A practical, bookmark-worthy guide to choosing supply vs return setpoints, fresh-air vents, data logger placement, and brand-by-brand tips so frozen Indonesian seafood arrives claim-free.
If there’s one topic that still triggers costly claims for frozen seafood in 2025, it’s reefer setpoint supply vs return. We’ve seen perfect products get questioned at destination simply because the controller was left in the wrong control mode, or a logger sat in the warmest spot by the door. The good news. A few practical rules prevent 90 percent of issues.
The one rule that saves most frozen seafood shipments
For frozen seafood, set—and control—the reefer on Supply Air. Not Return Air. Use -18°C supply air for most frozen fish and shrimp, unless your buyer specifically requires -20°C or colder. Keep the fresh-air vent closed at 0 m3/h.
Why? The unit controls the temperature of the air it supplies into the cargo space. That’s the air the product actually “sees” first. Return air inevitably reads warmer because it’s picking up heat from packaging and the product mass on the way back to the unit. A 2–3°C gap between return and supply is normal on a well-stowed load, and it can be higher during defrost or if airflow is restricted.
In our experience, when shippers switch their SOP from return to supply control, temperature claims plummet. We’ve watched customers go from multiple claims a quarter to none for a year. That’s not luck. That’s physics plus documentation.
Brand-by-brand. How to ensure you’re on Supply control
Most ocean reefer brands show both Supply (SUP/SAT) and Return (RET/RAT) temperatures on the display, but only one is the control reference. Here’s how it typically looks in 2025.
- Thermo King (Magnum Plus and newer). Display shows SUP and RET. In the controller menu, choose Temperature Control Mode = Supply. Set setpoint to -18°C. Don’t assume default is Supply. Confirm before gate-out.
- Carrier Transicold (Micro-Link). You’ll see SUPPLY and RETURN temps. In configuration, ensure Control = Supply Air. Set -18°C. Some Carrier units retain the last cargo’s control mode, so we always verify at stuffing.
- Daikin. Display typically labels SUP and RET. Choose Supply Control for frozen cargo. Set -18°C or your buyer’s spec.
- Star Cool (MCI). Star Cool is supply-air centric. You’ll see SAT and RAT. Frozen defaults to Supply control in most line SOPs. Set -18°C SAT.
Working with Maersk, CMA CGM, MSC or others. Their reefer manuals increasingly standardize on Supply control for frozen cargo. But equipment pools are mixed, and settings get changed at depots. Always confirm the control reference on the actual controller, not just the booking note. If you want a quick sanity check for your next stuffing, need help with the exact menu path, or want photos of the screens to train your team, just Contact us on whatsapp.
Translating buyer specs into setpoints that won’t get challenged
We still see POs that say “-20°C frozen shrimp.” Is that supply air or product core? Here’s how we interpret and confirm it.
- If the spec says “Setpoint -18°C” or “-20°C,” and nothing else, we default to Supply Air. We confirm this with the buyer in writing.
- If the spec says “Product/pulp at -18°C,” we still control by Supply Air but we verify and log core temperature at stuffing. That means cargo is pre-frozen to ≤ -18°C before loading. Reefers don’t freeze. They maintain.
Suggested wording we put on bookings and invoices. “Setpoint: -18°C Supply Air. Fresh Air Vent: 0 m3/h. Product core at stuffing: ≤ -18°C.” Simple language. Zero ambiguity.
Fresh-Air Vent setting for frozen seafood. Close it. 0 m3/h. Opening the vent for frozen shrimp or fish invites warm, humid air. That drives frost load, increases return air, and can create ice on cartons. We only open vents for chilled cargo that needs respiration gas exchange. Not frozen.
Precooling the empty reefer. Don’t. For frozen loads, an empty pre-cooled container warms during loading and condenses moisture onto cartons. Load quickly with pre-frozen product and start the unit as soon as the doors close.
Why your return air is warmer than the setpoint
Return air typically runs 2–4°C warmer than supply on a healthy frozen load. During defrost, the gap can widen temporarily. Common drivers.
- High ambient and long power-off transfers. The unit works harder. RA climbs first.
- Blocked airflow. Stacks touching the ceiling, load over the T-floor lips, or cargo tight against the front bulkhead.
- Wet packaging or high glaze. Extra moisture increases frost and coil load.
