15 Water Treatment, Pump Blower

Updated on 01/04/26 by Mananjay MahatoShare on WhatsApp

1. Water Treatment in RAC Systems

Water is primarily used in Cooling Towers and Chilled Water Systems. Untreated water causes scale, corrosion, and biological growth, reducing heat transfer efficiency.

Key Contaminants & Effects

  • Scale Formation: Caused by Calcium and Magnesium salts (Hardness). It acts as an insulator on condenser tubes, increasing head pressure.

  • Corrosion: Caused by dissolved oxygen, low pH (acidity), or chlorides. It leads to thinning of pipes and leaks.

  • Algae & Slime: Biological growth that blocks water flow and can harbor Legionella bacteria.

Common Treatment Methods

  • Mechanical Cleaning: Using brushes or “bullet” cleaning for condenser tubes.

  • Filtration: * Side-stream Filtration: Filters about 10% of circulating water to remove suspended solids.

  • Chemical Treatment:

    • Softening: Using Ion-Exchange resins to replace Ca/Mg with Sodium.

    • Inhibitors: Phosphates or Chromates (older) to form a protective film against corrosion.

    • Biocides: Chlorine, Bromine, or Algaecides to kill biological growth.

  • Blowdown (Bleed-off): Removing a portion of concentrated water to reduce TDS (Total Dissolved Solids) and replacing it with fresh “make-up” water.

    • Cycles of Concentration (C.O.C): The ratio of dissolved solids in tower water to solids in make-up water. High C.O.C saves water but increases scale risk.

2. Pumps in RAC Systems

Pumps circulate chilled water to AHUs (Air Handling Units) and condenser water to cooling towers.

Types of Pumps

  • Centrifugal Pumps: The most common in HVAC. They use a rotating impeller to add kinetic energy to the fluid.

    • Best for: High flow rates and medium pressure.

    • Key Parts: Impeller, Casing (Volute), Shaft, Mechanical Seal/Gland Packing.

  • Positive Displacement (Reciprocating): Rarely used for water circulation; mostly used for chemical dosing or high-pressure cleaning.

Important Terminology for Exams

  • Priming: Filling the pump casing and suction pipe with liquid to remove air before starting. Centrifugal pumps require priming if located above the water level.

  • Cavitation: Formation of vapor bubbles in low-pressure areas (impeller eye). It causes “pitting” damage and loud noise (like pumping gravel).

  • Net Positive Suction Head (NPSH): The minimum pressure required at the suction port to prevent cavitation.

  • Mechanical Seal: Prevents water leakage along the shaft. Superior to “Gland Packing” as it requires less maintenance and doesn’t leak.

3. Blowers and Fans

Blowers move air across evaporator coils (indoor) and condenser coils (outdoor).

Classification

  1. Centrifugal Blowers (Sirocco/Squirrel Cage):

    • Air enters the center and exits at 90°.

    • Used in AHUs and FCUs because they can overcome high “Static Pressure” (resistance from ducts/filters).

    • Types: Forward-curved (quietest), Backward-curved (most efficient).

  2. Axial Fans (Propeller Type):

    • Air moves parallel to the shaft.

    • Used in Air-Cooled Condensers and Window ACs.

    • Best for: Moving large volumes of air at low static pressure.

Drive Systems

  • Direct Drive: Motor shaft is connected directly to the blower. Common in small units (Window/Split AC).

  • Belt Drive: Uses V-belts and pulleys. Allows speed adjustment by changing pulley sizes. Common in large commercial AHUs.

Summary Table for Quick Revision

Component Main Goal Key Maintenance Point
Water Treatment Prevent Scale/Corrosion Check pH (7.0–9.0) and TDS levels regularly.
Centrifugal Pump Circulate Water Ensure proper Priming and check for leaks in Mechanical Seals.
Centrifugal Blower Move air through ducts Check Belt Tension and clean the “Squirrel Cage” blades.
Axial Fan Heat rejection (Condenser) Ensure no obstructions; check for blade balance to avoid vibration.

  • Example Fan Law: Airflow ($Q$) is directly proportional to RPM ($N$). $Q_1/Q_2 = N_1/N_2$.

15 Water Treatment, Pump Blower