Views: 0 Author: Site Editor Publish Time: 2026-10-22 Origin: Site
At CSTHEATEXCHANGER, we engineer and manufacture custom-built liquid circulation heat recovery systems (commonly known as pumped run-around coil loops) specifically designed for retrofitting existing commercial, industrial, and hospital Air Handling Units (AHUs), Dedicated Outdoor Air Systems (DOAS), and Make-Up Air Units (MAUs). When supply and exhaust air ducts are physically separated by distance, walls, or building floors—and where zero cross-contamination is strictly mandatory (such as hospitals, cleanrooms, and pharmaceutical labs)—CSTHEATEXCHANGER run-around systems connect custom-dimensioned exhaust and supply finned-tube coils via a closed-loop pumped water-glycol circuit. Delivering sensible energy recovery efficiencies up to 70%, low air-side pressure drops (ΔPair≤85 Pa to match existing AHU fan limits), and 100% slide-in physical drop-in accuracy, our retrofit solutions slash annual building chiller and boiler thermal loads by up to 55% without replacing existing AHU cabinets.
Across thousands of existing commercial high-rises, hospitals, universities, and industrial manufacturing plants, aging Air Handling Units (AHUs) consume massive amounts of boiler steam, natural gas, and chilled water to condition 100% raw outdoor air.
Building owners and energy service companies (ESCOs) seeking to upgrade existing ventilation systems to meet modern decarbonization mandates face three severe physical barriers:
CHALLENGES OF RETROFITTING HEAT RECOVERY INTO EXISTING BUILDINGS
┌────────────────────────────────────────────────────────────────────────┐
│ PHYSICAL DUCT SEPARATION: Supply AHU on Roof ◄──► Exhaust Duct in Base │
│ Rotary heat wheels or plate exchangers IMPOSSIBLE without duct reroute │
├────────────────────────────────────────────────────────────────────────┤
│ ZERO CROSS-CONTAMINATION MANDATE: Hospitals, Labs & Cleanrooms (HACCP) │
│ Exhaust air contains pathogens/VOCs that CANNOT touch supply air │
├────────────────────────────────────────────────────────────────────────┤
│ EXISTING AHU CASING & FAN LIMITS: Cannot exceed existing fan static │
│ Coils must slide into existing filter/coil tracks without cabinet work │
└──────────────────────────────────┬─────────────────────────────────────┘
│
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CSTHEATEXCHANGER® PUMPED LIQUID RUN-AROUND COIL (RAC) RETROFIT SOLUTIONPhysical Spatial Separation: In most existing buildings, supply AHUs are located in rooftop mechanical penthouses while exhaust air is discharged hundreds of meters away through basement shafts or facade louvers. Installing rotary heat recovery wheels or fixed plate air-to-air exchangers would require tearing down walls and rebuilding entire ductwork risers at prohibitive costs.
Strict Zero Cross-Contamination Mandates: In hospitals (operating suites, isolation wards), BSL-3/4 research laboratories, semiconductor cleanrooms, and paint spray booths, exhaust air contains hazardous pathogens, toxic chemicals, or volatile organic compounds (VOCs). Rotary wheels and membrane exchangers carry risk of seal leakage (2% to 8%2% to 8% carryover), which is legally prohibited.
Rigid Existing Fan Static Pressure Budgets: Existing supply and return fans were sized for the original system. Adding bulky, high-resistance recovery devices causes fan motor overload and starved airflow.
A Liquid Circulation Run-Around Coil (RAC) System completely bypasses these limitations by decoupling the supply and exhaust air streams into two independent finned-tube coils linked solely by closed hydronic piping:
CSTHEATEXCHANGER® PUMPED LIQUID RUN-AROUND COIL (RAC) LOOP
┌────────────────────────────────────────────────────────────────────────┐
│ EXHAUST AIR AHU / DUCT (22°C Stale Room Air Discharged to Atmosphere) │
│ ┌────────────────────────────────────────────────────────────────────┐ │
│ │ CSTHEATEXCHANGER® EXHAUST RECOVERY COIL (HEAT HARVESTER) │ │
│ └────────────────────────────────┬───────────────────────────────────┘ │
└──────────────────────────────────┼─────────────────────────────────────┘
│ Warm / Cool Glycol Loop (Piped)
▼
┌────────────────────────────────────────────────────────────────────────┐
│ SKID-MOUNTED HYDRONIC PUMP & CONTROL MODULE (PUMP + 3-WAY VALVE + PLC) │
└──────────────────────────────────┬─────────────────────────────────────┘
│ Closed-Circuit Fluid Circulation
▼
┌────────────────────────────────────────────────────────────────────────┐
│ SUPPLY AIR AHU (Raw Outside Air Pre-conditioned Before Main Coils) │
│ ┌────────────────────────────────────────────────────────────────────┐ │
│ │ CSTHEATEXCHANGER® SUPPLY PRE-CONDITIONING COIL (100% SLIDE-IN FIT) │ │
│ └────────────────────────────────────────────────────────────────────┘ │
└────────────────────────────────────────────────────────────────────────┘Winter Heating Mode: The exhaust coil extracts sensible heat from outgoing room air (22∘C22∘C), warming the circulating water-glycol solution. The pump circulates this warm fluid to the supply coil inside the fresh air AHU, pre-heating incoming freezing outdoor air (−10∘C−10∘C up to +12∘C+12∘C) before it reaches the main building heating coil.
