Mark SwainFood Refrigeration & Process Engineering Research Centre, University of Bristol

Energy savings in primary food cooling

National Motorcycle Museum3rd April 2009

Introduction - primary cooling

• Meat – primary carcass chilling• Dairy – milk cooling on the farm• Potatoes – bulk cooling and storage• Blast freezing – potato products

Energy – where used?

• Compressor – electric motor drive• Evaporator fans• Condenser fans• Defrost heaters – heating elements• Lighting• Pumps• Auxiliary fans

Energy saving – refrigeration

Only 3 ways to cut refrigeration energy use:

• Minimize heat load

• Minimize auxiliary electrical consumption

• Increase efficiency of refrigeration equip.

Energy saving – heat load

Made up of:• Product load – heat from food• Infiltration load – air leaks/open doors• Transmission load – through fabric• Heat from other sources in room –

• fan/pump motors• defrost heaters• racking/trolleys• people

Energy use in meat chilling

• Refrigeration accounts for 45 - 90% of electrical consumption in abattoirs

• Up to 80% is for primary carcass chilling

Energy use in meat chilling

• Beef primary chilling UK• Minimum 19 kWh/tonne• Average 32 kWh/tonne• Maximum 52 kWh/tonne

• Pork primary chilling UK• Minimum 25 kWh/tonne• Average 43 kWh/tonne• Maximum 72 kWh/tonne

Note - Large range

Energy saving – heat load

Made up of:• Product load – heat from carcass

Little can be done unless process changed? • Infiltration load – air leaks/open doors

Door discipline/air curtains?• Transmission load – through fabric

Set by design specification?• Heat from other sources in room

Evaporator fans and defrost optimisation?

Air infiltration - solutions

• Door maintenance - mechanisms, seals• Strip curtains (up to 90% effective)• Improved door discipline• Door monitoring and alarms• Automatic/rapid doors• Vestibules/air lock• Air curtains (up to 80%)• Shut doors – 100% effective

Infiltration – air curtain

Thermoscreens Ltd

Up to 80% as effective as door

Heat load from other sources

Evaporator fans:• Electric motors (counts x2)• Fan power increases with cube of velocity• Fan speed controllers – reduce speed• High efficiency fan motors/fan bladesDefrost heaters:• Minimize defrost cycles (counts x2)• Defrost on demand

Energy saving – refrigeration equipment

Increase efficiency of compressor etc:

Coefficient of Performance

COP = Refrigeration effect (kW) .

Power input to compressor (kW)

Condenser

Evaporator

Expansion valve Compressor

Refrigeration circuit components

Energy saving - evaporator

• Increase evaporating temperature(1°C = 1 to 4% saving)

• Large heat transfer surfaces• Efficient fan motors and control• Keep clean

Energy saving - condenser

• Decrease condensing temperature(1°C = 1 to 4% saving)

• Large heat transfer surfaces• Efficient fan motors and control• Keep clean

Variable load profile – meat chilling

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26-Feb 27-Feb 28-Feb 01-Mar 02-Mar 03-Mar 04-Mar 05-Mar

Energy saving - compressor

• Match capacity – peak load / average load• Compressors less efficient at part load• Variable speed drives• Multi-compressor pack

Milk cooling – raw milk on farm

• Leaves udder at approx 37°C• Cooled to below 4°C on farm• Penalty for higher temperature• Collected by tanker daily or every other

day

Milk cooling – ice bank cooler

• Stirred bulk storage tank• Chilled water spray• Ice builds on coil• Remote refrigeration unit

Source: DeLaval

Milk cooling – DX system

Source: DeLaval

• Stirred bulk storage tank• Integral evaporator• More efficient than ice bank• Remote refrigeration unit

Milk cooling – pre-cooling

Source: DeLaval

• Stirred bulk storage tank• Plate heat exchanger• Ice builder• Mains / well water + chilled• Remote refrigeration unit

FEC Services

Milk cooling – energy saving

• Ice bank – least efficient• DX system – better• Pre-cooling + DX system – best

• Use scroll compressor – 15 to 20% more efficient than reciprocating compressors

Potato cooling/storage - bulk raw

• Main issues:• Old stores - insulation and air infiltration

FEC Services

Potato cooling/storage - bulk raw

• New case study by Potato Council Ltd• Store X purpose-built potato store

53.2 kWh/tonne• Store Y converted grain store

116.3 kWh/tonne – closed & modified

Store X Store Y

Potato cooling/storage - bulk raw

• Main issues:Newer stores – compressor/evaporator fans

Potato cooling/storage - bulk raw

FEC Services

Blast freezing - potato products

• Potato products emerge from fryer at 80°C

• Need to be frozen to -12°C before packaging

• Process rate 4.5 tonnes/h• Spiral freezer incapable of

extracting initial heat load • Initial moisture loss

freezes on coils

Blast freezing - ambient cooling

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Start 5 minutes 10 minutes

Tem

pera

ture

(°C

)

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5 minutes 10 minutes

Ener

gy lo

ss (k

J/kg

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SensibleEvaporativeTotal

31% 44%of heat load

20°C, 1 m/s

Blast freezing - ambient cooling

• 5 minutes of ambient cooling• Removes 562,500 kJ of heat energy

from 4.5 tonnes of product per hour• Stops 60 kg/h of water freezing on coils• Insignificant increase in freezing time• Increases energy efficiency of process

Energy saving measures - summary

• Reduce infiltration• Ensure good insulation• Increase evaporating temperatures• Reduce condensing temperatures• Keep defrost cycles to a minimum• Reduce fan speeds once heat removed• Use high efficiency electric drives• Proper maintenance schedules• Monitor and target energy consumption

SPECIFY & DESIGN PROCESS/SYSTEM CORRECTLY