Cryogenic tunnel freezers for fish and seafood

01

Size capacity, belt, temperature and cryogen use

FAO's Fisheries Technical Paper 340 describes the liquid nitrogen freezer: fish on a stainless steel conveyor first meet a counter-current flow of nitrogen gas at about −50 °C, which extracts up to 50 percent of the product heat, then pass under the liquid spray where freezing is completed, and a last stage gives a few minutes for temperature to equalise. It lists operating temperatures of −50 to −196 °C for liquid nitrogen and −50 to −70 °C for liquid carbon dioxide, and quotes freezing times of 5 minutes for shrimp meat and 12 minutes for a 500 g whole lobster in a liquid nitrogen spray. [1]

Air Products' Freshline MP cryogenic tunnel is built from 3 m modules in belt lengths of 6 to 15 m and belt widths of 720 or 1,220 mm; its table gives freezing capacities of 550–1,400 kg/h on the 720 mm belt and 900–2,300 kg/h on the 1,220 mm belt, electrical requirements of 9–29 kW and weights of about 4,000–11,000 kg, and notes that capacity depends on product and operating conditions. Linde's CRYOLINE MT tunnel runs on liquid nitrogen or carbon dioxide with usable belt widths of 0.6, 1.0 and 1.25 m, freezing belt lengths of 4.0 to 13.0 m, retention times from 0.5 to 65 minutes by model, a maximum product height of 100 mm, and up to 30 stored recipes. [2] [3]

Cryogen use per kg depends on the freezing profile. In a patent test on golden pomfret of about 500 g in a cabinet liquid nitrogen freezer, freezing to a −18 °C core at a single −95 °C stage used 3.38 kg of liquid nitrogen per kg of fish, while two-stage profiles (−95 °C until a −5 °C core, then a warmer second stage) used 1.66–2.87 kg per kg. FAO adds that liquid nitrogen storage loses about 0.5 percent of contents a day and about 10 percent in tanker transfer, and that liquid nitrogen freezing can cost up to four times as much as conventional air blast freezing. [4] [1]

Editorial calculation: on the Freshline MP 1,220 mm belt, 900 kg/h over 6 m and 2,300 kg/h over 15 m are both about 150 kg/h per metre of belt; on Linde's 13.0 m belt, retention times of 2.0 to 65 minutes correspond to belt speeds of about 13.0 ÷ 65 = 0.2 to 13.0 ÷ 2.0 = 6.5 m/min; and a 1,000 kg/h line at 1.66–3.38 kg/kg would draw about 1,660–3,380 kg of liquid nitrogen an hour. [2] [3] [4]

Give the supplier the species and product form (whole fish, fillets, shrimp, molluscs), piece size and thickness, infeed temperature, target core and average temperature, required kg/h and operating hours per day, IQF or block freezing, cryogen (liquid nitrogen or CO2) and supply pressure, available floor space, and ask for a guaranteed kg of cryogen per kg of product from a trial on your product.

02

Plan oxygen safety, venting and hygienic design

Nitrogen and carbon dioxide released from a tunnel displace oxygen. OSHA's permit-required confined space standard, 29 CFR 1910.146, defines a hazardous atmosphere to include oxygen below 19.5 percent or above 23.5 percent and an oxygen-deficient atmosphere as one below 19.5 percent by volume; its sewer-entry appendix advises monitors that sound an audible alarm below 19.5 percent oxygen. FAO states that high carbon dioxide levels in factory air are dangerous, so a carbon dioxide freezer must be vented and the gas discharged outside the building. [5] [1]

Air Products describes the Freshline MP as opening vertically for cleaning, with a belt cleaning system fitted as standard, an optional assisted tunnel cleaning system, and a side-mounted exhaust below the belt with a duct cleaning system to avoid product contamination. Linde lists smooth surfaces that avoid food residues, a completely accessible interior for cleaning, frequency-controlled motors and downloadable production events for traceability on the CRYOLINE MT. [2] [3]

Editorial note: ask each supplier how exhaust gas is ducted out of the building, where fixed oxygen sensors and alarms are placed in the freezer room, how the tunnel interlocks on low oxygen, and whether any pit or enclosure must be treated as a permit-required confined space for cleaning.

03

Compare cryogenic and mechanical freezing tunnels

FAO lists continuous air blast freezers at −35 to −40 °C air and liquid nitrogen freezers at −50 to −196 °C, and gives average freezing rates of 5 to 30 mm/h for quick freezing in a tunnel air blast or plate freezer against 100 to 1000 mm/h for freezing in liquefied gases such as nitrogen and carbon dioxide. A liquid nitrogen freezer needs no compressors, condensers or coolers, so maintenance and power are low and the freezer is small, but it needs space for the cryogen tank and tanker access and a guaranteed supply; FAO advises not considering one where supply cannot be guaranteed. With large carbon dioxide units, about 80 percent of the refrigerant can be reliquefied. [1]

When requesting quotations, ask each supplier for capacity tables for your product, cryogen consumption from a trial, belt and tunnel dimensions, cleaning method and time, exhaust and oxygen-monitoring arrangements, and installation drawings.

04

Belt, precooling zone, spray zone and oxygen alarm along a cryogen tunnel

05

Freezing rate, cryogen use per kg of fish and tunnel footprint

Cryogen consumption is the row to pin down: the only published figure here, 1.66–3.38 kg of liquid nitrogen per kg of pomfret, comes from a cabinet freezer test rather than a tunnel, so ask for a guaranteed figure from a trial on your own species.

06

Send the product, capacity and site data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Product: [species; form; piece size; infeed temperature]
Target: [core temperature; average temperature; IQF or not]
Capacity: [kg/h; hours per day]
Cryogen: [liquid nitrogen or CO2; supply pressure; tank]
Site: [floor space; exhaust route; oxygen monitoring]
Hygiene: [cleaning method; materials]
Quantity: [units]

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