Engineering of Water Systems

Standby and Backup Systems: Engine Rooms

By Ed Butts, PE, CPI

We have reviewed the basics and sizing of standby and backup engines and generators in the past few columns. In this latest installment, we will review the fundamentals and design criteria for engine and generator rooms.

I caution you, however, that the following information is based on my personal experience and local codes. The applicable codes and regulations in your region may vary, so please verify them before embarking with a room design.

Engine and Generator Room Design

Design of an enclosed room or structure to house and operate a stationary engine or generator plant requires specific considerations of airflow, ventilation, limiting temperature rise, and fuel storage and supply piping.

Many of these variables are included in local or state fire or building codes. NFPA 37: Standard for the Installation and Use of Stationary Combustion Engines and Gas Turbines is the applicable code for most engines or generator sets within a building. Designers are encouraged to obtain a copy and follow all applicable codes. A designer must always adhere to these or more stringent regulations, when applicable.

This section is offered as the minimum design considerations for both engine and engine/generator (genset) service when alternate procedures or codes are unavailable.

Air Requirement: For gensets at ΔT < 20°F: Required CFM = kW rating × 45 + combustion air

Rule of Thumb: Combustion air estimate: gas: CFM = generator kW rating × 2.40 diesel: CFM = kW rating × 3.50

Example 1: For 100 kW diesel genset. What is the estimated CFM requirement for combustion and inlet air?

Solutions: a) Combustion air estimate: 100 kW × 3.50 = 350 CFM

b) Inlet air estimate for ΔT< 20°F: 100 kW × 45 + 350 CFM = 4850 CFM

Refer to Table 1a for approximate diesel and Table 1b for natural gas fuel consumption at various levels of genset loading.

Estimating Generator Heat Output

The heat radiated by the generator (HRG) itself can be estimated by the following formulas. This is added to the heat radiated by the engine to derive the total heat output:

Equation 1a: HRG (kW) = P × [(1 ÷ Ƞ) – 1]

Equation 1b: HRG (BTU/min.) = P × [(1 ÷ Ƞ) –1] × 56.9

Where: HRG = Heat radiated by the generator (kW) or (BTU/min.)

P = Generator output at maximum engine rating (kW)

Ƞ = Generator efficiency (% ÷ 100%)

Example 2: A 975 kW genset has an efficiency of 92%. Find the heat rejection in kW and BTU/min.

Solution: Given: P = 975 kW – Efficiency (Ƞ) = 92% ÷ 100% = 0.92

HRG (kW) = 975 × [(1 ÷ 0.92) – 1] = 84.78 kW

HRG (BTU/min.) = 975 × [(1 ÷ 0.92) – 1] × 56.9 = 4824 BTU/min.

Heat Balance

Engines and generator sets need to breathe, as they require large amounts of air to both cool and create combustion in the cylinders. It has been estimated that up to 20% of the heat content of the fuel consumed by the average internal combustion engine is dissipated to the surrounding area as heat. This heat must be removed from the site to maintain proper and safe engine and genset operation and life.

A properly designed engine room ventilation system will generally maintain engine room air temperatures within a range of 15°F to 25°F above the ambient air temperature, although up to a 30°F rise is occasionally used depending on the local maximum ambient temperature. A maximum temperature rise of 20°F is often selected.

When the engine is placed inside an enclosed environment or room, the air temperature can be maintained with induced draft or supplementary ventilation fans in the absence of a unit-mounted radiator fan. Heat balance is a term used to describe the heat input into the engine which equals the sum of the heat and work outputs. This relationship is shown in the following basic word equation:

Total heat input = Work output + Total exhaust heat + Radiant heat + Jacket water + Oil cooler + Aftercooler

Maintaining a proper heat balance is important for all engine-driven pumps or generators, but critical for an operating engine situated within an enclosed environment such as a closed room or building. The heat generated by the entire engine including the exhaust stack and muffler, radiant heat to atmosphere, battery and alternator, and heat emitted by the cooling system must be included in addition to the driven machine
such as a generator or water pump and electrical switchgear within the room.

The primary aspects of a properly designed engine room ventilation system are to provide an adequate volume of cooling and combustion air. Cooling air refers to the free flow of air that removes the radiant heat generated from the engine, which includes the radiant heat from the engine, muffler and
exhaust piping, turbocharger, aftercooler, and radiator, generator, and any other driven equipment, electrical equipment within the room, and all other heat-emitting engine room components.

Combustion air describes the volume of air the engine requires to completely burn the specific type of fuel needed to convert fuel energy to output power. Combined cooling and combustion air directly impacts an engine and package unit performance and dependable service life. These must always be considered in the design of an engine room ventilation system.

It is also important to consider all ancillary engine room equipment including switchgear in the design of a ventilation system and to provide a safe and comfortable environment for service personnel to perform any required maintenance while the engine is operating. Some driven equipment, such as a generator situated in a large engine installation, may require a dedicated ventilation source as both the engine and generator independently produce heat while running.

In many cases, supplementary airflow to an engine is required to create the necessary heat balance condition. This is particularly true when the engine is a stationary engine situated inside of an enclosed environment, such as a generator or engine room.