Summer Pre-Cooling Mode: In reverse, cool exhaust air (24∘C24∘C) cools the circulating fluid, which then pre-cools incoming hot outdoor air (35∘C35∘C down to +28∘C+28∘C), slashing peak chiller electrical draw.
Absolute 100% Isolation: The supply and exhaust air streams never cross paths. The only medium passing between them is clean, sealed water-glycol inside closed steel or copper pipes, guaranteeing 0.00% cross-contamination.
At CSTHEATEXCHANGER, we engineer custom run-around coil retrofit packages that maximize heat transfer recovery while maintaining low hydraulic and aerodynamic friction:
CSTHEATEXCHANGER® RUN-AROUND RETROFIT COIL ARCHITECTURE
┌────────────────────────────────────────────────────────────────────────┐
│ SLIDE-IN RETROFIT CASING (HEAVY Z275 GALVANIZED OR AISI 304 STAINLESS) │
│ • Exact Millimeter Dimensions Matching Existing AHU Internal Tracks │
│ • Slotted Side Flanges & Slide Rails for Zero-Modification Installation│
├────────────────────────────────────────────────────────────────────────┤
│ HIGH-SURFACE MULTI-ROW TUBE MATRIX (DEEP SENSIBLE RECOVERY) │
│ ┌────────────────────────────────────────────────────────────────────┐ │
│ │ Seamless Cu-DHP (5/8" OD) or AISI 316L Stainless Steel Tubes │ │
│ │ Optimized Multi-Pass Counter-Crossflow Circuiting (Low ΔP ≤ 35 kPa)│ │
│ │ High-Tonnage Mechanical Mandrel Expansion (>99.8% Permanent Bond) │ │
│ └────────────────────────────────────────────────────────────────────┘ │
├────────────────────────────────────────────────────────────────────────┤
│ ADVANCED HYDROPHILIC BLUE CORRUGATED LOUVERED ALUMINUM FINS │
│ • Fin Density: 10 to 14 Fins Per Inch (FPI) (Low Air ΔP ≤ 75 Pa) │
│ • Hydrophilic Silicate Coating (Water Contact Angle <15° No Drip) │
├────────────────────────────────────────────────────────────────────────┤
│ FORGED BRASS / STEEL MANIFOLDS WITH DUAL-SIDE VENT & DRAIN VALVES │
└──────────────────────────────────┬─────────────────────────────────────┘
│
▼
Sensible Recovery 60%–70% · Zero Air Mixing · 25+ Year LifeEvery CSTHEATEXCHANGER retrofit coil is custom-dimensioned to match your existing AHU's internal slide-rail tracks, finned face height, casing depth, and pipe connection locations.
Available with top, bottom, or side connection headers, allowing contractors to simply slide out the old blanking plate or damaged coil and slide in the new run-around coil during weekend shutdown windows with zero sheet-metal cutting.
Engineered with 4-row, 6-row, or 8-row deep coil configurations optimized for counter-crossflow heat transfer.
Computer-modeled internal circuitry balances fluid velocity between 0.9 m/s0.9 m/s and 1.5 m/s1.5 m/s, achieving maximum internal convective heat transfer while keeping liquid pressure drop strictly under 30 kPa−45 kPa30 kPa−45 kPa to minimize pump power.
To prevent overloading existing AHU supply and exhaust fans, CSTHEATEXCHANGER utilizes advanced sine-wave corrugated slit fins with optimized fin pitch (1.8 mm−2.5 mm1.8 mm−2.5 mm).
Keeps total air-side static pressure drop below 65 Pa−95 Pa65 Pa−95 Pa at design face velocities (2.0 m/s−2.5 m/s2.0 m/s−2.5 m/s), allowing existing building fans to operate at original airflow CFM without motor upgrades.
CSTHEATEXCHANGER supplies complete, pre-piped plug-and-play circulation skids equipped with:
Dual N+1 redundant variable-speed circulating pumps with high-efficiency IE4/IE5 EC motors.
Motorized 3-way modulating bypass control valve.
Intelligent microprocessor PLC with integrated frost-protection algorithms (automatically bypasses fluid in winter if exhaust air approaches freezing to prevent coil frost formation).
