10.3 Hydronic Chilled Water Systems, Boilers, and Process Cooling Fundamentals

Hydronic Chilled Water System Architecture

Commercial facilities, campus buildings, and industrial plants frequently utilize hydronic chilled water systems rather than direct expansion (DX) refrigerant systems to satisfy large-scale cooling loads. In a chilled water system, a central chiller cools water (or a glycol mixture), which is then pumped through insulated piping to air handling units (AHUs), fan coil units (FCUs), and chilled beams throughout the building.

Chiller Types & Compressor Configurations

Chillers are broadly categorized by their heat rejection method (Air-Cooled vs. Water-Cooled) and their compressor technology:

1. Air-Cooled Chillers: Packaged outdoor units incorporating propeller fans and finned condenser coils to reject heat directly to ambient air. Used in small to medium commercial buildings (20 to 500 tons). Lower installation cost but lower energy efficiency than water-cooled systems.
2. Water-Cooled Chillers: Installed indoors, rejecting condenser heat to a secondary water loop connected to an outdoor cooling tower. Superior energy efficiency (KW/ton), used in large commercial and industrial plants (200 to 5,000+ tons).

Standard Chilled Water Temperatures & Delta T

The HVAC industry standard design temperature parameters for hydronic chilled water cooling loops are:

• Chilled Water Supply (CWS) Temperature: 44°F leaving the chiller evaporator barrel.
• Chilled Water Return (CWR) Temperature: 54°F returning from building cooling coils.
• Standard Design Temperature Difference ($\Delta T$): 10°F ($\Delta T = 54^\circ ext{F} - 44^\circ ext{F} = 10^\circ ext{F}$). Modern high-efficiency systems may utilize high-Delta T designs (e.g., 42°F CWS / 58°F CWR for a 16°F $\Delta T$) to reduce water flow rates (GPM) and pump horsepower.

Primary / Secondary Pumping Loops & Decoupler Pipes

To ensure operational stability, central chilled water plants separate water circulation into two distinct hydraulic loops connected by a zero-pressure-drop decoupler pipe (common pipe):

  +--------------------+                    +--------------------+
  |  PRIMARY LOOP      |                    |  SECONDARY LOOP    |
  | (Constant Flow)    |   DECOUPLER PIPE   | (Variable Flow)    |
  |                    |    (Common Pipe)   |                    |
  | [Chiller Barrel]   |======[   ]========| [Building AHUs]    |
  |         |          |      |   |         |         |          |
  |  [Primary Pump]    |      |   |         |  [Secondary Pump   |
  |  (Constant Speed)  |      v   v         |      w/ VFD]       |
  +--------------------+                    +--------------------+

• Primary Loop (Constant Flow): Primary pumps circulate chilled water through chiller evaporator barrels at a constant flow rate. Chiller barrels require constant velocity to maintain laminar-to-turbulent heat transfer limits and prevent tube freezing.
• Secondary Loop (Variable Flow): Secondary pumps equipped with Variable Frequency Drives (VFDs) distribute chilled water to building coils based on two-way modulating valve demand, maintaining differential pressure across the system.
• Decoupler Pipe (Common Pipe): Connects the primary supply/return headers to the secondary supply/return headers. When secondary flow matches primary flow, net flow through the decoupler is zero. If secondary demand drops below primary pump output, excess chilled water flows backward through the decoupler to the primary return, maintaining constant flow through active chillers.

---

Hydronic Heating Boilers & Safety Controls

Hydronic heating systems use boilers to heat water or produce steam for space heating and domestic hot water.

Low-Pressure Hydronic Heating Boilers

Under ASME Boiler and Pressure Vessel Code Section IV (Heating Boilers) and local mechanical codes:

• Hot Water Heating Boilers: Designed to operate at pressures not exceeding 160 PSI and water temperatures not exceeding 250°F.
• Low-Pressure Steam Boilers: Designed to operate at steam pressures not exceeding 15 PSI.

Hydronic Expansion Tanks: Open vs. Diaphragm / Bladder

Water expands when heated. Because water is essentially incompressible, heating water in a closed piping system without expansion volume causes system pressure to skyrocket rapidly, triggering safety valves.

