The main purpose of an HVAC system is to provide the best indoor air quality and thermal comfort to the people who work, live, learn or play in the buildings we design. To make this happen, hydronic heating and cooling systems are used extensively. In cold temperature climates, hydronic fluids have to provide freeze protection for the system. In these instances, hydronic fluids are composed of three key ingredients:
- Water, to provide heat transfer
- A balanced inhibitor system, to protect common metals
- Ethylene glycol or propylene glycol, for freeze protection
Water is nature’s heat transfer fluid, a liquid used for centuries to provide heating and cooling. It freezes at 32°F and boils at 212°F. To extend this range, other chemicals like ethylene glycol and propylene glycol are added. Glycol-water mixtures are commonly used to provide protection in closed-loop heating and cooling systems. Glycol is also routinely used in thermal energy storage systems to allow low temperature operation – where ice is made at night and used during the day to provide cooling for the building.
But what criteria do you use to evaluate whether ethylene glycol or propylene glycol is best for a chilled and hot water system? Do you select a glycol based on heat transfer characteristics? Viscosity? Or environmental considerations? Perhaps it is simply a matter of cost.
Ethylene glycol and propylene glycol share many similar qualities; however they also have important differences. These differences can determine how you select and size a system. Your design selections impact the initial investment and costs of installing the system. Second to providing adequate building thermal comfort, systems should be designed for the owner in the most economically and energy-efficient way possible.
For example, the amount of material in the coil, number of fins, tubes, and overall size, determine the coil’s initial cost. The size of the coil also influences the air handler’s weight, size, and footprint. As the size of the coil increases, so does the size of the air handler needed to house it. A larger air handler could require a larger mechanical room, reducing the usable space in the building, or could compromise the arrangement of ductwork and piping in the building.
The chilled water coil is an integral component to the air distribution system. Its geometry, the size, number of rows, number of fins, and fin spacing, contributes to the airside pressure drop and affect the power required of the fans to circulate air through the duct system. The glycol selection can determine this geometry and can affect the pumping energy required to circulate fluid through the piping system, as the viscous effects and heat transfer characteristics differ for propylene glycol and ethylene glycol. Coil performance and glycol selection can even impact the energy efficiency of the chiller.
Ethylene vs. Propylene Glycol
The most important physical properties of both ethylene glycol and propylene glycol are:
- High boiling points
- Relatively low vapor pressure
- Ability to lower the freezing point of water
Ethylene glycol and propylene glycol differ when considering physical, environmental, and heat transfer characteristics. Propylene glycol is typically used in food processing facilities and applications where there is potential for contamination of potable water or food as it is non-toxic to humans and animals. However, propylene glycol has characteristics that inhibit heat transfer and decrease efficiencies of the systems.
Glycols are more viscous than water alone. Solution viscosity increases with an increase in glycol concentration or decrease in temperature. Viscous effects directly impact the pumping energy required of a system and affect the ability of a system to transfer heat. The higher viscosity of propylene glycol, especially at lower temperatures, directly translates into higher pumping costs. There is also a reduction in cooling capacity when using propylene glycol, due to its reduced heat transfer characteristics.
Effects on Chilled Water Coil Performance
It can be shown, when using propylene glycol versus ethylene glycol, that in order to achieve an equivalent amount of heat transfer, the surface area of the coil must be increased. An increase in surface area requires additional tube rows and fins per inch, increasing the size of the coil. As mentioned before coil size determines the coil’s initial cost, air handler’s weight, size and footprint, and could compromise the arrangement of ductwork and piping in the building. An increased coil size also creates a larger pressure drop across the coil and increases the fan energy needed to supply air throughout the building’s ductwork.
Effects on Chiller Performance
The viscous effects of glycol have similar effects on the chiller’s performance as they do on the chilled water coil. Just as the viscous effects of propylene glycol decreases the heat transfer rate in chilled water coils, the heat transfer rate in a chiller is decreased as well. Using propylene glycol versus ethylene glycol is shown to decrease the water’s delta T across the chiller, which decreases the capacity and efficiency of similar chillers.
