Energy usage of refrigeration plant is dependent on:

1. Heat loads on the cold store

2. Coefficient of performance (COP) of the refrigeration plant

This information pack covers the first issue

of minimising the heat loads on cold stores.

Other information packs cover optimising

refrigeration plant and use of high efficiency

components and equipment.

Heat loads on cold stores are covered under

the following headings:

1. Transmission

2. Infiltration

3. Fans

4. Defrosts

5. Food (including respiration

and packaging)

6. Lighting

7. Machinery

8. People

9. Radiation

10. Floor heating

Heat loads on cold stores can vary

considerably. The figure right shows heat

loads on 3 similar cold stores that were all

built around the same time on the same site.

Even though the purpose of each store was

almost identical, the heat loads on the stores

varied considerably. This demonstrates that

it is vital that the heat loads on each

0

10

20

30

40

50

60

70

80

Store 1 Store 2 Store 3

kW

Transmission

Evaporator fans

Defrosts

Lights

Product

Infiltration

Fork trucks

People

Calculating and minimising heat loads

individual store are assessed so that the

area(s) of highest heat gain can be targeted.

Heat loads can be calculated using the

information in this and other ICE-E information

packs or can be calculated using the free ICE-

E tools available to model the performance of

cold stores (http://www.khlim-

inet.be/drupalice/models).

Heat loads on

cold stores can

vary widely

It is therefore

important to

assess each

heat load to

determine

whether energy

savings can be

achieved

ICE-E

INFORMATION PACK

Transmission (insulation)

The heat load transmitted through the cold

store fabric can be calculated using the

following equation:

tUAQ

Where;

Q = heat flow rate (kW)

U = overall heat transfer coefficient (W.m-2.K)

obtained from:

k

x

hhU oi

111

Where:

hi = heat transfer coefficient on inside of room

(W.m-2.K)

ho = heat transfer coefficient on outside of

room (W.m-2.K)

x = thickness of insulation (m)

k = thermal conductivity of wall material

(W.m-1.K)

A = area of wall (m2)

t = air temperature difference inside and

outside the cold store (K)

Typical values for thermal conductivity

of cold store walls (W.m

-1.K):

Polyurethane 0.020

Polyisocyanate 0.020

Extruded polystyrene 0.024

Expanded polystyrene 0.036-0.038

Mineral wool 0.040

Modified phenolic 0.040

Most chilled cold stores will have wall panels

at least 100 mm thick whereas frozen stores

will have walls at least 150 mm thick.

Calculation of transmission across cold store

walls provides an idealised estimate of the

transmission heat load. In reality panels may

be damaged or degrade over time. Therefore it

is recommended that the integrity of walls is

checked on a regular basis by thermographic

scanning of the walls. This can highlight areas

where the insulation is damaged and should

be replaced/repaired.

Infiltration

Infiltration through entrances, gaps in doors

or through the walls can be a significant

heat load on stores. Moisture in air

infiltrated into the store can have a high

latent load on the evaporators and can also

freeze on floors or ceilings causing safety

problems. Reducing infiltration and

methods to calculate infiltration loads are

covered more fully in the ICE-E ‘Infiltration

through entrances’ information pack.

Although there are a number of methods

that can be used to assess infiltration the

Gosney and Olama model (Gosney and

Olama, 1975) is well tested and used by

ASHRAE in their refrigeration Handbook.

The calculation assumes that the air

temperature within the cold store remains

stable during door openings (this is

reasonable in a large store) and uses the

following equation:

tFmgHr

irhrhiAq

.1221.0

5.0

5.0

Where:

q = heat through infiltration (W)

A = Area of cold store door (m2)

hi = Enthalpy of ambient air (kJ.kg-1)

hr = Enthalpy of refrigerated air (kJ.kg-1)

ρr = Density of refrigerated air (kg.m-3)

calculated from = p / R T (where p =

pressure in Pa (assumed to be

100,000), T = temperature in K and R =

universal gas constant (287))

ρi = Density of ambient air (kg.m-3)

g = Acceleration due to gravity (9.81 m.s-2)

H = Height of cold store door (m)

Fm = (2/(1+(ρr/ρi)0.333))1.5

Options to reduce infiltration heat loads are

contained in the ICE-E ‘Infiltration through

entrances’ information pack and include:

1. Use of pedestrian doors

2. Better control of door openings

3. Automated doors

4. Rapid roll and fast opening doors

5. Air curtains

6. Automated cold store entrances

ICE-E INFO PACK

Transmission

Infiltration

Fans

Most fans in cold rooms are used on

evaporators to distribute the air around the

cold store. Occasionally additional fans are

used to distribute air in low air velocity areas

of a store (most usual in produce stores). The

energy used by the fan is dissipated into the

cold store as heat. Therefore the cold store

user is paying for the energy to operate the

fan plus the energy required to extract the

heat from fan (direct and indirect effects).

