Engineering of Water Systems

Standby and Backup Systems: Engine Rooms, Part 2

By Ed Butts, PE, CPI

We began a three-part series in the January issue on planning and designing rooms for engines and generator sets. In this month’s edition of The Water Works, we continue this series with ventilating and fuel storage design.

Cold Climate Considerations

Engines that operate in colder climates must generally prescribe to a set of special conditions. Extreme variations in room temperature can result in condensation and moisture accumulation in valve covers, freezing of ordinary waterlines, and battery failure.

Engines that must operate in these extremes are often additionally protected by incorporating a small unit heater and exhaust fan within the room to stabilize temperatures and humidity. Although not comfort heat by human standards, maintaining a constant room temperature between 45° to 50°F prevents the formation of condensation, provides stable temperatures for engine components, and avoids freezing conditions.

Applications with engine-mounted radiators using engine room air for cooling generally provide more airflow than is necessary for adequate ventilation. This high airflow combined with low ambient temperatures, below 70°F (21°C), can cause water to condense inside exposed engine components such as valve covers. This can result in oil and maintenance problems. Therefore, special installation considerations must be made in cold climates.

In many colder climates, the desired temperature rise to maintain the optimum engine operating temperature could be as much as 80°F (27°C) instead of the usual 15° to 20°F (9.4° to 11°C) rise. In these cases, it is better to specify and use a number of smaller room exhaust fans than one large fan to supply ventilation air. This not only provides a safety factor against failure of a single fan but also allows adjusting the total ventilation volume if the engine happens to operate at a lower horsepower output.

Adjustable louvers, positioned to redirect engine heat back into the room until the jacket water temperatures reach 190°F (88°C), may also be used. Once the temperature reaches a high enough value, these louvers can close so that all ventilation air is exhausted.

It is important to note that most engine failures are the result of overheating. This often occurs from hot air recirculation, which is a common problem with engine rooms. This occurs when the hot air from the unit’s radiator exhaust (and sometimes the hotter engine exhaust as well) is somehow able to quickly find its way back into the engine/genset room or enclosure, thus increasing the air temperature into the environment,
and ultimately, the radiator core inlet air temperature.

Exhaust system components, including mufflers and exhaust pipes, that are not properly insulated and adequately spaced from inlet louvers will likely lead to this same problem. Lastly, if the engine is an enclosed unit within an outdoor enclosure and does not have adequate clear space surrounding it, the engine will also begin to recirculate its nearby rejected heat and eventually overheat.

Common solutions to this problem include: (1) providing more clear space around the engine enclosure; (2) ducting the radiator exhaust to an exterior space through the roof where it cannot easily return to reach the engine intake; or (3) adding a turned-up duct to the front end to force the hot air up and away from the engine.

Finally, for estimating and planning airflow in an enclosed space, I offer a few rules of thumb:

• If the engine/genset is to be placed within an enclosed room, based on typical fixed louvers, the radiator exhaust louvers should be at least one and a half to two times larger than the surface area of the radiator.
• In this same type of environment, the air intake louvers should be at least one and a half to two times larger than the surface area of the exhaust louver as previously determined.
• Always provide at least 1200°F-rated thermal insulation and wrap over the entire exhaust system.
• Never locate exhaust components within 1.5 feet (18 inches) of any combustible materials or sprinkler heads.
• Never place intake and exhaust louvers in adjoining walls or proximity to each other.
• Space and direct engine and radiator cooling exhausts apart, upwards, and away from intake louvers.
• Consider prevailing winds and direction when locating radiator air intake louvers and exhaust outlets.
• Use beveled outlets and weighted flap gates on exhaust discharges to ensure full closure.
• If the engine is to be located inside of a room, ensure that intake or exhaust louvers cannot be blocked off. If powered intake louvers are used, consider adding spring-assisted louver blades to allow opening under a low room negative air pressure should the powered device fail.

