Shrimp Hatchery Chiller Water Level Control: Sizing a 5 m³ Seawater Tank System

In a shrimp hatchery and nursery, two factors decide whether larvae survive their most fragile weeks: stable water temperature and stable water level. A chiller that simply "gets cold" is not enough — the system has to pull a known heat load down within a set time window, then hold a constant level so pumps, aeration, and stocking densities keep working. This guide walks through a real-world seawater duty — a 5 m³ tank per pool — and shows how to size the chiller and design the water level control around it.

The Application at a Glance

Parameter	Design Value
Tank volume	5 m³ per pool
Water type	Natural seawater, salinity 30–35 ppt
Initial water temperature	26–30°C
Target water temperature	20–24°C
Required cooldown time	≤ 1.5–2 hours per 5 m³ tank
Required cooling capacity	200,000–240,000 BTU/hr (58.6–70.3 kW)
Water circulation flow	10–15 m³/h
Design ambient temperature	40°C and above

Step 1: Calculate the Theoretical Cooling Load

The sensible heat load is a straightforward calculation:

Q = m × cp × ΔT

• Mass of water (m): 5 m³ × 1,025 kg/m³ ≈ 5,125 kg of seawater
• Specific heat (cp): seawater at 30–35 ppt is about 4.0 kJ/kg·°C (slightly lower than freshwater at 4.186)
• Temperature drop (ΔT): worst case is 30°C down to 20°C = 10°C

Q = 5,125 × 4.0 × 10 = 205,000 kJ = 205 MJ

Spread that energy over the allowed cooldown window:

• Over 1.5 hours (5,400 s): 205,000 kJ ÷ 5,400 s ≈ 38 kW ≈ 130,000 BTU/hr
• Over 2 hours (7,200 s): 205,000 kJ ÷ 7,200 s ≈ 28.5 kW ≈ 97,000 BTU/hr

So the theoretical minimum load is roughly 97,000–130,000 BTU/hr. No real installation runs at the theoretical minimum, for the reasons below.

Step 2: Why Installed Capacity Runs Higher Than the Calculation

The specified duty — 200,000–240,000 BTU/hr (58.6–70.3 kW) — is about 1.8–1.9× the theoretical figure. That is not over-engineering; it is sound margin for real conditions:

1. 40°C+ ambient derating. An air-cooled condenser loses capacity as ambient temperature rises — condensing pressure climbs, compressor capacity falls. At 40°C and above, a unit rated at lower ambient can deliver 10–20% less than its nameplate. Oversizing guarantees full duty on the hottest days, which is when hatcheries need the chiller most.
2. Heat gains from the system itself. The circulation pump moving 10–15 m³/h adds heat directly to the water. Uninsulated tank walls and uninsulated piping in a 40°C environment add more.
3. Make-up water thermal shock. When warm seawater is added to replace evaporation or after a water change, the chiller must recover 20–24°C quickly without letting temperature spike. Recovery demand is often the peak load.
4. Fouling margin. Biofouling, algae, and scale on a seawater heat exchanger reduce heat transfer steadily. A clean-tube margin keeps the system performing between maintenance intervals.
5. Compressor longevity. A unit running at 60–70% load instead of 100% continuous sees lower discharge temperatures and far longer service life.

The result: a 200,000–240,000 BTU/hr unit is the correct installed capacity for reliably reaching 20–24°C from 26–30°C within 1.5–2 hours per 5 m³ tank, even at 40°C ambient.

Step 3: Seawater Means Titanium — No Compromise

Salinity of 30–35 ppt is aggressive. Copper, brass, and standard stainless steel corrode or pit quickly in direct seawater contact, and corrosion products are toxic to shrimp larvae — a single failed coil can poison a whole nursery.

For direct-contact seawater duty, the evaporator and heat exchanger must be titanium. Titanium offers:

• Outstanding corrosion resistance to chlorides and biofouling
• No copper ions released into the culture water
• Long coil life even at high salinity and elevated temperatures

Delin equips its seawater chillers with titanium coil evaporators (tank-style) or titanium shell-and-tube exchangers (for recirculating loops) so the heat-transfer surface is as durable as the biology it protects.