Besides the air required for combustion along with ambient cooling, a means of providing supplemental air is required if the engine is radiator and fan cooled and the radiator fan capacity is less than the engine’s heat rejection. This is needed to replace the air brought over the engine by the fan to cool the engine. Generally, this is a substantial and adequate volume (Figure 1).

Water jacket-cooled engines, although not needing an air supply for a fan and radiator, nonetheless require a continuous source of supplemental air in addition to the air required for engine combustion to remove the radiant heat generated by the operating engine, including a turbocharger and aftercooler. In these situations, supplemental air is also required to provide ample cooling to the generator, alternator, electrical components, and any other heat-emitting device within the environment.

For best performance, engines and generators enclosed within a room should not be exposed to an air temperature rise greater than 20°F to 30°F over the ambient air temperature. This means in an environment where the ambient air temperature is 80°F, the maximum temperature within the room should not exceed 100°F. In no case should the room’s operating temperature exceed 120°F. Any application in which the ambient temperature exceeds 100°F must be individually addressed to determine the need for supplemental cooling.

Ventilation and Engine Room Supplemental Air Movement

There are several methods used to draft air into an engine room for cooling. These various methods are one element of determining the volume of air needed to an enclosed room engine as the routing factor (F). The routing factor is a multiplier applied to the basic heat rejection value from Equation 2 (shown later) and reflects the relative efficiency of the different methods of passing air over the engine and driven equipment
to remove the generated heat from the room.

One of the most efficient methods of removing this heat is with a system that pulls air past and over the engine and generator (when present) from back to front. Outside air is brought into the engine room through a single louver or system of multiple ducts or louvers. These inlet air entry points should be routed behind the unit and direct air over the engine and driven equipment and, using a radiator pusher fan, discharge the air from the front of the engine as shown in Figure 1.

Ventilation air exhaust fans should be mounted or ducted at the highest point in the engine room and direct the air away from the radiator fan discharge to prevent short-circuiting. They should be mounted directly over the heat sources. This system has an F factor of 1 and provides one of the best ventilation methods 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 the engine room air temperature. If air curtains are used, the airflow should gather this radiant heat just above the engine, which offers greater efficiency and less exposure to high air velocities in other areas of the room.

Airflow should be upward around each engine, or in the case of engines with integral mounted radiators, across the back of the engine to the front. The ventilation system should sufficiently move air to equally control temperature in all areas of the engine room.

In addition to the ventilation method previously discussed, there are four other established methods.

Type 1 (Preferred Design) (Routing Factor F 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. Ventilation air exhaust fans should be mounted or ducted at the highest point in the engine room. They should be directly over the heat sources. This Type 1 system provides among the best ventilation with the least amount of air that is required.

Type 2 (Skid Design) (Routing Factor F of 1)
Outside air is brought into the engine room through a system of ducts and routes it between engines. Type 2, however, directs airflow under the engine and generator so the air is discharged upward at the engines. The most economical method to achieve this design is to use a service platform. The platform is built up around the engines and serves as the top of the duct. Ventilation air exhaust fans should be mounted or ducted at the highest point in the engine room. They should be directly over the heat sources. This system provides one of the best ventilation methods with the least amount of air required.

Type 3 (Alternate Design) (Routing Factor F of 1.5)
If ventilation Types 1 and 2 are not feasible, an alternative is Type 3. However, this routing configuration will require approximately 50% more airflow than Type 1. Outside air is brought into the engine room using fans or large intake ducts. The inlet is placed as far away as practical from heat sources and discharged into the engine room as low as possible. The air then flows across the engine room. Ventilation air exhaust fans should be mounted or ducted at the highest point in the engine room. Preferably, they should be directly over the heat sources

Type 4 (Less Effective Design) (Routing Factor F of 2.5)
If Types 1, 2, and 3 are not feasible, then the Type 4 method can be used, but it provides the least efficient ventilation and requires approximately two and a half times the airflow of Type 1. Outside air is brought into the engine room using supply fans and discharged toward the turbocharger air inlets on the engines. Ventilation exhaust fans should be mounted or ducted from the corners of the engine room. This system mixes the hottest air in the engine room with the incoming cool air, raising the temperature of all air in the engine room. It also interferes with the natural convection flow of hot air rising to exhaust fans. Engine rooms can be ventilated this way, but it requires the use of large capacity ventilating fans.

Determining the Heat Rejection

The designer at this point can determine the heat rejection value from the engine, generator (if used), and exhaust system. There are numerous methods used by various manufacturers. I developed a hybrid method used on numerous enclosed room engine or gensets with great success. It combines the need to reject the known heat from the room but adds a supplementary volume for protection and as a safety factor. This involves the following elements:

• Heat radiated from the engine, including a turbocharger and/or aftercooler, in BTU/min.
• Heat rejection from the engine-mounted generator, gearhead, or pump, in BTU/min.
• Heat rejection from the muffler and exhaust pipe, in BTU/min.
• A percentage of the heat rejection from the engine-mounted radiator, in BTU/min. This will vary with the unit, but a reasonable estimate is 15% of the coolant value. Disregard for externally located radiators or heat exchangers
• Any miscellaneous heat rejection from switchgear, transformers, etc. within the room.

