Potassium Formate and Acetate Solutions

Potassium formate (HCOOK) and potassium acetate (CH₃COOK) solutions represent a class of environmentally responsible secondary coolants developed as low-toxicity alternatives to conventional glycol-based heat transfer fluids. These organic salt solutions combine superior heat transfer characteristics with biodegradability, making them particularly suitable for applications where environmental impact, food safety, or human contact are critical concerns.

Chemical Composition and Properties

Potassium Formate (HCOOK)

Potassium formate is the potassium salt of formic acid with a molecular weight of 84.12 g/mol. The compound dissociates in aqueous solution to form K⁺ and HCOO⁻ ions.

Chemical characteristics:

• Molecular formula: HCOOK
• CAS number: 590-29-4
• Appearance: White crystalline powder (solid), clear colorless liquid (solution)
• pH of saturated solution: 8.2-8.6 (mildly alkaline)
• Maximum solubility: Approximately 75% by weight at 20°C
• Freezing point depression: -60°C achievable at maximum concentration

Production and purity:

• Manufactured by neutralization of formic acid with potassium hydroxide
• Commercial formulations typically 50-52% concentration for HVAC applications
• Food-grade potassium formate meets FCC (Food Chemicals Codex) specifications
• Technical-grade products contain corrosion inhibitor packages

Potassium Acetate (CH₃COOK)

Potassium acetate is the potassium salt of acetic acid with a molecular weight of 98.14 g/mol.

Chemical characteristics:

• Molecular formula: CH₃COOK
• CAS number: 127-08-2
• Appearance: White hygroscopic crystalline solid (pure), clear liquid (solution)
• pH of saturated solution: 7.5-9.0
• Maximum solubility: Approximately 68% by weight at 20°C
• Freezing point depression: -55°C achievable at maximum concentration

Production methods:

• Produced by neutralization of acetic acid with potassium hydroxide or potassium carbonate
• Available in food-grade (FCC) and technical grades
• Often blended with potassium formate for optimized performance

Thermophysical Properties

Potassium Formate Solutions

Potassium Acetate Solutions

Viscosity Characteristics

Potassium Formate Dynamic Viscosity (mPa·s):

Potassium Acetate Dynamic Viscosity (mPa·s):

Temperature-viscosity relationship: Viscosity follows an Arrhenius-type temperature dependence. For potassium formate at 48% concentration:

μ(T) = A × exp(B/T)

where:

• μ = dynamic viscosity (mPa·s)
• T = absolute temperature (K)
• A ≈ 0.085 mPa·s
• B ≈ 1850 K

This exponential relationship means viscosity increases rapidly at lower temperatures, impacting pumping requirements and heat transfer coefficients.

Heat Transfer Performance

Prandtl Number Analysis

The Prandtl number (Pr = μcₚ/k) indicates the ratio of momentum diffusivity to thermal diffusivity:

Potassium Formate 48% at various temperatures:

Higher Prandtl numbers at low temperatures indicate thicker thermal boundary layers and reduced heat transfer coefficients compared to operation at moderate temperatures.

Heat Transfer Coefficient Comparison

For turbulent flow in smooth tubes (Re > 10,000), using the Dittus-Boelter correlation:

Nu = 0.023 Re^0.8 Pr^0.4

Relative heat transfer coefficients (water at 20°C = 1.00):

• Potassium formate 48% at 20°C: 0.72
• Potassium formate 48% at -10°C: 0.48
• Potassium acetate 48% at 20°C: 0.74
• Ethylene glycol 40% at 20°C: 0.68
• Propylene glycol 40% at 20°C: 0.62

Potassium-based solutions demonstrate superior heat transfer compared to glycol solutions at equivalent concentrations, primarily due to lower viscosity and higher thermal conductivity.

Film Coefficient Considerations

For design calculations, the reduction in film coefficient relative to water ranges from:

• 25-30% at normal operating temperatures (5-20°C)
• 45-55% at freezing protection temperatures (-10 to -20°C)

This must be accounted for in heat exchanger sizing, particularly for low-temperature applications.

