 𝑷𝒆𝒏𝒅

Seff = Effective volume flow [m³/h]

Vchamber = Chamber volume [m³] Pstart = Starting pressure [1013 mbar]
t = Evacuation time [h] Pend = Final pressure (e.g. 8.7 mbar for cooling to 5°C)

Example: Chamber volume: 10 m³ Evacuation time: 5 min

Seff = 570 m³/h

The vacuum system must have an average pumping speed from atm pressure to 8,7 mbar of
570 m³/h to achieve 5 minutes evacuation time.

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Vacuum System Sizing for Vacuum Cooling
How big must be the vacuum system?
Total requirements for example case:

The mechanical pumping system must be designed for following operation points:

For pump-down:
 >570 m³/h from 1013 mbar to 8.72 mbar

For vapour and leakage handling behind condenser:

 Veff = 153 m³/h @ 31.7 mbar
 Veff = 1500 m³/h @ 8.72 mbar

RUTA WAU2001 / DV650 A

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Vacuum Cooling: Theoretical background, experiences and trends
Agenda

1 Introduction and theoretical background

2 Conventional cooling vs. vacuum cooling

3 Vacuum system sizing for vacuum cooling

5 Vacuum pump principles – advantages and challenges for vacuum cooling

4 Practical vacuum cooling examples:

- Vegetable cooling
- Grass cooling
- Bread and pastries cooling
5 Summary and trends

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Pump Principles – Advantages and Disadvantages

Rotary-Vane Pumps (RVP), e.g. SOGEVAC

 RVP are reliable low-cost products offering good performance for industrial applications

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Pump Principles – Advantages and Disadvantages

Rotary-Vane Pumps (RVP), e.g. SOGEVAC

Advantages Disadvantages

 Good water vapor tolerance  Requires protection against particles (inlet filter
 Fully air-cooled design (easier for mobile systems) usage is advisable)
 Low noise level  Regular oil and exhaust demister exchanges
necessary
 Reliable operation
 Pump could be damaged if overloaded with
 Compact design
vapors
 Low investment

Cost effective standard technology for vacuum cooling!

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 Pump Principles – Advantages and Disadvantages

Screw-type Pumps, e.g. DRYVAC or SCREWLINE

 Dry compressing vacuum pumps are used whenever the
application causes high maintenance and service efforts on oil
sealed pumps
 Screw-pumps are the most up-to-date dry pump technology

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Pump Principles – Advantages and Disadvantages

Screw-type Pumps, e.g. DRYVAC or SCREWLINE

Advantages Disadvantages

 Highest process robustness  Moderate higher investment compared to oil-

 High handling performance for vapors and sealed pumps
particles  Most pump-versions require water-cooling
 Tolerance even for limited vapor overload
 Lowest noise level
 Lowest power consumption
 Extreme compact design
 Very low maintenance demand
 Build-in frequency converter for improved process
control

“Install and forget” solution for highest demand!

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 Pump Principles – Advantages and Disadvantages

Roots-blowers, e.g. RUVAC WA, WS or WH

Roots type vacuum pumps are used to boost up the pumping speed of the fore-vacuum pump at low
pressures and to decrease end-pressure.

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Pump Principles – Advantages and Disadvantages

Roots-blowers, e.g. RUVAC WA, WS or WH

Higher
Pumping
Speed

Lower End
Pressure

Roots pumps “boost” up the fore-vacuum pump!

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Pump Principles – Advantages and Disadvantages

Roots-blowers, e.g. RUVAC WA, WS or WH

Advantages Disadvantages

 Good handling performance for vapors and  Adds only significant pumping speed at pressures
particles <50 mbar
 Pump combinations are very energy efficient
 Very low maintenance demand
 Extreme compact design

Extreme compact, power saving & reliable!

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Vacuum Cooling: Theoretical background, experiences and trends
Agenda

1 Introduction and theoretical background

2 Conventional cooling vs. vacuum cooling

3 Vacuum system sizing for vacuum cooling

5 Vacuum pump principles – advantages and challenges for vacuum cooling

4 Practical vacuum cooling examples:

− Vegetable cooling
− Grass cooling
− Bread and pastries cooling
5 Summary and trends

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Process examples – Salad cooling
Typical vacuum cooling systems for salads

 Systems for leafy vegetables and flowers have very

similar design.
 Mobile (trailer) or fixed systems.
 Chamber is equipped with sliding or hinged door.
Depending on the level of automation, opening and
closing is made automatically.
 Huge chamber allows to cool 1 or several pallets per
cycle. For the biggest ones (not mobile), up to 20
Euro pallets can be loaded at the same time and
capacity can be more than 300 tons per day.