1. Open Expansion Tanks: Older gravity systems utilizing an open tank positioned above the highest system radiator, vented to atmosphere. Prone to oxygen absorption and corrosion.
2. Closed Diaphragm / Bladder Expansion Tanks: Modern sealed steel pressure vessels containing a synthetic rubber diaphragm or full-acceptance bladder separating an air cushion from system water.
   • Pre-Charge Setting: The air side of the bladder must be pre-charged with air or nitrogen to match the static cold fill pressure of the system (typically 12 PSI for a standard two-story residence, calculated as static height + 5 PSI).

Air Separation & Deaeration

Entrained air in hydronic piping causes pump cavitation, water hammer, radiator gurgling, and flow blockage:

• Air Separators: Installed in the main supply piping where water is warmest and pressure is lowest (at the boiler outlet). Microbubble air separators (such as Spirovent) utilize wire mesh or coalescing media to capture dissolved microbubbles, venting them out through automatic air vents.
• Automatic Air Vents: Installed at all high points in the piping network where air naturally collects.

ASME Safety Relief Valves

Every hydronic boiler MUST be equipped with an approved ASME Code Safety Relief Valve to prevent catastrophic vessel explosion from overpressurization:

• Residential / Light Commercial Hot Water Boilers: Standard ASME relief valve pressure setting is 30 PSI.
• Commercial Hydronic Systems: Set at higher ratings (e.g., 60 PSI, 125 PSI, or 150 PSI) matching system design pressure.
• Discharge Piping Rules: Safety relief valve discharge piping must extend full size (no restriction or reduction) to within 6 inches of the floor or to an approved drain, pointing downward. No shutoff valve may ever be installed between the boiler and the relief valve or in the discharge line.

---

Process Cooling Applications & Heat Exchangers

Industrial process cooling applications—such as plastic injection molding, laser cutting, food processing, and data center cooling—require continuous, precise fluid temperature management.

Industrial Fluid Coolers & Closed-Circuit Towers

Industrial fluid coolers combine a closed tube coil bundle with an outdoor evaporative cooling tower structure. Process fluid (water/glycol) stays sealed inside the coil, while spray water and fans cool the tube exterior evaporatively, eliminating process fluid contamination.

Glycol Loop Concentration & Freeze Protection

In Texas outdoor chillers, rooftop fluid coolers, and sub-freezing process loops, anti-freeze solutions (glycols) are mixed with distilled water to prevent equipment damage.

• Ethylene Glycol: Provides excellent heat transfer and low viscosity. However, it is TOXIC and strictly prohibited in systems with any potential exposure to potable water or food processing.
• Propylene Glycol: Non-toxic (GRAS - Generally Recognized As Safe). Required in food processing, beverage plants, and domestic water heat exchangers.
• Freeze Protection vs. Burst Protection:
  • Freeze Point: The temperature where ice crystals begin to form (slush stage).
  • Burst Point: The temperature where the mixture freezes solid and expands, rupturing pipes and chiller barrels.
• Refractometer Testing: Glycol concentration MUST be measured using a optical refractometer (specific gravity hydrometers are inaccurate due to temperature variation). A 30% to 40% glycol concentration is typical for outdoor freeze protection.
• Derating Performance: Adding glycol reduces heat transfer capacity and increases fluid viscosity, requiring higher pump head pressure and larger pipe sizing.

Heat Exchanger Selection: Plate-and-Frame vs. Shell-and-Tube

Heat exchangers transfer thermal energy between two isolated fluid streams.

  PLATE-AND-FRAME HEAT EXCHANGER       SHELL-AND-TUBE HEAT EXCHANGER

      Fluid A In   Fluid B Out              Fluid A In (Shell)
           |           ^                          |  +------------+
           v           |                          v  | ====Tubes==|
    [ | | | | | | | | | | ]              [ =====================  ] ===> Fluid B
           |           ^                             | ====Tubes==|
           v           |                             +------------+
      Fluid B In   Fluid A Out                    Fluid A Out

• Plate-and-Frame Heat Exchangers: Consist of a series of gasketed, corrugated stainless steel plates clamped together on a frame. Provides turbulent counter-current flow, extremely high heat transfer coefficients, compact footprint, and tight approach temperatures (1°F to 2°F). Plates can be added or removed to alter capacity.
• Shell-and-Tube Heat Exchangers: Consist of a heavy outer steel shell housing a bundle of copper or alloy tubes. Highly robust, handles extremely high pressures and dirty fluids, but requires a large footprint and maintenance pulling space.