Orient doors to require outward opening to enable opening of doors under low interior pressure or add a pressure relief window to relieve any negative pressure within the room. Incorporate a low atmospheric pressure cutoff switch in the engine room to disengage and shut down the engine if a low atmospheric or negative pressure condition occurs inside the engine room.

• Inlet and exhaust louvers typically include a screen or baffles which limit the open area of the device. This can vary between 33% to 75% of the total (gross) area. When determining the airflow through the louver, the free air velocity in feet per minute (FPM) must be ascertained from the net open area of the louver, and not the gross cross-sectional dimensions.

Generally, to prevent rain penetration into the room, the inlet air velocity through the net open area should
be between 500 to 750 FPM, but in no case exceed 1000 FPM. Head loss through the intake louvers should
not exceed 0.30-feet at the rated airflow rate to ensure the engine room pressure drop does not exceed a total of 0.50-inch of equivalent water column, but never less than the rated engine fan capacity.

A safety factor margin of 5% to 10% is often added to the inlet louver net size to account for variabilities
of fan capacity, engine heat rejection, and to prevent negative room pressures. Exhaust louvers are designed for the specific exhaust airflow (fan capacity), room pressure conditions, and permissible resistance but are typically designed and sized for a maximum air velocity of 2000 FPM with a corresponding maximum air resistance of 0.40-inch to 0.50-inch of water column head loss.

• For a radiator/fan-cooled engine, the volume of air required from a supplementary room fan will equal the total heat rejection from the engine and driven equipment (pump or generator) in CFM minus the volume of air from the pusher fan plus exhaust discharge on the engine, both in CFM. In many cases, the pusher fan can safely remove the heat rejection from the engine without the need for supplementary fan capacity.

Supplemental Air Requirement

In addition to providing an adequate volume of cooling and combustion air through the inlet louvers to maintain the room temperature below 120°F, the operating environment must not be allowed to incur an excessive atmospheric pressure drop that could result in negative pressures within the room, either from head loss through the inlet louver or an unbalance of inlet capacity versus exhaust or ventilation air discharge.

This is necessary to provide an adequate air supply and positive pressure to the engine under fully loaded conditions. Although this value may vary slightly between manufacturers and engine models, the typical maximum drop in allowable air pressure is 0.50-inch of water column.

In addition to potentially starving the engine of air, excessive pressure drop through the louver can cause penetration of water into the room. This is generally avoided by limiting the inlet louver air velocity to 1000 FPM or less. A proper heat balance is provided by removing the residual heat rejection developed by the engine and driven device by using a supplementary exhaust fan.

Following a reduction of heat emitted from the fan and exhaust stack, any residual heat should be extracted using the following equation.

Equation:

Supplemental capacity (CFM) = Total inlet air (CFM) – Radiator fan (CFM) + Exhaust flow (CFM)

Example 1:

a) What is the total required inlet air flow to an engine (in CFM) that requires 340 CFM of combustion air with an engine net heat rejection of 4600 BTU/min and desired maximum temperature rise of 20°F which drives a 110 kW genset at 92% efficiency while operating at sea level? Assume a routing factor of 1.

Solution: Gen heat rejection: kW × [(1/Eff) –1] × 56.9 = 110 kW × [(1/0.92) – 1] × 56.9 = 544 BTU/min
CFM = 4600 + 544 BTU/min = 15,094 CFM
________________
0.071 × 0.24 × 20°F
+ 340 CFM × 1.0 = 15,434 CFM at sea level

b) What square footage and minimum square dimensions of an inlet louver is required if the net open area is 33%?

Solution: 15,434 CFM ÷ 1000 FPM = 15.434 ft2 of net OA ÷ 0.33 = 46.77 ft2 gross OA = 6.84′ × 6.84′ sq.

Example 2: What capacity of supplementary fan is needed for the above example if the radiator cooling fan exhausts 8000 CFM and the exhaust system discharges 600 CFM?

Solution: Supplemental fan capacity = 15,434 CFM of inlet air – (8000 CFM + 600 CFM) = 6834 CFM

It is important to note that most engine failures are the result of overheating.

