Heat Load & Air Circulation NHB - Training 15.10.2013 - Delhi - - PowerPoint PPT Presentation

Heat Load & Air Circulation

NHB - Training 15.10.2013 - Delhi Himanshu Sheth

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REFREGERATION CYCLE FOR POTATO COLD STORAGE

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Layout : Four Chamber Potato Cold Store

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OLD – BUNKER SYSTEM

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COIL ARRANGMENT - BUNKER

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NEW COLD STORAGE WITH AIR UNITS

Slide 7

Fruit and vegetables

keep on living, even during storage!

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Slide 8

Metabolism of vegetables and fruit after harvesting

Oxygen Respiratory heat Carbon dioxide Water C2H4 Transpiration

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Slide 9

Respiratory heat

• Depends on the product
• Depends on the ambient temperature

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Slide 10

Heat generation of various products

Heat generation (kcal/t in 24 h) depending on the storage temperature (°C)

5 10 15 20

Potatoes

380 325 400 575 700

Apples

165 355 530 885 1,200

Tomatoes

320 425 750 1,450 1,875

White cabbage

340 475 700 1,125 2,250

Cucumbers

405 600 1,150 2,225 3,375

Asparagus

1,275 1,650 3,150 5,000 6,750

Chinese cabbage

1,100 1,680 3,480 5,600 8,500

Brussels sprouts

1,200 2,400 4,075 5,630 10,400

Mushrooms

2,400 3,200 5,100 9,800 12,800

Endives

2,480 4,000 5,450 7,300 11,000

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Slide 11

Quality requirements

• It is essential to reduce respiration in order to

maintain a high quality.

• Once harvested, fruit and vegetables must

therefore be stored as soon as possible at optimum temperature and humidity levels for the respective product.

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Slide 12

Two types of storage

• Short-term storage

During short-term storage, several products may be kept together at comparatively high temperatures.

• Long-term storage

Long-term storage involves particularly high demands with regard to the air conditioning of the refrigeration system. Both the temperature and the air humidity must be adapted to the respective products.

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Slide 13

Empirical values

• The smaller the temperature difference, the lower the dehydration

rate and the higher the air humidity. A temperature difference of 4 º C can only be achieved with an electronic expansion valve.

• Caution: Make sure that the cooling capacity is designed to match

this low temperature difference !!!

cold room temperature evaporating temperature temperature difference

• rel. air humidity

measured values +1 °C −7 °C 8 °C 85-92 % +1 °C −4 °C 5 °C 92-94 % +1 °C −3 °C 4 °C 97-98 %

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Slide 14

Heat to be extracted by the evaporator

Field heat and packaging heat

The product is stored at field temperature. The heat released is called field heat. The same applies to the packaging material.

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Slide 15

Heat to be extracted by the evaporator

Respiratory heat

So-called respiration or dessimilation is associated with any living product and releases respiratory heat. Rules of thumb: – Lots of heat at high temperatures – Little heat at low temperatures – More or less heat depending

• n the type of product

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Slide 16

Heat to be extracted by the evaporator

Air exchange rate

During air exchange, the entire air content of an empty cold room is exchanged and replaced by fresh air, an effect that is mainly caused by the

• pening of the cold room door.

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Slide 17

Heat to be extracted by the evaporator

Ventilation heat

During ventilation, a certain air volume in the room is exchanged and replaced by fresh air from outside, which also introduces a certain amount of heat. At the same time, a certain amount of ventilation is to be expected if the cold room is not absolutely tight (e.g., if a door does not close tightly, or if one of the walls is permeable). Important: Permeability means heat supply!

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Slide 18

Heat to be extracted by the evaportor

Conduction heat

entering through walls, ceilings, and

• floors. The difference between the inside

and outside temperature causes a certain heat flow into the cold room.

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Slide 19

Heat to be extracted by the evaporator

Door heat

The door is opened regularly to load products into the cooling cell, causing cold air to escape from the cold room and warm air to enter.

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Slide 20

Heat to be extracted by the evaporator

Fan motor heat

The electrical power of the evaporator fan is converted into heat and must be removed.

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Slide 21

Heat to be extracted by the evaporator

Personnel and fork lift heat

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Slide 22

Heat to be extractor by the evaporator

Lighting

Lights should be left on only as long as there are people in the cold room (e.g., during loading and unloading).

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Slide 23

Heat to be extracted by the evaporator

Respiratory heat Field and packaging

heat

Fan motor heat Door heat Conduction heat Ventilation heat Lighting heat Air exchange Personnel & fork lift heat

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Slide 24

Potato Cold Store

Chamber Volume 3.4 cu m/MT Design Conditions : Air Flow: Temperature 3 + 1 deg C Loading & Pull down 85 cmh/MT Relative Humidity 90 to 95 % Holding Around 50% Outdoor air change 2-6 air changes/day CO2 Level : During loading upto 4000 ppm During holding upto 2000 ppm Cooling Rate: For CIPC application: Cooling upto 10 deg C in 24 hours. Cooling upto 15 deg C in 24 hours. Cooling upto 3 + 1 deg C within 8 days. Cooling @ 0.5 deg C/day upto 11+1 degC. Typical operating conditions: Loading Rate: Air on 3 deg C 4 % at 25 deg C Air out 1 deg C 5 % at 20 deg C Evaporating temp (-)2 deg C

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Slide 25

Determining size of the cold room

The size of the cold room depends on:

• The filling capacity
• The air circulation system
• Cold rooms should have a rectangular shape if

possible (length-to-width ratio = 3:2)

• The height of the room depends on the packaging.

