ice-e info pack 13 operation of doors and door protection

8/13/2019 ICE-E Info Pack 13 Operation of Doors and Door Protection

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Cold store door protection and infiltration

Infiltration of warmmoist air through

doorways into coldstorage rooms duringloading and unloadingcauses manyproblems to theoperators

These include:

increased costs for running [1] and

defrosting the refrigeration system

safety problems associated with the mist

formed in the doorway, as the cold air

mixes with the ambient air [2].

safety problems associated with ice

forming around the door opening, on the

floor and on the ceiling [Error!

Bookmark not defined.]

food quality, safety and weight loss

caused by temperature fluctuations.

In many food cold stores door openings

contribute a high percentage of the room heat

load. The exceptions to this are the more

automated cold stores where the entry is fully

air locked and product is often moved in and

out of the store using automatic picking and

placing systems. Air locks are dehumidified

this will reduce the latent load on the room

further reducing the heat load and also the

energy required to defrost any of the moisturefrozen onto the evaporator.

In most cold rooms food must enter via a main

door that is often opened for lengthy periods

during loading and unloading of the product.

Operation of doors

Doors can be operated in a number of

different ways, each having their own

advantages and disadvantages.

Manual

This is common on small cold stores, where

access is usually for pedestrians, not vehicles.

The sliding or hinged door will be opened and

closed manually. The quantity of infiltration will

depend on the user. A good user will open the

door and close it immediately when they are

inside. A bad user may keep the door open

whilst they are inside, only closing the door

when they leave the store. It is also possibleto leave the door open during a shift, this can

happen when there are many repeated entries

to the store. In this case, the way to reduce

infiltration is by proper management, making

sure users do not leave the door open any

longer than required.

Automatic doors

In larger cold stores, especially where fork lift

trucks are used, a door will be opened by

either activating a switch or sensor. The door

will then fully open for a set length of time

In many foodcold stores dooropeningscontribute a high

percentage ofthe room heatload

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before closing. The quantity of infiltration will

depend on the timer. To reduce infiltration, the

timer should be just long enough for the

person or fork lift truck to enter. It is not

uncommon for the timer to be set long enough

for the user to both enter, pick up or deliver the

product and leave. This is usually the case

with very slow opening/closing doors, as this

reduces the time in waiting for the door to

open.

Rapid roll and fast opening

Rapid roll and fast opening doors are

becoming more popular especially where there

is a high frequency of forklift movements [1].

Flexible doors can be opened much fasterthan solid doors can operate at about 1 m/s,

therefore taking only about 2 to 3 seconds to

fully open. Rapid roll doors are generally

designed to survive impact with a fork lift truck

and they also cause less damage to the truck

and driver. This allows them to be set with a

smaller safety margin reducing the time the

door is open with nothing passing through.

The simplest door will be a single layer of PVC

material (which may be transparent) for chilled

stores. For frozen stores, a double skinned

material with dehumidification between the

layers may be used.

Automated cold stores

Some cold stores are fully automated, using

an automated handling system to move

product in and out of the store. Automating the

movement of product in and out of the store

allows some of the variability and safety

concerns of manual operation to be

addressed, with the potential to reduce

infiltration.

Infiltration during door

opening

Due to the stack effect caused by the cold air

in the store being more dense than the warm

air outside, the cold air will try and escape

from the cold store, to be replaced by warm

moist air from the environment. Opening the

cold store door allows this exchange of air

(infiltration) to happen. During the opening of

the door the amount of heat entering the

door is many times higher than any other

heat load in the store and the ability of the

refrigeration system to remove it, and

generally leads to a rise in air temperature

inside the store.

For a small cold store with a small volume

of air, the effect is most noticed by a rapid

increase in air temperature, followed by

increases in food temperature. For a large

cold store, the volume of air is such that

temperatures only increases locally near

the door. However, the effect on energy

consumption on moisture on the

evaporators is more of an issue.

To design a cold store refrigeration system

demands calculating the rate of heat

entering the cold store during a door

opening, by estimating the proportion of

time the door will be open. To calculate the

heat load on the room during door

openings a number of analytical models

are available:

Brown and Solvason (1963) [3]

This model assumes a neutral level in the

doorway at which the pressure inside the

store is equal to the pressure outside. The

major assumption used, is that the height

of the neutral level is half the height of the

doorway. The model is expressed as;

H

b0.498-1

)-((gH)A0.343I

0.5

avg

oi0.5

Tamm (1966) [4]Tamm improved the Brown and Solvason

model by calculating the height of the

neutral level and using iinstead of avg.