Fuel Storage Methods and Capacity

The required capacity and methods used to store fuel for onsite or portable engine or generator sets vary as to the type of fuel, safety considerations, highway and trailer capacities (for portable units), need for supplementary fuel for other purposes, required duration of operation, and need for fuel rotation and turnover to prevent contamination.

Generally, the principal types of fuel this applies to are diesel, gasoline, and propane (LPG). Safety considerations not only include the prevention of explosion or fire due to vapor or fuel ignition, but ensuring access to the fuel storage means are limited to prevent vandalism, contamination, or accidental contact and leakage.

At one time, many steel fuel tanks were buried to avoid these potential problems, but subsequent leakage to surrounding soil as well as groundwater has severely restricted or banned this type of storage. Currently, for liquid fuel storage, such as diesel and gasoline, the three primary fuel storage methods are:

1) Free standing, above-ground reinforced concrete-encased steel storage vessel

2) Free standing, above-ground steel storage vessel

3) Buried (below-ground) fiberglass storage vessel (should never be used in close proximity to a well).

Most governmental regulations now require a primary and secondary means of protection against fuel leakage to the environment. This is often referred to as “dual containment” and is a particularly important aspect when a fuel tank is to be located near or adjacent to a water well or surface water body.

Of the three, the reinforced concrete-encased, steel storage tank (i.e., a Convault style) offers the highest protection value and greatest resistance to vandalism since the tank itself is encased within a 4-inch to 6-inch reinforced concrete encasement. Fuel storage is provided by a steel tank, set within the encasement, as the primary means. Secondary protection is provided by the encasement plus a void between the steel tank
and encasement, which provides a zone for leak detection emanating from the steel tank.

Concrete-encased fuel tanks can be designed to resist rifle shots and ballistic damage as well as provide blast and impact resistance. Primary and secondary containment for conventional steel and fiberglass tanks are usually provided through double-wall construction. With this method, any fuel leakage from the interior tank is stopped by the exterior tank wall (i.e., second wall).

The area between the two walls provides the ability for a leak detection switch to trigger an alarm before the leakage can breach the second wall and escape the tank environment. This type of construction using a steel tank is typically what is used for portable engines on trailers.

It is critical that all fuel storage tanks be equipped with the proper fill means, overflow protection, venting, and fuel pickup to the engine. A day tank with 6 to 12 hours of equivalent fuel capacity is often used to provide an intermediary or emergency fuel storage between the engine and bulk fuel storage.

Day tanks are usually provided with a separate fuel pump that are often self-priming and designed to draw fuel from the bulk storage tank as well as onboard alarms for conditions such as tank overfill and low level to alert personnel. This prevents the engine’s fuel pump from having to draw high rates of fuel from an excessive distance or lift, which generally exceeds a standard fuel pump’s ability and can cause partial
or complete fuel starvation to the engine.

Regardless of the method, bulk liquid fuel storage should never be placed adjacent to a water well or structure/residence, with minimum clear distances of 50 feet to 100 feet generally required.

Under certain conditions, such as the type of well construction, dual-containment, leakage alarms, vandalism protection, and limited access, closer proximities may be permitted with regulatory agency approval.

All fuel storage tanks must comply with UL 142 and 2085 standards for fuel storage tanks, NFPA 30, and Uniform Fire Code for a two-hour minimum fire rating, and all local codes.

Propane storage is generally provided by using UL- or FM-approved fuel tanks for this specific purpose. Natural gas fuel is generally provided by the local gas utility through a standard 2 psi natural gas service to the meter. A solenoid valve, tied to the engine control panel, is generally used to shut off the fuel supply when the engine fails or is otherwise not in operation.

The permitted method of fuel storage always varies with individual jurisdictions as well as storage volume. Therefore, designers must verify the specific requirements with local authorities.

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This wraps up this installment of The Water Works. We will conclude our discussion on fuel storage methods and capacity in the July edition.

Until then, keep them pumping!

Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He was honored by the National Ground
Water Association with the 2025 Technology Award. He can be reached at epbpe@juno.com.