With courtesy
of Weber
Cooling NL
Process examples – Salad cooling
Typical cycle description
 Pallets with products are loaded in the vacuum chamber
 As soon as door is closed, vacuum system starts. Pressure
drops from 1000 mbar to 15/20 mbar in about 5 minutes. At that
pressure (as products are at around 20°C), water start to
evaporate and the cooling process starts.
 Over 15 to 20 minutes, pressure continues to drop down to 5/6
mbar which means the product has reached a temperature of
about 2°C.
 During the process, a condenser (with glycol water -6/-10°C)
traps most of the water vapour and protects the pumps.
 Pumping and cooling systems are stopped. Chamber is vented
gradually to atmospheric pressure within 2/3 minutes
 After the process, the salads are stored in a cool chamber and
can be kept 2/3 weeks w/o spoilage (longer than non vacuum-
cooled products).
Process examples – Salad cooling
Vacuum pump systems / Challenges
 As long as condenser works fine, process is relatively “friendly”
for the vacuum pumps as starting temperature is rather low and
water quantity to evaporate is quite limited.
 Some dirt particles or small vegetable parts.
 Typical vacuum system is made of several rotary vanes pumps:
− From 300 m³/h to 1000 m³/h
− Standard gas ballast
− Standard mineral oil
− Inlet filter with polyester cartridge
 Typical maintenance and service requirement :
− Oil and oil filter exchange: 2x/year
− Exhaust filter exchange: 1x/year
− Pump overhaul: 5 to 10 years

Top: 4 x Sogevac SV1200

Bottom: 2 x Sogevac SV1200 & 1 WAU2001
Process examples – Grass cooling

 Grass in football stadiums has not grown inside the

stadium, it is produced in special farms and harvested
in rolls.
 These rolls are often vacuum cooled! This allows the
grass to resist transportation before installation in the
stadium until next watering.
 Application is quite similar to vegetable cooling, but the
amount of water to extract to reach the desired
temperature is much higher due to the mass of the
product (including soil and mud).
 Inlet filter with polyester cartridge is compulsory.
 For oil sealed pump, maintenance and service will also
require more efforts.
Process examples – Bread and pastries

Why vacuum baking and cooling of pastries?

 Improved product appearance:
− Crispy crust (brittle, yet soft, hearable cracking)
− Loose crumb (fluffy crumb, uniform and elastic texture)
− Volume consistency (stable dimension)
− More enjoyment when eating
− Higher attractiveness in display Foto: Cetravac AG

 Advantages for the bakers:

− Faster cooling (from 0,5 – 2,5 h down to 2 – 6 min)
− Only marginal infestation with germs and mold spores
− Less complex (costly) packaging (No need for MAP)
− No cooling chain in supply necessary
−
Foto: Cetravac AG
Significant space saving (up to 90%)

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Process examples – Bread and pastries

Challenges for the vacuum pump and process control:

 Contamination of pump by outgassing baking ingredients  Robust dry screw pump with inlet filter
and vacuum transport of crumbs, seeds, etc.
 High vapor tolerance dry screw pump
 High and hot water vapor load in line with condenser
 Big diversity of bread and pastries, each type requires  Suction speed control by build-in
a different controlled pressure drop frequency converter
 Repeatable vacuum supply, independent from fluctuating  Stable vacuum is a design benefit of
ambient conditions or utilities as e.g. cooling water dry screw pumps

Particle Filter

Condenser

DRYVAC

37
Process examples – Bread and pastries

DRYVAC Qualification:
 Intensive qualification and long time test
at CETRAVAC and end-users
 In opposite to other pumps, DRYVAC has
proven to withstand the challenges of the
process Source: CETRAVAC

 >100 pumps in operation already

 DRYVAC as best vacuum pump

technology:
 Robust performance, even in harsh processes
 Compact pump design
 Integrated frequency drive
Source: CETRAVAC Source: CETRAVAC
 Diversity in electrical control Source: CETRAVAC /
backspiegel September/2017

DRYVAC DV 650 – oil free screw vacuum pump

38
Vacuum Cooling: Theoretical background, experiences and trends
Agenda

1 Introduction and theoretical background

2 Conventional cooling vs. vacuum cooling

3 Vacuum system sizing for vacuum cooling

5 Vacuum pump principles – advantages and challenges for vacuum cooling

4 Practical vacuum cooling examples:

− Vegetable cooling
− Grass cooling
− Bread and pastries cooling
5 Summary and trends

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 Summary and trends

 Vacuum cooling is a fast and energy efficient cooling methods for a wide range of food and non-
food products.

 Additionally, it increases food safety and extends product shelf life.

 Challenges for the vacuum pumps or vacuum systems depend a lot on the products nature.

 Oil sealed rotary vane pumps have proven to be effective on vegetable cooling but new and more
demanding processes can today be overcome by using dry technology !

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 FRESH.
FRESHER.
LEYBOLD

Thanks for your attention !

Any question ?