Semi dry flue gas scrubbing systems use a spray dryer, coupled with either a precipitator or a fabric filter for a combined acid gas and particulate removal system.

Semi-welded plate heat exchanger is a welded plate heat exchanger designed specially for handling aggressive media. The semi-welded plate pack consists of welded channels, which alternate with traditional casketed channels. It is typically used for heating and cooling of petrochemicals, cooling of sulphuric acid, evaporation and condensing in refrigeration plants.

Semi-Aqueous Cleaners are a group of cleaning solutions composed of natural or synthetic organic solvents, surfactants, rust inhibitors and other additives. The term semi-aqueous refers to the use of water in some part of the cleaning process, such as washing, rinsing, or both. In general, they have a good ability to remove heavy grease, waxes and tar. Their low surface tension allows them to penetrate small spaces. Some also have low vapor pressure and thus have low volatile organic compound (VOC) emissions. Depending on the type of cleaner, adverse effects will vary; some may be toxic to aquatic life, others are highly flammable or need rinsing and drying.

Semi-Aqueous Emulsion Cleaners contain non-chlorinated solvents along with emulsifiers and inhibitors. Since they contain non-volatile materials, a rinse is essential. Many semi-aqueous cleaners are flammable and some have worker exposure limits. Most semi-aqueous cleaners emulsify soils, dispersing them throughout the liquid; as the cleaner is used, the entire bath becomes saturated. Many of the citrus based cleaners are considered semi-aqueous.