Knowledge / Ship systems

Marine HVAC: sensible load, dehumidification and heat recovery

Understand temperature and moisture loads, dry-air-basis calculations, part-load behaviour and heat-recovery limits in ship accommodation.

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Reaching the desired room temperature aboard a ship does not establish suitable humidity. Outside air, occupants, showers and door openings introduce water vapour, while sunshine, equipment and hot surfaces create heat loads in different proportions. Combining these effects under one cooling-capacity label can conceal significant problems, particularly at part load.

Airflow, temperature and moisture have separate balances

Sensible heat is associated with temperature change. Latent load is associated with moisture removal. Humidity ratio w is kilograms of water vapour per kilogram of dry air; it is not relative humidity. Heating air containing the same water vapour can lower its relative humidity without removing any water.

Supply airflow must address temperature control, moisture transport, ventilation and pressure balance together. Increasing fan flow alone does not resolve every problem. If supply humidity ratio remains too high, extra air may not provide adequate dehumidification. A change in the exhaust-to-outside-air balance can also increase uncontrolled moisture entry from corridors or outdoors.

Cooling and dehumidification at the coil

Condensing water vapour requires a relevant surface temperature below the air's dew point. Not all air reaches the same surface temperature in a real coil; surface area, contact, water or refrigerant conditions and airflow distribution influence the leaving state. Nominal coil capacity and dehumidification capacity at a particular condition therefore need separate evaluation.

Trane's technical newsletter on humidity control explains how reducing capacity in response to temperature can reduce condensation at part load. This land-based HVAC explanation describes physical behaviour applicable to ship accommodation too; it is not used as a ship ventilation rule or a current climatic design table.

A calculation on a dry-air basis

For an illustrative coil calculation at approximately 101.3 kPa, assume dry-air mass flow of 1.20 kg/s. Entering conditions are T = 32°C and w = 0.018 kg/kg dry air; leaving conditions are T = 14°C and w = 0.00950 kg/kg dry air. At steady state, condensate flow is 1.20 × (0.018 − 0.00950) = 0.01020 kg/s, approximately 36.7 kg/h. These are example state points, not targets for a particular vessel or product.

Use the approximate moist-air relation h = 1.006 T + w(2501 + 1.86 T), with T in Celsius and h in kJ/kg dry air; the constant 2501 is two thousand five hundred and one. Entering enthalpy is 78.28 and leaving enthalpy 38.09 kJ/kg dry air. The air-side enthalpy reduction is 1.20 × (78.28 − 38.09) ≈ 48.2 kW. This simplified balance neglects enthalpy carried by liquid condensate, fan heat and external heat transfer.

Using only temperature difference with an assumed specific heat of 1.02 kJ/(kg·K) gives approximately 22.0 kW. That second figure is only a rough estimate of the sensible component, demonstrating why moisture removal must also be considered. Confusing total moist-air flow with dry-air flow introduces a systematic error into an enthalpy calculation.

Which load does heat recovery reduce?

Supply and exhaust streams can exchange sensible energy and, in suitable equipment, moisture. The EnergyPlus engineering reference treats sensible and latent effectiveness as separate model quantities. Using sensible effectiveness as though it were total enthalpy effectiveness can overstate the reduction in moisture load.

Assume balanced heat-capacity rates, outside air at 32°C, exhaust air at 24°C and sensible effectiveness of 0.60. Fresh-air outlet temperature is 32 − 0.60 × (32 − 24) = 27.2°C. Under a sensible-only assumption, humidity ratio is unchanged. At 1.20 kg/s and 1.02 kJ/(kg·K), recovered sensible power is approximately 5.88 kW. Fan power, leakage, fouling and control losses are excluded from this ideal calculation.

Additional limits of the marine environment

Salt-laden air and sea spray can affect maintenance needs for coils, droplet separation and outside-air inlets. Vessel motion makes condensate drainage, slope and trap arrangements important; fire divisions affect duct and damper arrangements. A successful comfort calculation does not establish suitable fire protection or ventilation of hazardous spaces. Those systems require assessment within their own approved arrangements.

Daikin's marine HVAC explanation is a manufacturer example of adapting equipment to marine applications. It does not establish a universal outside-air fraction, filter or equipment recommendation. For heat recovery, contaminants in the exhaust stream and possible cross-leakage must be considered separately from the energy benefit.

Which measurements distinguish humidity problems?

If room relative humidity rises while temperature remains normal, examine outside-air humidity ratio, coil entering and leaving conditions, valve or compressor command and condensate behaviour together. Drainage failure, insufficient coil capacity and unsuitable control logic can produce similar symptoms. A sensor close to a local hot surface can also distort the relative-humidity record.

An energy assessment counts fans, pumps and any reheat as well as compressor power. Overcooling followed by reheating can control humidity, but total energy and control stability need evaluation. A useful performance record includes the outside-air and operating conditions producing the room state. One daily average temperature does not explain dehumidification performance.

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