The model is expressed as;

5.1

333.0

o

0.5

i

oi0.5

/+1

2)-((gH)A0.333I

i

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Typical door types:

Strip curtains

Automated doors

Rapid roll doors

Docking doors


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Fritzsche and Lilienblum (1968) [5]

Fritzsche and Lilienblum, who conducted

experiments using vane anemometers, added

a correction factor to Tamms equation. The

correction factor takes into account the

contraction of the flow, friction and thermal

effects. The correction factor given is

expressed as;

Kf,L = 0.48 + 0.004(To-Ti) (4)

The model is expressed as;

5.1

333.0

o

0.5

i

oi0.5

Lf,/+1

2)-((gH)AK0.333I

i

Gosney and Olama (1975) [6]Fritzsche and Lilienblums equation assumed

that the volume flow rate into and out of the

room were the same. This is only the case if

the air entering the room does not cool. If it

does cool then the volume flow rates will not

be the same, however, the mass of air in the

cold store will remain constant because both

the volume and density of air inside the room

remains constant.

Gosney and Olama provided an equation for

constant mass flow rate and by fitting

measurements with their model provided a

different coefficient. This means that i /o

has changed to

o /i in the following

equation;

5.1

333.0

i

0.5

i

oi0.5

/+1

2)-((gH)A0.221I

o

The Gosney and Olama model is well tested

and used by ASHRAE in their refrigeration

Handbook

Pham and Oliver (1983) [7]

Pham and Oliver conducted experiments on

air flow through cold store doors and

produced a factor of 0.68 which should be

applied to Tamms equation to fit their

experimental data, this new equation they

called Tamms modified equation and is

shown below;

5.1

333.0

o

0.5

i

oi0.5

/+1

2)-((gH)A0.226I

i

The relative merits of each model have been

tested by Foster et al [8] using a tracer gas

on a cold store.

All of these models assume that the cold

store is well sealed, in a good cold store

this should be the case, except if anotherdoor is opened. If another door is

opened, different mechanisms from the

stack effect take over, generally

producing a much higher infiltration. This

can be seen on a windy day, when one

door is open, there is little effect,

however, if another door or window is

opened at the back of the building a large

through draft can be created. In a cold

store this can very quickly remove the

cold air. Therefore multiple door

openings, especially on opposite sides of

the building should be avoided.

Infiltration when

door closed

Cold stores are not absolutely air tight,

even when the door is closed. Air can

leak through door seals and also the

pressure equalisation ports. This leakagerate is very small compared to when the

door is open, however, if the door is

opened seldom, then this door leakage

can be a significant part of the heat load.

It is more difficult to provide adequate

sealing of a rapid roll door when it is

closed. Chen et al [1] showed that the

infiltration through a closed sliding door

was 13 to 25% of that of a rapid-roll door.

He also showed that infiltration through a

closed door was a function of the lengthand condition of the seal.

Where there are frequent door openings,

a rapid-roll door has the advantage of

reducing the time the door is open.

Where there are in-frequent openings a

sliding door will reduce the infiltration

whilst the door is closed.

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The Gosney and

Olama model iswell tested and

used by

ASHRAE in their

refrigeration

Handbook


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Reducing infiltration

Obviously the longer a door is opened for,

and the larger the door area, the greater

the heat load on the room. Decreasing thedoor area available for air exchange can

reduce the heat gain through the door. This

is commonly achieved by use of strip or air

curtains that can have varying

effectiveness to air movement of up to

0.85-0.9.

The effectiveness of a strip curtain is

defined as the reduction in air flow through

the door caused by the strip curtain.

Therefore an effectiveness of 0.9 (or 90%)

means the flow is reduced by 90%

compared with that through an open door.

Strip curtainsalthough highly effective in

reducing infiltration have other

disadvantages. In a chilled cold store, they

will often have condensation forming on

them. If this condensation is not regularly

removed there is a risk of bacterial growth.

If unwrapped food is moved through the

strips, this can fall onto the food

contaminating it. For frozen stores, the

condensation will freeze; this causes the

strips to become less flexible. The strips

can easily become damaged because theyare brittle and the frozen strips can cause

damage to personnel as they move

backwards and forwards. Strips are also

difficult to see through especially when

they have condensation or ice, causing

potential hazards as fork lift drivers cannotsee through the entrance.

Air curtainsoffer the advantage of being

non intrusive whilst still reducing

infiltration. Air curtains provide a jet of air,

generally from the top, but can also be

from the side. Air curtains must be

correctly fitted to achieve high

effectiveness. A very badly fitted air

curtain can actually be worse than an

open door.

Other alternative strategies are to

reduce the amount of time the door is

opened.The use of automatic doors, high

speed doors, rapid roll doors and doors

with air locks all reduce door opening

times and therefore heat loads. If air locks

are dehumidified this will reduce the latent

load on the room further reducing the heat

load and also the energy required to

defrost any of the moisture frozen onto

the evaporator.