Native BACnet MS/TP or Modbus RTU interface for direct connection to central Building Management Systems (BMS).
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CSTHEATEXCHANGER RUN-AROUND COIL RETROFIT (HOSPITAL 10,000 CFM DOAS CASE STUDY)
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Operating Parameter Winter Heating Mode Summer Cooling Mode
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Fresh Outdoor Air Volume 10,000 CFM (17,000 m³/h) 10,000 CFM (17,000 m³/h)
Outdoor Air Entering Temp -10.0 °C (14.0 °F) +35.0 °C (95.0 °F)
Supply Air Leaving Temp (Preheated) +10.5 °C (50.9 °F) +27.8 °C (82.0 °F)
Exhaust Air Entering Temp +22.0 °C (71.6 °F) +24.0 °C (75.2 °F)
Circulating Fluid 30% Propylene Glycol 30% Propylene Glycol
Sensible Energy Recovery Rate 64.1 % 62.8 %
Thermal Heat Duty Recovered 118.5 kW (404,000 BTU/hr) 41.2 kW (140,500 BTU/hr)
Air-Side Pressure Drop (Coil) 78 Pa (0.31 in.wg) 82 Pa (0.33 in.wg)
Fluid-Side Pressure Drop 32.5 kPa (4.7 psi) 34.0 kPa (4.9 psi)
Annual Heating/Cooling Energy Saved 285,000 kWh / Year $28,500 USD / Year
Equipment Payback Period 14.2 Months 14.2 Months
================================================================================Airflow Capacity Range: 1,000 CFM1,000 CFM to 80,000+ CFM80,000+ CFM (1,700 m3/h1,700 m3/h to 136,000+ m3/h136,000+ m3/h) per AHU.
Thermal Recovery Efficiency: Sensible Effectiveness ηsensible=55% to 72%ηsensible=55% to 72%.
Coil Row Depths: 4-Row, 6-Row, 8-Row, or 10-Row Multi-Pass Architectures.
Tube Metallurgies: High-Purity Seamless Copper (Cu-DHP / C12200, 5/8" OD), AISI 304, or AISI 316L Stainless Steel.
Fin Spacing / Geometry: 8 to 16 FPI8 to 16 FPI (1.6 mm−3.2 mm1.6 mm−3.2 mm pitch) Corrugated Sine-Wave Aluminum or Hydrophilic Blue Fins.
Casing Material: 1.5 mm−2.0 mm1.5 mm−2.0 mm Hot-Dip Galvanized Steel (Z275 Coating) or AISI 304 Stainless Steel.
Working Pressure: Rated to 1.6 MPa1.6 MPa (230 psig230 psig); Tested hydrostatically and pneumatically underwater to 2.5 MPa2.5 MPa (360 psig360 psig).
Operating Fluid: Water, Propylene Glycol / Ethylene Glycol Solutions (10%−40%10%−40%).
Quality Certifications: AHRI Standard 410 Certified Performance, ISO 9001:2015, CE.
Hospitals have strict infection control standards that strictly forbid recirculating or mixing exhaust air with fresh air. Rotary enthalpy wheels and fixed plate heat exchangers have physical seals that can leak up to 5%5% of airborne bacteria or viruses into the supply stream. CSTHEATEXCHANGER run-around systems use completely isolated coils connected solely by closed water pipes, guaranteeing 0.00% cross-contamination while harvesting heat from operating room and ward exhaust ducts.
Our engineering team utilizes laser dimensional surveying and precision CAD modeling based on your existing AHU model numbers or on-site field measurements. We replicate the exact overall casing frame height, width, slide rail depths, and header connection centerlines, ensuring the replacement coils slide directly into existing internal coil compartments.
When outdoor air is severely cold (e.g., −15∘C), the cold circulating fluid entering the exhaust coil can cool exhaust air below 0∘C0∘C, causing condensate on the exhaust coil to freeze into frost. CSTHEATEXCHANGER intelligent pump skids feature an automated 3-way modulating bypass valve that blends warm return fluid to maintain coil surface temperatures strictly above +2.0∘C, guaranteeing 100% frost-free operation without shutoffs.
Unlock massive energy savings and meet building decarbonization targets without replacing your Air Handling Units with CSTHEATEXCHANGER:
Custom-Engineered 100% Slide-In Run-Around Replacement Coils
Zero Air Mixing & 100% Hygiene-Compliant Energy Recovery
Low Air-Side Pressure Drops Tailored to Existing AHU Fan Curves
Complete Turnkey Skidded Pumping & BMS Automation Controls
Contact our commercial HVAC retrofit engineering team today with your existing AHU dimensions, airflow CFM, and design temperatures for a complete thermodynamic payback analysis and customized CAD proposal.
Website: https://www.cstheatexchanger.com/
Technical Inquiries: info@cstheatexchanger.com