Corrosion Characteristics

Material Compatibility

Potassium formate and acetate solutions exhibit excellent corrosion resistance when properly formulated with appropriate inhibitor packages.

Corrosion rates for uninhibited solutions (mils per year at 60°C):

Corrosion rates for properly inhibited solutions (mils per year at 60°C):

Corrosion Inhibitor Systems

Commercial formulations incorporate multi-metal corrosion inhibitors:

Common inhibitor components:

• Sodium molybdate (Na₂MoO₄): Anodic inhibitor for ferrous metals, 200-500 ppm Mo
• Sodium silicate (Na₂SiO₃): Forms protective films, 50-200 ppm Si
• Benzotriazole (BTA): Copper and brass protection, 5-20 ppm
• Tolyltriazole (TTA): Enhanced copper protection, 5-20 ppm
• pH buffers: Maintain alkaline pH (8.0-9.5) for optimal inhibitor performance

Inhibitor depletion mechanisms:

• Consumption by corrosion reactions (primary mechanism)
• Thermal degradation at elevated temperatures (>120°C)
• Dilution from makeup water addition
• Precipitation in hard water conditions

Material Selection Guidelines

Ferrous metals: Carbon steel and cast iron are acceptable for inhibited potassium formate/acetate systems. Ensure inhibitor concentration is maintained per manufacturer specifications.

Copper and copper alloys: Fully compatible with inhibited solutions. Copper provides excellent thermal performance for heat exchangers and coils.

Aluminum: Not recommended for continuous contact even with inhibited solutions. Corrosion rates remain elevated compared to other metals. Use alternative materials for heat exchangers.

Stainless steel: Excellent compatibility. Recommended for high-reliability applications, pharmaceutical, food processing, and where long service life is required.

Elastomers and gaskets:

• EPDM (ethylene propylene diene monomer): Excellent compatibility
• Nitrile (Buna-N): Good compatibility
• Viton (fluoroelastomer): Excellent compatibility
• Neoprene: Good compatibility
• Natural rubber: Not recommended, degrades over time

Environmental and Safety Considerations

Environmental Profile

Biodegradability: Both potassium formate and acetate are readily biodegradable under aerobic conditions:

• BOD₅/ThOD ratio: >60% (readily biodegradable per OECD 301 criteria)
• Complete mineralization to CO₂, H₂O, and potassium salts
• No bioaccumulation potential
• Low aquatic toxicity: LC₅₀ > 1000 mg/L for most aquatic organisms

Comparison to glycols:

• Ethylene glycol: Toxic (LD₅₀ oral rat: 4700 mg/kg), moderately biodegradable
• Propylene glycol: Low toxicity (LD₅₀ oral rat: 20,000 mg/kg), readily biodegradable
• Potassium formate: Low toxicity (LD₅₀ oral rat: 1500-2000 mg/kg), readily biodegradable
• Potassium acetate: Low toxicity (LD₅₀ oral rat: 3250 mg/kg), readily biodegradable

Groundwater protection: Potassium formate and acetate do not contribute to groundwater contamination:

• Rapid biodegradation in soil (half-life: <7 days)
• No persistent metabolites
• Potassium is an essential plant nutrient
• Suitable for wellhead protection zones and sensitive watersheds

Safety Characteristics

Toxicity:

• Non-toxic at concentrations used in HVAC applications
• Food-grade formulations approved for indirect food contact (FDA 21 CFR 178.3570)
• No carcinogenic, mutagenic, or reproductive toxicity
• Minimal skin and eye irritation potential

Handling requirements:

• No special PPE required beyond standard industrial hygiene practices
• Safety glasses and gloves recommended during concentrated fluid handling
• Solutions are non-flammable (flash point: none)
• No DOT hazardous materials classification for typical concentrations

Freezing point safety margin: Select concentration to provide 10-15°C safety margin below lowest expected system temperature. Unlike glycols, crystallization produces slush rather than solid ice, reducing risk of pipe rupture.