Although the incremental values for heat rejection from the engine, coolant, and generator are usually readily available and should always be obtained from the manufacturer, they can be estimated when possible for a standby-rated, 60 Hz, diesel-fueled, genset or pumping application using the following values:

• Engine radiant heat rejection to ambient: 5 BTU/min. (0.09 kW) per engine gross BHP
• Genset radiant heat rejection to ambient: 30 BTU/min. (0.53 kW) per generator kW
• Generator or gearhead radiant heat rejection: 7 BTU/ min. (0.123 kW) per HP/kW
• Heat to coolant: 20 BTU/min. (0.35 kW) ÷ Engine gross BHP; 65 BTU/min. (1.15 kW) per generator kW
• Heat to exhaust: 30 BTU/min. (0.53 kW) ÷ Engine gross BHP; 100 BTU/min. (1.75 kW) per generator kW

As previously cited, the above values are strictly for estimation purposes only, as each value can vary widely between manufacturers. Aftercoolers, turbochargers, oil cooler, and electrical equipment within the room can also vary and must be determined directly from the applicable manufacturer and added.

The largest contributor to heat rejection is obviously the muffler and exhaust stack. Although the above cited estimates will provide a reasonable value for the exhaust system output, it is possible to more closely estimate the actual heat rejection by using the exhaust outlet size. The data in Table 2 can be used for estimating these values for 1½-inch through 12-inch diameters.

The values in Table 2 reflect the approximate heat rejection for uninsulated mufflers and exhaust pipe. The heat rejected to the engine room will almost always be reduced through insulating the muffler and exhaust pipe, but aftercoolers and turbochargers should never be insulated. Although the percentage reduction for lowering the heat value will vary with the type and thickness of insulation, for approximation values, a reduction of 50% from the values shown in Table 2 will generally provide reasonable values.

The following equation provides the proper CFM airflow for a given engine assuming a 100°F maximum ambient temperature. When a generator is the driven equipment, the heat rejection from this must also be added to the engine. This value is typically based on the generator’s efficiency which is generally available from the manufacturer.

Equation 2a: Ventilating Inlet Air (CFM) = H                      + ECA × F
______
D × SH × ΔT

Equation 2b: Ventilating Inlet Air (CFM) at Standard Air Conditions = 55 × H + ECA × F
______
ΔT

Where:

H = Total engine heat radiation or rejection (BTU/min.), generally available from engine manufacturers

D = Density of air: at 100°F = 0.071 lb./cu ft.

SH = Specific heat of air = 0.24 BTU/lbs./°F

ΔT = Permissible temperature rise in engine room (Δ degrees in °F)

(Note: The maximum permissible engine room temperature is generally considered to be 120°F.)

ECA = Engine combustion air (air volume required for fuel combustion in CFM) (Approximate range of combustion [intake] air required for a diesel engine is 2.0 to 2.5 CFM per HP.)

F = Ventilation routing factor, Types 1-4 (Note: Ventilation airflow (CFM) should increase 10% for every 2500 feet (760 m) above sea level.)

Example 3: What is the net heat rejection and required inlet air flow from a 40 kW engine/generator set with the following manufacturer supplied parameters (assume the muffler and exhaust pipe is insulated)?

• Heat Rejection: 850 BTU/min. to atmosphere from engine, 300 BTU/min. from generator, 5-inch muffler and 12 feet of 5-inch exhaust pipe, 2700 BTU/min. from coolant.
• Air and Exhaust Flows: combustion air required: 250 CFM, exhaust flow: 500 CFM, radiator fan capacity: 5000 CFM.
• No other electrical equipment in room. Assume a 25°F maximum temperature rise and a routing factor of 1.

Solution: Add the separate values of applicable heat rejection:

850 BTU/min. from engine

300 BTU/min. from generator

1650 BTU/min. from insulated muffler and exhaust (use 50% of the value from Table 2)

405 BTU/min. from coolant (use 15% of the original value
__________
3205 BTU/min. of heat rejection                              of 2700 BTU/min.)

Apply this value to Equation 2b:

(55)(3205 BTU/min.) = 176,275 = 7051 + 250 × 1.0
_______________     _____
Max. Rise of 25°F                 25

= 7301 CFM

Since the radiator fan capacity is 5000 CFM, supplementary fan is required to remove the difference:

Supplementary fan capacity = 7301 CFM of required air flow – 5000 CFM (radiator fan) = 2301 CFM

The supplementary fan can be either upper wall or roof mounted but the designer must verify any head loss that exists during engine and fan operation. This is generally shown as inches of mercury pressure drop, such as 0.50″ Hg. This is important as even a slight pressure drop can dramatically impact a fan’s performance.

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This concludes the first of three columns I am going to author on engine and generator room design. In the April edition of The Water Works, we will continue this overview on ventilation requirements and begin a discussion on fuel storage methods and calculating the required volume.

Until then, keep them pumping!

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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 can be reached at epbpe@juno.com.