Calculation of Engine Room Ventilation

GENERAL

The primary aspects of a properly designed engine room ventilation system are cooling air and combustion air. Cooling air refers to the flow of air that removes radiant heat from the engine, generator, other driven equipment and other engine room components. 
Combustion air describes the air the engine requires to burn fuel.

Cooling and combustion air directly impact engine and package unit performance and dependable service life; these must be considered in the design of an engine room ventilation system. It is also important to consider all engine room equipment in the design of a ventilation system and provide a comfortable environment for service personnel to perform maintenance.

Some driven equipment, such as a generator in a large engine installation,may require a dedicated ventilation source.

Sizing Considerations

Cooling Air
A portion of fuel consumed by an engine is lost to the environment in the form of heat radiated to the surrounding air. In addition, heat from generator inefficiencies and exhaust piping can easily equal engine-radiated heat. Any resulting elevated temperatures in the engine room may adversely affect maintenance, personnel, switch gear, and engine or generator set performance.

Engine room ventilation air (cooling air) has two basic purposes.

• To provide an environment that permits the machinery and equipment to function properly with dependable service life.
• To provide an environment in which personnel can work comfortably and effectively.

It is important to note that cooling air is needed for more than just the engine; the generator intake also requires cool clean air. The most effective way to do this is to provide a ventilation air source low to the ground at the rear of the package.

The use of insulation on exhaust pipes, silencers, and jacket water pipes will reduce the amount of heat radiated by auxiliary sources.

Radiated heat from the engines and other machinery in the engine room is absorbed by engine room surfaces. Some of the heat is transferred to atmosphere or, on marine installations, to the sea through the ship’s hull. The remaining radiated heat must be carried away by the ventilation system.

A system for exhausting ventilation air from the engine room must be included in the ventilation system design.


Combustion Air 
In many installations, combustion air is drawn from outside the engine room via duct work that is designed to move a large amount of air with very little restriction. These installations have very little impact on engine room ventilation design.

Other installations, however, require that combustion air be drawn directly from the engine room. In these installations,combustion air requirements become a significant ventilation system design parameter.

Approximate consumption of combustion air for a diesel engine is 0.1 m3 of air/min/brake kW (2.5 ft3 of air/min/bhp) produced. Engine specific combustion air requirements can be found using the resources mentioned in the foreword of this guide.

Ventilation Airflow
Required ventilation airflow depends on the desired engine room air temperature as well as the cooling air and combustion air requirements outlined above. While it is understood that total engine room ventilation airflow must take all equipment and machinery into account, the following sections provide a means for estimating the airflow required for the successful operation of Cat engines and packages.

Calculating Required Ventilation Airflow
Engine room ventilation air required for engines and packages can be estimated by the following formula.

V = [ (H / (D x Cp x T)) + Combustion Air] x F

Where:
V = Ventilating Air (m3/min), (cfm)
H = Heat Radiation i.e. engine,generator, aux (kW), (Btu/min)
D = Density of Air at air temperature 38°C (100°F). The density is equal to 1.099 kg/m3
       (0.071 lb/ft3)
CP = Specific Heat of Air (0.017 kW x min/kg x °C), (0.24 Btu/LBS/°F)
T = Permissible temperature rise in engine room (°C), (°F)
(Note: Max engine room temperature is 120°F)
F = Routing factor based on the ventilation type.

Note: If combustion air is supplied to the engine through dedicated duct work, “Combustion Air” should be omitted from the formula.

Example:
The engine room for a 12V, 32/40 Stx Man has a Type 1 ventilation routing configuration and a dedicated duct for combustion air.

It has a heat rejection value of 659 kW (37,478 Btu/min) and a permissible rise in engine room temperature of 11°C (20°F).

Solution:
The estimated engine room ventilation required for this arrangement:
 metric :
V = [ (659/1.099 x 0.017 x 11) + 0] x 1

V = 3206.61 m3/min

British :
V = [ (37478/0.071 x 0.24 x 20) + 0] x 1
V = 109970.7 cfm

Proper ventilation is heavily dependent on the path of the ventilation air. Applications involving high load factors and continuous full power operation require a rigorous approach based on classical heat transfer calculations accounting for radiant heat and allowable room temperature rise and adjusting with a ventilation routing factor.

Engine Room/Enclosure Temperature
The primary reason for maintaining engine room temperature at an appropriate level is to protect various components from excessive temperatures. Items that require cool air are:

• Electrical and electronic components.
• Cool air to the air cleaner inlet.
• Cool air to the torsional vibration damper.
• Habitable temperatures for the engine operator or service personnel.
• Cooling air for the generator or other driven equipment.

A properly designed engine room ventilation system will maintain engine room air temperatures within 8.5 to 12.5°C (15 to 22.5°F) above the ambient air temperature.

For example, if the engine room temperature is 24°C (75°F) without the engine running, the ventilation system should maintain the room temperature between 32.5°C (90°F) and 36.5°C (97.5°F) while the engine is in operation.

A ventilation design that ensures engine room temperature does not exceed 49°C (120°F). If the engine room temperature cannot be maintained below 49°C (120°F), cooler air should be ducted directly to the engine air cleaners.