Application Guidelines

System Design Considerations

Concentration selection methodology:

1. Determine minimum design temperature (ambient or process requirement)
2. Add 10-15°C safety margin for freeze protection
3. Select concentration from freezing point tables
4. Verify viscosity is acceptable for pumping and heat transfer at minimum temperature
5. Consider higher concentration may be needed for viscosity-limited systems

Example calculation:

• Minimum ambient temperature: -25°C
• Safety margin: -10°C
• Design freeze point: -35°C
• Required concentration: 48% potassium formate or 50% potassium acetate
• Verify viscosity at -25°C is acceptable: 28.5 mPa·s (acceptable for most systems)

Pumping System Design

Head loss correction factors: Pressure drop in potassium formate/acetate solutions exceeds water due to higher density and viscosity.

For 48% potassium formate at 20°C:

• Density ratio (ρ_fluid/ρ_water): 1.345
• Kinematic viscosity ratio (ν_fluid/ν_water): 6.5

Turbulent flow pressure drop ratio: Δp_fluid/Δp_water ≈ (ρ_fluid/ρ_water) × (f_fluid/f_water)

Using Colebrook equation for friction factor, at Re = 50,000:

• f_water ≈ 0.0180
• f_48%formate ≈ 0.0195

Pressure drop multiplier: 1.345 × (0.0195/0.0180) ≈ 1.46

Pump sizing guidelines:

• Calculate head and flow based on water
• Multiply head by density ratio and friction factor ratio (typically 1.4-1.6 for formate/acetate)
• Select pump to provide required head at corrected specific gravity
• Verify NPSH available exceeds NPSH required with solution density
• Use minimum 10% head margin for fouling and aging

Expansion tank sizing: Thermal expansion coefficient for potassium formate 48%: 0.00065 per °C

Expansion volume from -20°C to 80°C: ΔV/V = 0.00065 × 100 = 6.5%

For 1000-liter system: expansion volume = 65 liters minimum With 50% fill level: expansion tank capacity = 130 liters minimum

Heat Exchanger Selection

Shell-and-tube heat exchangers:

• Potassium formate/acetate on tube side preferred for easier cleaning
• Consider tube-side velocity 1.0-2.5 m/s for balance of heat transfer and pressure drop
• Use enhanced tubes (low-fin, corrugated) to offset lower film coefficients
• Apply fouling factors: 0.00018 m²·K/W (0.001 h·ft²·°F/BTU) for clean systems

Plate heat exchangers:

• Excellent performance with potassium-based solutions due to high turbulence
• Select gasket materials compatible: EPDM or Nitrile recommended
• Pressure drop can be 2-3× higher than water; verify pump capacity
• Brazed plate heat exchangers suitable for high-pressure or no-gasket requirements

Brazed aluminum heat exchangers: Not recommended due to aluminum corrosion concerns even with inhibited solutions. Use stainless steel brazed units or copper-brazed plate exchangers instead.

Chillers and Cooling Equipment

Fluid cooler sizing: Account for reduced heat transfer coefficient:

• Use manufacturer software with actual fluid properties if available
• Otherwise apply 15-25% additional surface area vs. water selection
• Verify glycol compatibility rating from manufacturer (most fluid coolers accept formate/acetate)

Chiller evaporators:

• Flooded evaporators: excellent performance, minimal heat transfer penalty
• Direct expansion: larger evaporator required, typically 20-30% more surface
• Falling film: acceptable with proper distribution
• Plate-and-frame: preferred for potassium solutions, compact and efficient

Temperature limitations:

• Maximum continuous operating temperature: 95°C for standard inhibitor packages
• Extended life formulations available for 110-120°C operation
• Below 150°C, formate solutions remain stable
• Above 150°C, thermal decomposition accelerates; use alternative fluids