What’s acceptable. Many buyers and auditors look for continuous Supply Air at setpoint (±0.5–1.0°C) and stable Return Air. EU frozen-food rules allow short-term transport fluctuations up to 3°C in air temperature, but product should remain at -18°C or colder. We avoid giving legal interpretations, but we design SOPs so our logs show supply at setpoint and product cores verified at stuffing.
Data logger placement that actually reflects product temperature
I’ve seen claims built on a single logger sitting near the doors. That’s the warmest place in the box. Our baseline setup for frozen seafood.
- One primary logger inside a representative carton located in the middle of the load, mid-height, about one third back from the doors. That approximates product behavior.
- One secondary logger in free air near the doors, mid-height. That captures the warmest air pocket and helps diagnose handling during transfer.
- Optional third logger near the return-air side, mid-height, for airflow diagnostics on sensitive cargoes or long routes.
Sampling interval. 10–15 minutes is practical for long voyages and keeps file sizes manageable. Calibrate loggers annually and label their locations on the packing list. If your buyer insists on a single location, negotiate for the primary in-carton position. It’s the fairest indicator.
Stowage that protects your temperature curve
Airflow is your best friend.
- Keep the T-floor clear. No slip-sheets or sagging cartons blocking the channels.
- Leave 8–10 cm headspace below the ceiling so supply air can move. Don’t press-load to the roof.
- Keep 10–15 cm off the front bulkhead to avoid choking the return.
- Use good pallets with open slats. If floor-loading, keep aligned channels front to back.
We apply the same rules whether we’re shipping Frozen Shrimp (Black Tiger, Vannamei & Wild Caught) or IQF finfish like Grouper Fillet (IQF) and Mahi Mahi Fillet. Airflow discipline prevents most “mystery” RA spikes.
Can you mix frozen shrimp and squid in one reefer?
Yes, if both are truly frozen and share the same setpoint. We’ve run mixed loads of shrimp and squid at -18°C Supply Air with vents closed and clean arrival. Our caveats.
- Odor and drip risk. Strong-odor items or high-glaze products should be wrapped or segregated.
- Weight and carton strength. Squid blocks can be dense. Don’t crush shrimp cartons underneath.
- If any item requires -20°C Supply Air or colder, use -20°C for the whole load or separate containers.
Straight answers to the questions we get every week
For frozen shrimp at -18°C, do I set the reefer on supply air or return air?
Supply air. Set -18°C Supply and control by Supply. Vents closed.
Why is my return air 2–3°C warmer than the setpoint on frozen seafood shipments?
Because return air picks up heat from product, packaging and defrost. A 2–4°C gap is normal on a healthy load with correct airflow.
What fresh-air vent setting should I use for frozen fish and shrimp?
0 m3/h. Closed for the entire voyage.
How do Thermo King, Carrier, Daikin, and Star Cool label supply vs return setpoints?
You’ll see SUP/SAT and RET/RAT on screens. Choose Supply Control. Thermo King and Carrier let you toggle control mode. Daikin similar. Star Cool defaults to Supply for frozen in most fleets.
Is -20°C buyer requirement a supply-air setpoint or a product/pulp temperature?
Clarify in writing. We treat it as Supply Air unless “pulp/core” is stated. We also verify product core at stuffing.
Where should I place data loggers in a reefer carrying frozen seafood?
One inside a mid-load carton. One in free air near the doors. Optional third near return-air mid-height.
Can I mix frozen shrimp and squid in one reefer without changing the setpoint?
Yes, if both are frozen and compatible at -18°C Supply with vents closed. If any SKU needs -20°C, use -20°C for all or split the shipment.
Final Indonesia-origin checklist for claim-free frozen shipments
- PTI valid and controller in Supply control at the depot. Confirm on-screen before gate-out.
- Setpoint as per PO. Most frozen seafood runs -18°C Supply. Doors sealed immediately after stuffing.
- Fresh-air vent closed. Drain holes clear. Genset fuel checked for domestic legs.
- Cargo pre-frozen to ≤ -18°C. No pre-cooling of the empty container.
- Loggers placed and documented on packing list. Photos of controller screens saved.
We’ve shipped everything from Yellowfin Saku (Sushi Grade) to Swordfish Steak (IQF) on the same simple principles. Control by Supply. Vent closed. Airflow protected. Loggers where they matter. Do those four things, and you’ve solved most of the “why is my return air warmer” drama before it starts.
Questions about your buyer spec or your next mixed-load plan. Call us and we’ll walk you through the exact settings by equipment brand and carrier SOP so you load once and sleep better.