Question 11 3AE02 - Third Assistant Engineer (Alt)

The Correct Answer is A **Why Option A is Correct:** Option A states that steam heating coils are usually the **header type** and **vertically installed**. This configuration is standard practice for steam heating coils (or steam preheat coils/reheat coils) in HVAC air handling units for several critical reasons related to efficient heat transfer and proper condensate management: 1. **Header Type (or Distribution Tube Type):** Steam heating coils are designed with a primary distributing header that feeds steam into multiple smaller tubes. Crucially, these smaller tubes often incorporate inner distributor tubes (known as steam distributing or "non-freeze" coils). This design ensures the steam is evenly distributed across the face of the coil and, most importantly, allows the steam to enter the tube at high velocity and heat the length of the tube quickly, preventing localized freezing of condensate, which is a significant risk in outdoor air applications. 2. **Vertically Installed:** Positioning the tubes vertically is essential for **gravity drainage of condensate**. Steam condenses back into water (condensate) inside the coil tubes. For efficient and safe operation, this condensate must drain out of the tubes immediately. A vertical installation ensures that the condensate flows straight down to the lower header and out through the trap, preventing waterlogging (known as "short cycling" or "hammering") and potential freeze-up damage. **Why Other Options Are Incorrect:** * **B) Steam heating coils are usually the serpentine type and vertically installed.** * **Incorrect (Serpentine Type):** Serpentine coils (where a single tube winds back and forth across the coil face) are common for hot water and chilled water applications. They are highly inefficient and dangerous for steam heating, especially in preheat applications, because they hinder even steam distribution and make condensate drainage extremely difficult, leading to water hammer and freeze damage. * **C) Steam heating coils are usually the header type and horizontally installed.** * **Incorrect (Horizontally Installed):** While the header type is correct, horizontal installation is highly problematic for steam. If the tubes are installed horizontally, condensate tends to pool along the bottom of the tube rather than draining immediately. This pooling causes inefficient heat transfer, leads to premature erosion, and significantly increases the risk of the trapped water freezing and rupturing the coil in cold weather applications. * **D) Steam heating coils are usually the serpentine type and horizontally installed.** * **Incorrect (Both Serpentine and Horizontal):** This combines the two worst possible features for steam coil design. Serpentine coils are difficult to drain even vertically; when oriented horizontally, proper condensate removal becomes virtually impossible, guaranteeing efficiency loss and severe damage due to water hammer or freezing.