Reference: stacker height plus a clearance of at least 10% of the total room height.

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Slide 26

Clearances

60 cm 25 cm Stacker height 525 cm Case height 75 cm

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Slide 27

25 cm 35 cm

Clearance for sloping ceilings

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Slide 28

Sample calculation for 250 t apples

• Determining the size of the cold room

– Stacking cases (wood, 60 kg) containing 300 kg apples – Dimensions l x w x h = 1 x 1.2 x 0.75 m

• Number of cases

– 250,000 kg : 300 kg per case = 840 cases

• Stacking of cases

– Stacked 7 cases high, the bottom level covers a floor space of 10 x 12 cases. – Space between cases = 5 cm – Space between walls = 10 cm

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Slide 29

Dimensions of the cell

• Length

12 x 1 = 12.00 m + 11 x 0.05 = 0.55 m + 2 x 0.10 = 0.20 m Inner length = 12.75 m

• Width

10 x 1.2 = 12.00 m + 9 x 0.05 = 0.45 m + 2 x 0.10 = 0.20 m Inner width = 12.65 m

• Height

7 x 0.75 = 5.25 m +

• min. 10%

= 0.75 m Inner height = 6.00 m

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Slide 30

Stacking plan for apple crates

10 5 1265 100 1275 10 5 Dimensions in cm

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Slide 31

Required cooling capacity

Cooling capacity kW Day 10 Loading Long-term storage

Conduction heat Ventilator heat Air exchange heat Respiratory heat Field heat Lighting, fork lift, personnel Respiratory heat loading

5 10 15 20 25 30 35 40 45

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Slide 32

ULO Principle (Ultra Low Oxygen)

Conversion of O2 to CO2 ca 1% every 24 h

O2 CO2

O2 & CO2 analyser

CO2 CO2 Scrubber O2 N2 generator N2 Air O2 Air

Vacuum safety valve Relief valve

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Slide 33

Capacity / air volume ratio

50 kW 50 kW

Sufficient air Insufficient air

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Slide 34 Air in +1 °C Air out

• 1 °C

Evaporating temp.

• 6 °C

Surface 284 m2 K value 24 TD air, motor side 0.2 °K

Draw-through or blow-through air coolers

+1 °C +1.2 °C

• 1 °C

+1 °C

• 1.2 °C
• 1 °C

Blow through Draw through

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• 2
• 3
• 4
• 5
• 7
• 6
• 8
• 9

+1 +1.2 Air temperature °C 7.2 K 5 K ∆T1,2 = LOG ∆T = 6.03 Capacity = m2 x K value x LOG ∆T = 284 x 24 x 6.03 = 41.1 kW

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• 1.2
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+1 Air temperature °C 7 K 4.8 K ∆T 1,2 = LOG ∆T = 5.83 Capacity = m2 x K value x LOG ∆T = 284 x 24 x 5.83 = 39.8 kW

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Slide 35

RH draw-through Air Coolers

Air-off -1 °C 89 % RH

+1 °C

• 1 °C

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Slide 36

RH blow-through air coolers

Air-off 0 °C 98 % RH (drawing: 89%)

+1 °C

• 1 °C

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Slide 37

4 5 6 7 8 9 10 11 12

DT1 in K

70 75 80 85 90 95 100

• Rel. humidity in %

Relative humidity as a function of DT1

DT1 = diff. Between air inlet and evaporating temperature.

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Slide 38

Influence of the fan speed on RH

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Slide 39

Extract of hx diagram at 1013.25 mbar

• rel. humidity: 90%

3 2 1

• 1
• 2
• 3
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• 5
• 6

Temperature [°C]

1 2 3 4 5

• abs. humidity x [measured: H20/kg dry air]

Air cooler inlet tL1 = 2°C

• rel. humidity = 92%

Average coil temperature tsurface= - 4.1°C ∆t1=11K VL= 50% (n=50%)

• Av. coil block temperature

Tsurface = - 0.9°C ∆t1= 6K VL= 100% (n=100%) Absolute dehumidification cooler fan n= 50% Absolute dehumidification cooler fan n=100% ∆x=0,5 [g/kg] x=1.3 [g/kg] 100%

RH at high (n=100%) & low (n=50%) fan speeds

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Slide 40

Evaporator calculation, apple storage

Required capacity 46 kW

• Temp. difference TD

7 K Evaporator surface A = Q / K x TD = 46000 / 24 x 7 = 273 m2 Air exchange rate 30- to 40 times the room volume (968 m2) Selected evaporator type THOR-F 276-7 Capacity 46 kW Air volume 36,000 m3/h Surface area 284 m2 Fin spacing 7 mm Air exchange rate air volume/room volume = 36,000 / 968 = 37-fold

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Slide 41

Agricultural Storage Coolers modelsTHOR-F & LFX / TYR-F

• 3 to 8 fans
• Fin spacing 7 mm
• Capacities 7 - 52 kW
• Temperature area

+5 tot -10°C

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Coolers for long term storage room apples and pears

Slide 42

Thanks for your attention !