Filtration and Fluid Maintenance

Initial system cleaning:

1. Flush system with water to remove construction debris
2. Clean with mild alkaline detergent solution
3. Rinse thoroughly with clean water
4. Pressure test
5. Drain completely
6. Charge with potassium formate/acetate solution

Filtration requirements:

• Install full-flow or side-stream filtration: 25-50 micron bag or cartridge filters
• Magnetic filters recommended for ferrous systems to capture corrosion products
• Replace filter elements when pressure drop exceeds 10 psi or annually

Fluid monitoring program:

Fluid life expectancy: With proper maintenance and monitoring:

• 5-10 years typical service life in closed systems
• Indefinite life possible with regular inhibitor replenishment
• Replace fluid if contaminated with oil, dirt, or other chemicals
• Replace if concentration cannot be maintained due to unknown leaks

Economic Analysis

Cost Comparison (Approximate Material Costs)

Life-cycle cost factors:

1. Initial fluid cost: Higher for potassium formate/acetate (20-40% premium vs. glycol)
2. Equipment sizing: Potential 10-20% larger heat exchangers offset by reduced maintenance
3. Energy consumption: Lower pumping energy than glycol (10-15% reduction) due to lower viscosity
4. Maintenance costs: Reduced corrosion monitoring and treatment
5. Disposal costs: Lower or zero (biodegradable, no special disposal)
6. Liability and insurance: Reduced environmental risk, potential insurance savings

Total cost of ownership: For environmentally sensitive applications, life-cycle costs of potassium formate/acetate typically equal or are lower than glycol solutions when all factors are considered.

Market Applications

Food and beverage processing:

• FDA-approved for indirect food contact
• USDA-accepted for meat and poultry plants
• Brewery glycol replacement
• Dairy processing facility chillers
• Fruit and vegetable cooling

Pharmaceutical and healthcare:

• Clean room HVAC systems
• Process cooling in pharmaceutical manufacturing
• Hospital and laboratory secondary coolant loops
• Contamination-sensitive environments

District cooling and heating:

• Campus central plants serving food service facilities
• Municipal systems with environmental mandates
• Systems passing through wellhead protection zones
• Backup to potable water where cross-connection risk exists

Snow melting and deicing:

• Airport runway and taxiway deicing (potassium acetate primary application)
• Hydronic snow melting systems
• Bridge deck heating systems
• Critical access roads and emergency vehicle routes

Solar thermal and renewable energy:

• Solar thermal collector loops (low toxicity advantage)
• Geothermal heat pump ground loops
• Renewable energy demonstration projects

Data centers:

• Free cooling economizer loops
• Computer room air handler secondary circuits
• Reduced environmental impact for corporate sustainability goals

Standards and References

ASHRAE Standards and Guidelines

ASHRAE Standard 15-2022: Refrigerant system design requires secondary coolants to be compatible with materials of construction. Potassium formate and acetate meet material compatibility requirements when properly inhibited.

ASHRAE Standard 90.1-2022: Energy efficiency considerations include:

• Proper fluid selection for minimizing pumping energy (Section 6.5.4)
• Heat exchanger efficiency with secondary coolants
• Temperature reset strategies to reduce viscosity penalties

ASHRAE Guideline 3-2018: Recommends fluid property verification and proper concentration maintenance for heat transfer fluids. Outlines monitoring procedures applicable to potassium formate/acetate systems.

ASHRAE Handbook - HVAC Systems and Equipment: Chapter 13 (Medium- and Low-Temperature Refrigeration) discusses secondary coolant properties including potassium-based solutions.

ASHRAE Handbook - Fundamentals: Chapter 31 (Physical Properties of Secondary Coolants) provides property data for potassium formate solutions.

Industry Standards

NSF/ANSI Standard 60: Potassium formate and acetate formulations are available with NSF 60 certification for use in systems with potential for potable water contact.