Temperature limits of the driven equipment must also be considered.
For model-specific electronic components, reference the appropriate service manual or EDS sheet for that component’s allowed surface temperatures.

If the engine room temperature exceeds 40°C (104°F), the generator must be derated per the generator derate schedule and cool outside air must be ducted directly to the generator air intake. 

Alternatively, custom generators can be sized to handle specific ambient conditions.

In larger multiple engine sites, the normal 8.5 to 12.5°C (15 to 22.5°F) temperature rise guidelines for engine rooms may require unobtainable or uncomfortable air velocities. For these larger sites, a ventilation system needs to give priority to the five items listed above and provides a bottom to top airflow. 

In all cases, engine room/enclosure design must ensure that air temperature around the engine will not exceed 50°C (122°F). Critical locations include the engine torsional damper and generator coupling. Air temperature reading should be taken no more than 6 inches away from these components. Note that in these extreme situations, it may be necessary to duct cool air directly to these critical components.

Atmospheric Heat Rejection Correction Factor
Atmospheric heat rejection values published in TMI are based on ambient cell conditions between 25°C and 29°C. Engine rooms can be designed at much higher ambient conditions; therefore a correction factor can be utilized to define the atmospheric heat rejection at the higher ambient condition. The correction factors defined below have been developed using fundamentals of heat transfer and can be applied to any object under the same conditions.

There are two distinct correction factors, one is used with wet exhaust and turbo manifolds, the other is used with dry exhaust and turbo manifolds. The skin temperature utilized in the dry manifold calculation is 200°C, approx value of the wrapped, or insulated manifold.

Wet exhaust and turbo manifold correction factor.
WCF = -.0156 * TER + 1.4505
Where:
WCF = Wet Correction Factor
TER = Ambient Engine Room Ambient (°C)

Dry exhaust and turbo manifold correction factor.
DCF = -.011* TER +1.3187
Where:
DCF = Dry Correction Factor
TER = Ambient Engine Room Temperature (°C)

To obtain the corrected atmospheric heat rejection value, multiply the TMI value by the WCF or DCF.

Radiant Heat
Radiant heat values for the engine and driven equipment is needed to calculate the required ventilation airflow.

Note: For the packaged generator sets, ensure that there is adequate airflow near the engine torsional damper. Excessive piping and cooling system structures may prohibit proper airflow near the torsional damper.

Engine
Engine generated radiant heat (heat rejection to atmosphere) is routinely provided with published engine technical data. Values are typically nominal with their tolerance noted.
Tolerance should always be added before using published data in calculations.

Generator
For generator set installations, the heat radiated by the generator can be estimated by the following formulas.

HRG (kW) = P x [(1/Eff) – 1]

HRG (Btu/min) = P x [(1/Eff) – 1] x 56.9

Where:
HRG = Heat Radiated by the Generator (kW), (Btu/min)
P = Generator Output at Maximum Engine Rating (ekW)
Eff = Generator Efficiency % / 100%

Example:
A 3512B, 975 ekW standby generator set has a generator efficiency of 92%. The generator radiant heat for this genset can be calculated as follows.

Solution:
P = 975 ekW
Efficiency = 92% / 100% = 0.92
HRG = 975 x (0.92 – 1)
HRG = 84.78 kW
HRG = 975 x (0.92 – 1) x 56.9
HRG = 4824 Btu/min
Note: This data is available in the TMI for generator sets.

 Ventilation Fans
Except for special applications, natural draft ventilation is too bulky for practical consideration. Adequate quantities of fresh air are best supplied by powered (fan-assisted) ventilation systems.

 Fan Types
The following types of ventilation fans are typically used.
• Vane-axial
• Tube-axial
• Propeller
• Centrifugal
(squirrel cage blowers)

The selection of fan type is usually determined by ventilation air volume, pressure requirements and space limitations within the engine room. The fans have various qualities that make them better
suited to certain applications.

 Fan Location
Fans are most effective when they withdraw ventilation air from the engine room and exhaust the hot air to the atmosphere. However, ideal engine room ventilation systems will utilize both supply and exhaust fans. This will allow the system designer the maximum amount of control over ventilation air distribution.
The fan motors should be mounted outside the direct flow of hot ventilating air for longest motor life.
The design of centrifugal fans (squirrel cage blowers) is ideal in this regard, but their size, relative to the vane-axial or tube-axial fans, sometimes puts them at a disadvantage.

Fan Sizing
Fan sizing involves much more than just selecting a fan that will deliver the airflow volume needed to meet the cooling air and combustion air requirements. It requires a basic understanding of fan performance characteristics and ventilation system design parameters.

Similar to a centrifugal pump, a fan operates along a specific fan curve that relates a fan’s volume flow rate(m3/min or cfm) to pressure rise (mm H2O or in. H2O) at a constant fan speed.
Therefore, fan selection not only requires that the volume flow rate be known, but also that the ventilation distribution system be known in order to estimate the system pressure rise. This information allows the optimum fan to be selected from a set of manufacturers’ fan curves or tables.