Some examples of reported effectiveness

values for infiltration protection devicesare shown in the table below [9].

Device Type Effectiveness

Strip curtain0.86 to 0.96 static

0.83 to 0.93 traffic

Dual 0.92 to 0.98

Air curtain Vertical non-recirculating

-0.44 to 0.83

-1.58 to 0.54 negativepressure

Vertical double non-recirculating 0.6 to 0.93

Vertical recirculating 0.36 to 0.8

Horizontal recirculating 0.59 to 0.82

Horizontal double recirculating 0.74

Combined air andstrip curtains

Horizontal air curtain 0.9 to 0.92

Fast doors 0.63 to 0.93


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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 andaccuracy 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 deemedto 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: Alan Foster ([email protected])

References

[1] Chen P, Cleland DJ, Lovatt SJ, Bassett MR. Air

infiltration into refrigerated stores through rapid-roll

doors. Proc. Of the 20th International Congress of

Refrigeration, Sydney, Australia. 1999, 19-24

September.

[2] Ligtenburg PJJH, Wijjfels DJ. Innovative air

curtains for frozen food stores. Proc. 16th Int. Cons.

Refrig. Storage, Transport and Distribution, 1995

August 20-25; The Hague, The Netherlands. p. 420-

437

[3] Brown WG, Solvason, KR. Natural convection in

openings through partitions-1, vertical partitions. Int.

J. Heat and Mass Transfer 1963;5;859-868.

[4] Tamm W. Kalterveluste durch

kuhlraumoffnungen. Kaltetechnik-Klimatisierung

1966;18;142-144.

[5] Fritzsche C, Lilienblum W. Neue messengun zur

bestimmung der kalterluste an kuhlraumturen.

Kaltetechnik-Klimatiserung 1968;20;279-286.

[6] Gosney WB, Olama HAL. Heat and enthalpy

gains through cold room doorways. Proc. Inst. of

Refrig 1975;72;31-41.

[7] Pham QT, Oliver, DW. Infiltration of air into cold

stores. Proc. 16th Int. Cons. Refrig. 1983;4;67-72.

[8] Foster, A.M., Swain, M.J., Barrett, R. and James,

S.J. Experimental verification of analytical and CFD

predictions of infiltration through cold store

entrances. International Journal of Refrigeration: 26:8: 918-925, 2003.

[9] Foster, A.M. Chapter 7. CFD optimization of air

movement through doorways in refrigerated rooms.

In Computational Fluid Dynamics in Food

NomenclatureA cross sectional area of entrance, m

2

b thickness of door frame, m

g acceleration due to gravity, 9.81 m s-2

Kf,L Correction factor, dimensionless

H height of entrance, m

I Infiltration rate, m3s

-1

t time, s

To, Ti temperature outside and inside coldsstore,

oC

V volume of air within the room, m3

Greek letters

i, o, avg density inside and outside cold

store and average, kg m

-3

An alternative strategy to reduce door

opening times and open area is to assess

the way in which the room is used. In

many rooms the door openings for

product entry are relatively low in

comparison to the number of door

openings for pedestrians only. If this is the

case the use of small pedestrian doors

can save considerable amount of heat

gain on the room.

The way cold store doors are maintained

and managed has a large impact on

energy use. Damaged door seals, poorly

maintained door protection and poor door

discipline all add to energy use and can

easily be improved by regular

maintenance and operator training. An

example of the reductions in heat that can

be achieved in 3 typical cold stores are

shown in the figure below.

In this case there were significant

numbers of door openings for pedestrians

and so fitting pedestrian doors had a

significant impact on reducing heat loads.

Further reductions in heat load were

achieved by fitting strip curtains. The

impact of fitting rapid roll doors was less

significant and in this case would be

primarily for convenience rather than heat

load reduction.

In all cases an assessment should be

made of the situation at a particular cold

store as the best options on door

protection may vary from cold store to

cold store.

Insulation

When the door is closed, heat gain is

through infiltration through the door seals

and conduction through the fabric of the

door. Conduction can be minimised by

using insulated panels for the doors of the

same thickness and the cold store walls.

Flexible doors do not provide this level of

insulation and do not seal as well as solid

doors, increasing heat gain through both

conduction and infiltration when closed.

Chilled or frozen

You should consider whether the cold

store is chilled or frozen when choosing a

door. Doors for freezers will generally

have thicker insulation and a heater tape

around the dear seal to stop it freezing

solid. Rapid roll doors may need to

incorporate some method to keep the

door flexible; this will have a heat input.

Chilled doors may also need heaters to

stop condensation which can form around

the seals and lead to hygiene concerns.

PED=Pedestrian doors

ice-e info pack 13 operation of doors and door protection

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