Information on energy used by fans can

either be obtained from manufactures data or

measured directly (recommended). In most

large cold stores the fans are operated using

3-phase induction motors. These are

relatively efficient compared to shaded pole

motors that may be used in smaller small cold

stores, retail display cabinets or small

commercial appliances. Electronically

Commutated (EC) motor technology fans are

available but are only around 10% more

efficient than 3-phase induction motor fans.

Therefore if fan load is high there may be

sufficient financial benefits to justify fitting EC

fans.

Alternatively in some stores fans can be

pulsed to just ensure that air is distributed to

all parts of the store and the required

temperature maintained. This is often a

strategy applied in produce stores where

there are minimal door openings and the heat

loads are mainly from transmission.

Defrosts

Like fans, defrosts can have a direct and an

indirect heat load effect on cold stores. This is

primarily related to electric and hot gas

defrost (active) systems and not off-cycle or

passive defrost systems. If appropriate,

passive systems should be used as they do

not add any heat load to the cold store.

However, for all frozen and some chilled

stores this is not possible.

In active defrosts systems energy is

consumed by the defrost and also adds heat

into the store if the defrost is not 100%

efficient (i.e. defrost energy is ‘wasted’ by

having to overheat parts of the

evaporator to ensure all ice is melted

across the whole evaporator). Often only

20-30% of the energy used for defrosting

evaporators is actually used to melt the

ice on the evaporator. To calculate the

effect defrost heaters have on cold store

heat loads an assessment needs to be

made of the defrost heater power (either

from manufacturers data or measured)

and the efficiency of the defrost

estimated. This could simply be

calculated by recording the volume of

water melted during a defrost and

comparing this to the energy required to

melt the ice.

Often evaporators are defrosted

unevenly and this can be caused by

defrosts heaters failing or by

inappropriate positioning of defrosts

heaters. Evaporators are also often over

defrosted to provide a level of ‘safety’ to

ensure no ice buildup. Often defrost

periods can be reduced, especially if

other latent loads (from infiltration, food

and packaging) are reduced.

Food (including respiration

and packaging)

Heat load from the food can be

calculated using the following equations:

For sensible heat:

tmcQ

For latent heat:

mL

Q

Where:

Q=heat extracted (kW)

m=mass of food (kg)

c=specific heat of food (kJ.kg-1.K-1)

t=temperature difference between food

and room air (K)

L=latent heat of freezing (kJ.kg-1)

∆t= reference time (s)

Although specific and latent heats for

ICE-E INFO PACK

Fans

Defrosts

ICE-E INFO PACK

individual food products can be obtained a

simplification is to base heat load

calculations on product water content and

to use the specific and latent heat

capacities for water or ice:

kJ.kg-1.K-1

Specific heat capacity of water 4.187

Specific heat capacity of ice 2.108

Latent heat of fusion 334

In produce stores respiration is also a heat

load on the store. Respiration of produce is

dependent primarily of storage temperature

with generally higher temperatures

generating higher respiration rates. In

some products such as potatoes

respiration can increase at temperatures

approaching the initial freezing point for the

product. In the case of potatoes minimum

respiration rate is achieved at

approximately 3°C.

Respiration rates for a range of products

are contained in the ICE-E tools available

to model the performance of cold stores

(http://www.khlim-

inet.be/drupalice/models).

Food packaging can adsorb or desorb

moisture. Paper based packaging is

hydroscopic and the moisture loss from

packaging, if frozen onto the evaporator,

can be a latent load on the cold store.

Packaging can also have a sensible heat

load. Although these can be calculated

their contribution to overall heat loads is

usually relatively small. Further information

can be found in Cleland (2012).

To reduce heat loads from food ideally food

should enter the cold store at a

temperature close to the temperature of the

cold store. This requires food to be rapidly

unloaded from lorries and the use of

docking bays or refrigerated loading bays.