Step 4: Water Level Control — The Part That Protects Everything

A chiller keeps water at the right temperature, but the water level determines whether the whole loop runs safely. In a shrimp hatchery, level control does three jobs at once.

Why level stability matters in hatchery tanks

• Stocking density is set by volume. The water level directly sets tank volume and therefore larval density. A drifting level silently changes stocking density and, with it, feeding and DO management.
• Heavy aeration causes foaming and overflow. Hatcheries aerate aggressively. Without level control, foam and wave action push water out of the tank, wasting chilled water and stressing larvae.
• Pump protection. The chiller's circulation pump needs a stable flooded suction. A low level causes air entrainment, cavitation, and eventually dry-run damage to the pump and chiller.
• Evaporation is significant at 40°C. Water loss must be replaced continuously — but never with raw ambient-temperature water, which would shock the larvae and spike the cooling load.

A practical control arrangement

1. Level sensing per tank. A float switch, pressure transmitter, or ultrasonic level sensor on each tank reports the live level.
2. Auto top-up with pre-chilled seawater. A solenoid valve admits pre-conditioned seawater into the tank, actuated by the level sensor — replacing water at target temperature, not ambient temperature.
3. Overflow back to a sump, not to drain. Return overflow to a chilled-water reservoir so nothing is wasted and the level stays inside a narrow band.
4. Low-level pump interlock. Below the minimum safe level, the chiller or circulation pump trips to protect against dry running.
5. A buffer tank on the chiller loop. A small buffer/expansion tank absorbs level fluctuations between the hatchery tank and the chiller, so the pump always sees a stable suction and the chiller cycles smoothly.

Circulation: matching flow to tank volume

The specified 10–15 m³/h against a 5 m³ tank means a full tank turnover every 20–30 minutes. That is the right range for even temperature distribution without creating currents strong enough to stress larvae. Two details make it work:

• Return-water diffusers distribute chilled water slowly and uniformly instead of jetting into the tank.
• A strainer on the pump suction protects the chiller from larval debris and biofouling entering the loop.

Step 5: The Recommended Machine — Delin 15 HP Seawater Chiller

For a single 5 m³ tank (or a comparable batch), Delin supplies a 15 HP seawater chiller engineered specifically for this duty class. It is sized to hold the 200,000–240,000 BTU/hr (58.6–70.3 kW) requirement even when the design ambient is 40°C and above, and it is built for direct seawater contact:

• Titanium evaporator / titanium coil for 30–35 ppt salinity
• High-ambient condenser design with adequate surface area so capacity holds at 40°C+
• Digital temperature control with hysteresis to hold 20–24°C without short cycling
• Low-level and flow protection to interlock with the hatchery's level-control system
• Marine-grade pumps and fittings sized for 10–15 m³/h circulation

The unit pulls a 5 m³ tank from 30°C to 20–24°C inside the required 1.5–2 hour window with reserve, then modulates to hold temperature while water level control keeps the loop safe.

Summary

Design Question	Answer
Theoretical load, 5 m³ tank, 10°C drop	~97,000–130,000 BTU/hr
Installed capacity (with real-world margins)	200,000–240,000 BTU/hr (58.6–70.3 kW)
Why the higher number	40°C ambient derating, pump/tank heat gain, make-up water shock, fouling, compressor life
Seawater contact material	Titanium only
Circulation	10–15 m³/h ≈ full turnover every 20–30 min
Recommended unit	Delin 15 HP seawater chiller

Temperature and level control are two halves of the same system: the chiller carries the heat load, and level control keeps the loop running safely at the density the tank was designed for. Specify the load, protect the contact surface with titanium, and control the level — the rest is routine.

Ready to size a chiller for your hatchery? Send us your tank layout and volumes — Delin will match a 15 HP (or larger) seawater chiller to your nursery, complete with the level-control interlocks it needs.