Airflow 101

Introduction

Airflow within the engine room space is critical for both the performance of a vessel’s engines and generators as well as for the safety of the people who work within the engine room itself. Fans are generally used to provide combustion and cooling air to the engine room and to exhaust the warm air from the engine room space. A typical setup for engine room ventilation would be to use a pair of intake fans to bring cooling and combustion air into the engine room and a separate slightly smaller pair of fans would function to exhaust heated air from the engine room space. The size and air flow characteristics of the intake fans are engineered based on the amount of air that the engines and generators will need for combustion plus the cooling air that is required for the engine room space.

High Velocity vs Low Velocity Airflow

The engine room’s powerful intake fans produce high velocity airflow into the engine room. High Velocity airflow does not behave like low velocity airflow. The faster the air flow is moving the harder it is to change it’s direction. Unlike the slow moving airflow in a home air conditioning system, high velocity air flow will not make turns in ducts as easily as low velocity air flow; this can result in substantial losses in flow volume and the build up of static back pressure within the air distribution system. These losses of air volume and the build up of static pressure can be minimized by proper ventilation system design and fabrication.

Bending Air

Air is a fluid. A common misnomer is to consider air to be light and free and able to easily be distributed where it is needed. This is partially true when we discuss low velocity airflow, however, the higher the velocity of the airflow, such as that required for engine room ventilation, the more that it behaves like a more dense fluid. Consequently, fast moving air can not be efficiently turned around sharp corners in the same way that slow moving air can. Changing the direction of high velocity airflows requires a gradual sweeping turn in order to maintain the airflow’s high rate of speed and not cause a build up of static pressure. Airflow diverters are typically used in engine room applications with high velocity airflow in order to turn the air and distribute it effectively throughout the engine room. These diverters bolt on to the fan housing and use large sweeping vanes to turn the air in the desired direction(s). There are five basic types of airflow diverters, which can split the airflow up and guide it in the desired directions. Check the products section of this site under Fans and Blower Accessories for more information on airflow diverter types.

Combustion Air

The need for air introduction into the engine room is critical for the proper performance, longevity, and warranty of a vessel’s main engines and generators. Too little airflow into the engine room will result in poor engine and generator performance, higher maintenance and fuel costs and potential warranty issues, and excessive negative pressure. An engine room ventilation system should be designed so that there is always enough air to feed the vessel’s engines and generators and there should never be an excessive negative pressure or “vacuum” within the engine room space. A vacuum is indicative of inadequate airflow into the engine room and this restriction is will show itself not only through poor engine performance, but also by making the engine room door difficult or impossible to open while the vessel is under way and running toward the top end of it’s speed range. The fans that are used for engine room intake air should be engineered and designed so that they will provide adequate air to the vessel’s main engines and generators at all throttle settings. Too much airflow into an engine room, which results in a positive pressure in the engine room, is also an undesirable condition as well and should be avoided.

Cooling Air

Along with combustion air, your vessel’s engine room will also require cooling air in order to help displace the heat build up within the space. Todays large engines radiate more heat and require more ventilation than their predecessors of yesterday. Exhaust fans are used for this purpose and their function is to exhaust warm air out of the engine room before it can build up to a critical level. The removal of heat from the engine room is important not only for the performance of the engines and generators, but it is also critical in maintaining a safe working environment for all personnel who work within the engine room as well. While a vessel is running hard, the engines are drawing a substantial portion of the engine room heat in through their air intakes and removing it to the benefit of the engine room environment. It is when the vessel is slowed down after it has been running that the engines will radiate large quantities of heat and the engines are drawing a much smaller volume of air in through their intakes, that the need for ventilation exhaust fans is the most important. Under this condition, the engine rooms exhaust fans are typically working at their maximum level to remove heat from the engine room.

Engine Room Air Exchange Rate

It is critical that large volumes of air both enter and exit the engine room in order to keep the environment safe for both man and machine and in order to maintain approximately the same pressure within and outside of the engine room. The rule of thumb that has been extrapolated from many years worth of empirical data calls for a complete change out of the engine room's air volume 1.8 times per minute. Every engine room is different and has different requirements however this rule of thumb value will allow the engineers to make a quick assessment on a vessel's ventilation effectiveness prior to doing an in depth engineering evaluation of the space.

Removing Salt and Water from Engine Room Air

It is critical that large volumes of air both enter and exit the engine room in order to keep the environment safe for both man and machine and in order to maintain the desired “slight negative” pressure within the engine room. Delta T Systems, Inc. has developed proprietary calculation software and methods to achieve desired results. The calculations take in to account the ambient air outside the vessel, radiated heat of the machinery, cubic area of the space being treated, combustion air requirements and the desired “slight negative” pressure values within the space. The calculations determine the optimal value of cooling air required for the project. Every engine room is different and has unique requirements so all factors are taken into consideration on a case-by-case basis. We also offer heated units to eliminate ice build-up on the moisture eliminator, along with chilled units for critical applications in extreme high outside air temperature environments.

Understanding Static Pressure Losses in Airflows

Air in general, and airflows in particular, are very misunderstood elements of engine room ventilation. The fans that move air within the engine room are designed not only to move a specific volume of air, but they also need to be able to handle a certain level of static pressure in order to produce the desired outcome. The more resistance to airflow, the smaller the volume of air that will flow from the fan per unit of time. Airflow is commonly measured in units of cubic feet per minute (or cubic meters per minute). The static pressure that a fan is designed to handle is usually measured in units of inches of water column (in the same manner that barometric pressure is measured in inches of mercury). Things that can cause resistance to airflow include ducts, turns in ducts, air filters and any other obstruction to the airflow itself. Therefore, the higher the resistance to the airflow, the higher the static pressure that the fan will need to be designed to handle. Failure to properly engineer the engine room ventilation system to take the above parameters into account will result in a poorly performing system that will not meet the needs of the vessel’s engines and generators in regard to combustion and cooling air requirements.

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