Manage Load to Avoid Wet Stacking

Diesel engines are the most common prime mover for backup power systems. When run with insufficient load, diesels can suffer from accumulation of unburned residues in their exhaust systems, a condition known as wet stacking. This article explains how load banks can be used to avoid this condition.

The Problem with Wet Stacking

Diesel engines combust fuel without the aid of an ignition system, such as those used for gasoline-fueled engines. Instead, diesel engines compress an air/fuel mixture at high ratios, causing a temperature rise that causes fuel to self-ignite. Recall the Ideal Gas Law:

Pressure x Volume = Amount of Substance x Ideal Gas Constant x Temperature

With all other variables constant, increasing pressure via compression raises the temperature, causing the fuel to combust. Because of their design, diesels provide efficient operation at high levels of output.

For ac power generation, the frequency of the ac sinewave is proportional to engine speed. Consequently, governors control engine-generator speed to keep frequency constant. They do this by adjusting the amount of power required to meet changes in load. When less or more power is needed, the governor provides less or more fuel to meet the corresponding load demand, without changing engine speed. Running at high loads produces a greater amount of heat than running at low loads, even though engines run at the same speed either way.

Diagram of combustion in a diesel generator
From left to right, low loads can result in cool combustion temperatures and incomplete combustion, depositing carbon residues and unburnt fuel on internal engine components and downstream exhaust equipment. This can cause performance and reliability problems and increase emissions.

The characteristics that make diesels so effective at high loads also pose a potential reliability and performance issue when run lightly loaded. Running at low load can produce engine operating temperatures below designed levels. When that occurs, fuel combustion is incomplete. The products of incomplete combustion include carbon substances that deposit onto internal engine parts such as pistons, piston rings, and exhaust valves. In addition, wet, unburned fuel residues can collect downstream in the exhaust system, a condition termed “wet stacking”.

The problem with wet-stacking is that the sooty sludge from incomplete combustion accumulates on surfaces throughout the exhaust system. This causes several issues. Where deposits are heavy, they can restrict exhaust flow, causing excess back pressure that can reduce performance or even cause shutdown. They can also contaminate pollution control devices, which increases pollutant emissions. Under some conditions, the residues can also present a fire hazard.

Solutions for Wet Stacking

The most basic solution for addressing wet stacking is to run engines “harder”, that is, with greater amounts of load, thus generating more heat. On this point, some manufacturers state that, following several hours of low-load operation, a best practice is to run engines for a prescribed period of time at high load to increase engine exhaust temperatures and burn off accumulated sludge. However, this solution does not address the reason a genset was running at low-load to start with. If there was insufficient load during a generator run, where will additional load be found to correct the resulting condition? The reason for running low load must be evaluated first.

Facilities run gensets at low load for any of several reasons. The amount of facility load varies by temporal changes in facility operations as well as time of day, season of the year, weather conditions, and more. For instance, total load at a hospital on a summer afternoon when the temperature is 85°F (29°C) will far exceed the total load during the following night when ambient temperatures dip to 55°F (13°C). When using building load to test generators, managers may elect to run tests at night precisely because there is less operational activity to disrupt. However, this can result in insufficient load for testing. Furthermore, many facilities prefer not to use building load to avoid any potential disruption caused by testing. In either case, sufficient load must be obtained.

Load banks are a straightforward solution for developing needed amounts of load. They convert electricity to heat that is dissipated to surrounding environments. This process places load on the power source. The amount of load can vary by controlling the load bank. The type of electrical load can be addressed by specifying the appropriate load bank type (resistive, inductive, capacitive, or combinations). This offers testing flexibility in the following ways:

  1. Facilities can use supplemental load banks to achieve minimum loads at any time, regardless of real-time total building load.
  2. Facilities can avoid transferring building load for testing by using load banks to supply the entire amount of load.


For regulated facilities, prevailing North American codes require monthly testing at a minimum of 30% of generator capacity for a minimum of 30 minutes. Other parameters apply to other types of testing. Regardless of loading and duration, load banks offer straightforward solutions for testing in compliance with requirements and streamlining testing operations.

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