Exhaust Fans
Ventilation air exhaust systems should be designed to maintain a slight positive or negative pressure in the engine room, depending on the specific application.
Positive pressure should normally not exceed .050 kPa or (0.2 in. H2O).
This positive pressure provides the following advantages.
• It prevents the ingress of dust and dirt, which is especially beneficial for those applications involving engines that draw their combustion air from the engine room.
• It creates an out draft to expel heat and odor from the engine room.

Some applications, such as a marine application where the engine room is adjacent to living quarters,
require that a slight negative pressure be maintained in the engine room. This negative pressure should not normally exceed 0.1275 kPa (0.5 in. H2O). The excess exhaust ventilation provides the following advantages.
• It compensates for the thermal expansion of incoming air.
• It creates an in draft to confine heat and odor to the engine room.

Two Speed Fan Motors
Operation in extreme cold weather may require reducing ventilation airflow to avoid uncomfortably cold working conditions in the engine room. This can be easily done by providing ventilation fans with two speed (100% and 50% or 67% speeds) motors.

Routing Considerations

General Routing Principles
Correct ventilation air routing is vital for proper operation of engines and packaged units.
Maintaining recommended air temperatures in the engine room is impossible without proper routing of the ventilation air. The following principles should be considered when designing an engine room ventilation system.

• Fresh air inlets should be located as far from the sources of heat as practical and as low as possible.

• Ventilation air should be exhausted from the engine room at the highest point possible, preferably directly over the engine.

• Ventilation air inlets and outlets should be positioned to prevent exhaust air from being drawn into the ventilation inlets (recirculation).

• Ventilation air inlets and outlets should be positioned to prevent pockets of stagnant or recirculating air, especially in the vicinity of the generator air inlet.

• Where possible, individual exhaust suction points should be located directly above the primary heat sources. This will remove heat before it has a chance to mix with engine room air and raise the average temperature. It must be noted that this practice will also require that ventilation supply air be properly distributed around the primary heat sources.

• Avoid ventilation air supply ducts that blow cool air directly toward hot engine components.

This mixes the hottest air in the engine room with incoming cool air, raising the average engine room temperature. This also leaves areas of the engine room with no appreciable ventilation.

• For installations where engines draw combustion air from inside the engine room, the routing should provide the coolest possible combustion air to the turbocharger inlets.

• For marine and offshore applications, the potential exists for seawater to be drawn into the ventilation air supply; systems for these applications must be designed to prevent seawater from being drawn into the air intake filters and ingested by the turbocharger. Generator cooling air must also be filtered to minimize the ingestion of salt.

These general routing principles, while driven by the same basic principles of heat transfer, will vary with the specific application.
This section discusses the general considerations relating to single and dual engine applications, multiple engine (3+) applications, and several special applications.

Single & Dual Engine Applications
Single and dual engine applications are arguably the most common applications encountered, regardless of engine market. These applications will generally require smaller engine rooms,which are especially challenging in regard to the use of good routing practices.
Recommended ventilation systems for these applications, presented in order of preference, are Type 1, Type 2, Type 3 and Type 4.

Ventilation Type 1 (Preferred Design)
Note: In ventilation airflow calculations, Type 1 systems have a Routing Factor of 1.

Outside air is brought into the engine room through a system of ducts. These ducts should be routed between engines, at floor level, and discharge air near the bottom of the engine and generator as shown below.
Ventilation air exhaust fans should be mounted or ducted at the highest point in the engine room. They should be directly over heat sources.
This system provides the best ventilation with the least amount of air required. In addition, the upward flow of air around the engine serves as a shield which minimizes the amount of heat released into the engine room. Air temperature in the exhaust air duct will be higher than engine room air temperature.

Ventilation type 1



Ventilation Type 2 (Skid Design)
Note: In ventilation airflow calculations, Type 2 systemshave a Routing Factor of 1.

A skid design may be preferred in petroleum applications. Similar to the Type 1 system, Type 2
brings outside air into the engine room through a system of ducts and routes it between engines.

Type 2, however, directs air flowunder the engine and generator so the air is discharged upward at the engines and generators as shown below.

Ventilation Type 2

Ventilation Type 3 (Alternate Design)
If Ventilation Type 1 or Type 2is not feasible, an alternative is Type 3; however, this routing configuration will require approximately 50% more airflow than Type 1.

Note: In ventilation airflow calculations, Type 3 systems have a Routing Factor of 1.5.

Ventilation type 3
 


Ventilation Type 4 (Less Effective Design)
 If Ventilation Type 1, Type 2 and Type 3 are not feasible, the following method can be used;
however, it provides the least efficient ventilation and requires approximately two and a half times the airflow of Ventilation Type 1.

Note: In ventilation airflow calculations, Type 4 systems have a Routing Factor of 2.5.

Ventilation Type 4





 Incorrect Airflow
Figure below illustrates an incorrect method to vent engine room heat.
Although the inlet duct has louvers to direct airflow toward the engine, rising heat will warm the cool air before it can reach the engine.

Incorrect Airflow












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