Secondary Coolant (Glycol) Loop Flow and Pump Penalty

The flow a secondary glycol loop actually needs, the pumping that costs, and the temperature penalty the loop imposes on the compressor. The flow correction is the part that gets skipped. Water's familiar 500 constant is 8.33 lb per gallon times 60 minutes per hour times water's specific heat of 1.0, so it is water's number: a propylene glycol solution has a specific heat well below one and a specific gravity slightly above it, and the product SG x cp is what corrects the constant. Because that product is below one, the same duty at the same delta-T needs MORE flow, not less -- around 13 percent more for a 40 percent propylene glycol solution. More flow at higher viscosity is more head, and pump power climbs faster than the flow does. The fluid factor reported here, 500 x SG x cp, is exactly the number the hydronic flow-from-load-and-delta-T calculator asks a user to supply, so what that one takes on faith is computed here. The temperature penalty compounds separately from the flow. A direct-expansion coil sees the refrigerant; a glycol coil sees glycol that is itself warmer than the refrigerant by the chiller's approach, so the evaporator must run several degrees colder for the same room condition, and every one of those degrees is compressor power. The design case for a secondary loop is charge reduction and safety -- fewer pounds of ammonia, confined to a machine room -- and that is often the right trade. This puts its operating cost in front of the designer rather than leaving it implicit. Fluid properties, head, and the approach temperatures are ENTERED; it does not select the fluid, compute viscosity or the pressure drop, size the pump or the chiller, or evaluate freeze protection, which is what glycol-mix is for.

water-basis flow = Q / (500 x dT); the fluid factor is 500 x specific gravity x specific heat and the corrected flow is Q / (fluid factor x dT); pump bhp = gpm x head x specific gravity / (3,960 x efficiency); the temperature penalty is the chiller approach plus the coil approach, times the entered percent per degF.

The water-side transport relation with the SG x cp correction for a secondary fluid, and pump brake horsepower as pump practice writes it. The 500 constant is 8.33 lb/gal x 60 min/h x water's specific heat of 1.0, and 3,960 is 33,000 ft-lb/min per hp over that same water basis. Fluid properties, head and the approach temperatures are ENTERED. It does not compute viscosity or pressure drop, size the pump or chiller, or set freeze protection.

Two flow relations and one pump relation; no proprietary fluid data is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: load_btuh, delta_t_f, glycol_cp, glycol_sg, head_ft, pump_efficiency, chiller_approach_f, coil_approach_f, compressor_pct_per_deg_f, annual_hours, energy_rate_per_kwh, water_gpm, glycol_gpm, flow_increase_pct, fluid_factor, pump_bhp, total_approach_f, compressor_penalty_pct

• Fluid properties are entered specific heat and specific gravity vary with glycol type, concentration and temperaturethe fluid manufacturer's data
• Head is entered viscosity at low temperature raises it substantially and is not computed herethe piping design
• The compressor sensitivity is entered it depends on refrigerant, suction condition and machinethe compressor's performance data