FDA 21 CFR 178.3570: Food-grade potassium formate approved as indirect food additive for use in heat transfer fluids in food processing facilities.

USDA approved: Select formulations approved for use in federally inspected meat and poultry plants.

SAE AMS 1435: Aerospace Material Specification for potassium acetate deicing fluid (airport ground operations).

Environmental Regulations

OSHA 29 CFR 1910.1000: Potassium formate and acetate are not listed as regulated substances. Standard industrial hygiene practices apply.

EPA regulations:

• Not listed as hazardous waste (40 CFR 261)
• Not subject to EPCRA Section 313 (TRI) reporting
• Not a CERCLA hazardous substance
• Biodegradable per EPA guidelines

State and local regulations: Some jurisdictions mandate biodegradable heat transfer fluids for specific applications:

• California: Environmental sensitive areas
• New York: NYC DEP wellhead protection zones
• Various states: Near public water supply sources

Design Checklist

System Planning Phase

• Determine minimum design temperature including safety margin
• Calculate required freeze point and select concentration
• Verify fluid meets environmental requirements for site location
• Confirm material compatibility for all wetted components
• Obtain approval from equipment manufacturers (chillers, heat exchangers)
• Review food-grade requirements if applicable
• Calculate life-cycle costs including initial fluid, equipment sizing, and maintenance

Design Development Phase

• Calculate pressure drop with corrected fluid properties
• Size pumps for actual fluid density and viscosity
• Apply heat transfer correction factors to all heat exchangers
• Size expansion tank for fluid thermal expansion coefficient
• Specify corrosion-resistant materials or compatible coatings
• Design filtration system (full-flow or side-stream)
• Specify fluid quality: inhibited, concentration, food-grade if required
• Provide makeup fluid storage and fill/drain connections
• Include fluid testing ports for concentration and inhibitor monitoring

Construction and Commissioning Phase

• Verify system cleanliness before fluid charge
• Pressure test system with water before final fluid charge
• Drain all water completely to prevent concentration dilution
• Fill system with premixed fluid at specified concentration
• Verify concentration with refractometer after filling
• Test pH and adjust if necessary
• Document initial fluid properties (concentration, pH, inhibitor level)
• Establish baseline pressure drops and temperatures
• Train operations staff on monitoring and maintenance procedures
• Provide Material Safety Data Sheets (SDS) and technical literature

Operations and Maintenance Phase

• Monitor fluid concentration quarterly (minimum)
• Test pH quarterly
• Perform annual comprehensive fluid analysis (concentration, pH, inhibitor, metals)
• Maintain fluid monitoring log
• Add inhibitor or adjust concentration per test results
• Inspect for leaks and repair promptly
• Change filters per schedule or pressure drop indication
• Maintain spare fluid inventory (5-10% of system volume)
• Plan fluid replacement at end of service life (5-10 years typical)

Conclusion

Potassium formate and potassium acetate secondary coolants provide an environmentally responsible alternative to conventional glycol-based heat transfer fluids. The principal advantages include biodegradability, low toxicity, superior heat transfer characteristics, and excellent corrosion resistance when properly inhibited. These benefits make potassium-based solutions particularly well-suited for food processing, pharmaceutical manufacturing, environmentally sensitive locations, and applications where human or environmental safety is paramount.

Design considerations include higher initial fluid cost offset by potential equipment size reductions due to better heat transfer, lower energy consumption from reduced viscosity, and simplified environmental compliance. Proper system design requires accounting for fluid property differences from water, including density, viscosity, specific heat, and thermal conductivity effects on pumping and heat transfer.

Material compatibility is excellent for ferrous metals, copper, and stainless steel when inhibited formulations are used. Aluminum should be avoided. Routine monitoring of concentration, pH, and inhibitor levels ensures long-term system reliability and corrosion protection. With appropriate design, installation, and maintenance practices, potassium formate and acetate solutions deliver reliable, environmentally responsible heat transfer performance for 5-10 years or longer in closed-loop HVAC applications.