In some cases food is chilled inside a cold

store. This is common with produce where

product is commonly taken directly from

the field and placed in a store. This means

that the store has 2 functions: one to chill

the product and the second to hold the

product at the reduced temperature. Ideally

product should be cooled separately

before entering the store but this is often

not practical. As rapid chilling of the

product is often required for food quality

reasons the store therefore needs to

operate efficiently to extract the initial high

product load and then to operate

efficiently in the storage mode. This can

only be achieved by using multiple

compressors or by using an inverter

controlled compressor.

Lighting

Lighting has a direct effect (through the

electrical energy used to power the lights)

and an indirect effect through the heat

generated from the lights that is a heat

load on the cold store. The heat load from

lighting is generally calculated as being

equal to the power of the lights multiplied

by a time factor related to usage of the

lights. The power used by each light is

simply obtained from the light fitting or

manufacturers data.

Lighting load can often be a high heat

load in cold stores. Depending on the

original lighting, the use of LED lighting

can save approximately 70-80% of the

heat load from lighting. Further reduction

in heat load can be achieved by the use of

sensor controlled LED lights to operate

only when operators are in the vicinity

(unlike most conventional lighting used in

cold stores LED lights can turn on and off

rapidly).

Machinery

Generally heat loads from machinery and

equipment can be obtained from

manufacturers’ data. The heat load from

most electric forklifts is between 2-6 kW,

dependent on size.

Reducing heat loads from machinery is

often difficult as the machinery is an

essential part of a cold store operation.

However, if taken into account in the

design stage machinery can be

minimised.

Lighting

Machinery

The work associated with this information pack has been carried out in accordance with the highest academic standards and reasonable endeavours have been made to achieve the degree of reliability and accuracy appropriate to work of this kind. However, the ICE-E project does not have control over the use to which the results of this work may be put by the Company and the Company will therefore be deemed to have satisfied itself in every respect as to the suitability and fitness of the work for any particular purpose or application. In no circumstances will the ICE-E project, its servants or agents accept liability however

caused arising from any error or inaccuracy in any operation, advice or report arising from this work, nor from any resulting damage, loss, expenses or claim. © ICE-E 2012

For more information, please contact: Judith Evans (j.a.evans@lsbu.ac.uk)

References

ASHRAE Refrigeration (2006),

Chapter 13.

Cleland, D.J. The effect of water

vapour on food refrigeration systems.

Proc. Inst. R 2001-12, 5-1.

Gosney W.B., Olama H.A.L. Heat and

enthalpy gains through cold room

doorways. Proc. Inst. of Refrig 1975;

72;31-41.

People

The sensible heat loads from pedestrian

access can be calculated the following

equation (from ASHRAE):

).6272( tNQ peosen

Where:

Qsen= sensible heat load (W)

Npeo = number of people

t=temperature of room (°C)

Latent heat from pedestrians can be

calculated from the following equation

senlat QQ .5.0

Where:

Qlat = latent heat (W)

Radiation

External cold stores experience

considerable fluctuation in temperature of

the external cold store surface. This can

cause stress in panels and eventual panel

breakdown in extreme cases. White

cladding is generally considered to be

best as it has low emissivity and reflects

radiant heat. The influence of radiant heat

can be taken into account in the

transmission heat load by increasing the

outside air temperature (ASHRAE).

Figures from ASRAE are shown below

where the figures are applied over a

24hour period when calculation wall heat

gain. When using the figures it should be

remembered that these are designed for

‘average’ conditions and so for high or low

solar radiation countries they may not be

totally suitable.

The above figures are part of the ICE-E

simple model. By using the ICE-E

complex model to calculate heat loads the

effect of location and position of the cold

store relative to the sun and the effect of

the solar passage throughout the day and

year can also be taken into account.

Floor heating

Floor heating is essential in stores where

temperature of the store is below 0°C as

water in the ground below the store may

eventually freeze and cause ‘frost heave’

(cracking and raising of the cold store

floor). Depending on the type of heating

system used the heat load can either be

estimated from the cold store designers’

data or real measurements.

Often cold store floors are over heated as

operators are concerned that they may

cause frost heave. The temperature of

cold store floors can be checked using IR

thermometers and reduced in temperature

if the floor is warmer than desired.

Example of solar radiation on cold store roof and effect on surface temperature throughout the day

People

Radiation

Floor heating

Wall (K)

Surface types

East South West Flat

Dark coloured:

Slate roof

Tar roof

Black paint

5 3 5 11

Medium coloured:

Unpainted wood

Brick

Red tile

Dark cement

Red, grey or green paint

4 3 4 9

Light coloured:

White stone

Light coloured cement

White paint

3 2 3 5

Allowance for effect of solar radiation (ASHRAE)