To demonstrate these effects, an experiment was performed using the DAIKIN Coil Selection software. Identical coils and operating parameters were used in the experiment. The coil performance was first tested with 30% propylene glycol followed by 30% ethylene glycol. The operating and physical characteristics were as follows:
The results from the tests on the chilled water coil are shown in the following table:
The results from the tests showed that by using ethylene versus propylene, the heat transfer of the coil increased by 47936 Btu/hr or 8.9%. In order to achieve the same amount of heat transfer when using propylene glycol, under the same operating conditions, a coil with an increased surface area of 8.9% would have to be used.
We performed a similar test on a water-cooled chiller, first run with 30% propylene glycol followed by 30% ethylene glycol. The results from the tests are shown in the following table:
The results showed that when using ethylene glycol versus propylene glycol for the same chiller, its capacity increased by 20 Tons or 5.6%. In addition, the delta T of the fluid through the chiller increased and the Integrated Part Load Value (IPLV) decreased.
These factors play a significant role when designing a system. We performed a yearly simulation with this same chiller. Using propylene glycol instead of ethylene glycol lead to 5.2% increased chiller costs. Over the life of the chiller these costs add up significantly for the building owner.
Chiller and Chilled Water Coil Selection
We also performed a study on equipment selection (Air Cooled Chiller and Air Handling Unit), for both ethylene glycol and propylene glycol, for a building requiring approximately 22,000 CFM airflow and approximately 95 Tons of cooling.
When comparing the two air handlers, it can be seen that the weight of the ethylene glycol air handler is significantly less than the one with propylene glycol. As it was mentioned before, propylene glycol will need additional rows and fins to achieve the similar heat transfer to that of ethylene glycol. In this example the propylene glycol chilled water coil requires 8 Rows of tubes for the required heat transfer, while the ethylene glycol coil only requires 5 Rows. The additional rows of tubes and fins add almost 900 pounds to the weight of the air handler.
The air pressure drop across the chilled water coil is significantly higher, approximately 0.3” w.c. for the propylene glycol, due to the additional rows of tube and fins. This directly impacts the required horsepower of the supply fan, as the air handler with propylene requires 23.92 BHP and ethylene only 22.86 BHP. This equates to 5% greater fan energy costs for using propylene.
The flow rate through the chilled water coil is greater for ethylene glycol, however upon analysis of the two systems, the higher viscous effects of propylene would cause about 12.5% greater Head for the propylene system. A yearly analysis shows this higher head leads to 8% higher pumping energy costs.
The effects on the chiller can be seen from the decreased efficiency and capacity when using propylene compared to ethylene. The capacity decreases from 95 Tons to 93.8 Tons, while the efficiency decreases from 9.9 EER to 9.8 EER, this may not seem like much of a difference but in larger buildings and applications this decreased efficiency could be much larger. Upon review the decreased efficiency for using propylene glycol equates to about 3% higher chiller energy costs.
This analysis of the effects of fluid selection on a chilled water system reminds us of the extent to which we preordain the capital and lifecycle costs of an HVAC system. Specifying the fluid not only determines the heat transfer characteristics but also the costs of moving the air and fluid.
Selecting chilled water coils for propylene glycol versus ethylene glycol requires a larger coil surface area, (additional rows and fins). Not only does this selection effect the increase the size and weight of air handler but results in an increase of about 0.3” w.c. in airside pressure drop. If assuming a Total Static Pressure required of 5” w.c., this is a static pressure increase of 6%, which equates to 0.05 BHP/1000 CFM, or an increase of 5% fan energy costs. Also for similar sized coils, ethylene glycol provides superior cooling capacity compared to propylene glycol.
The higher viscosity of propylene glycol increases the fluid friction in the chilled water coils which requires approximately 8% higher pumping energy costs due to the addition head loss from the fluid friction. The effects on the chiller are similar, such that the higher viscosity of propylene glycol decreases the efficiency and capacity of the chiller when compared to ethylene glycol. Propylene glycol also causes higher leaving water temperatures from the chiller to the chilled water coils. Energy costs associated with the chiller will be 3-5% lower when using ethylene glycol instead of propylene glycol. Overall, energy consumption with propylene glycol as compared to ethylene glycol will result in approximately a 5% cooling energy increase.
The next time you’re considering what fluid to specify in a chilled water system for freeze protection, invest a few extra minutes to explore the effects the fluid will have on the overall system (system size, cooling capacity, fan and pumping costs, and chiller performance). The potential benefits from ethylene glycol are simply too attractive to ignore.