How to Size Chillers for Condensing Applications
Selecting the proper cooling capacity for chillers used for condensing applications helps ensure high performance and process reliability.
Sizing a chiller for condensing applications in pilot and production settings requires careful consideration of various factors. Ensuring delivery of the proper cooling capacity, safety features and other options will provide high performance and reliability.
Condensers: Calculate Size First
Understanding the distillation requirements for the process is paramount to sizing the proper chiller. Applications running the same process continually (i.e., no variation of the solvent) are the easiest case.
Factors to identify include:
- Rate of evaporation (ΔV/Δt).
- Mixture temperature.
- Condensate temperature.
Solvent properties and the desired distillation rate factor into the two following calculations.
qcondensing = ΔvH° x ΔV/Δt x ρ / MW
qsubcooling = ΔV/Δt x ρ x cp x (Tmixture – Tcondensate)
qcondensing is the heat load from condensing the vapor.
ΔvH° is the enthalpy of vaporization of the solvent at the mixture temperature.
ρ is the density of the solvent.
MW is the molecular weight of the solvent.
ΔV/Δt is the rate of evaporation.
cp is the constant-pressure specific heat of the solvent
Tmixture is the mixture temperature.
Tcondensate is the condensate temperature.
For example, consider a water distillation process desiring a rate of evaporation (ΔvH°) of 20 liters per hour (L/hr) with a mixture temperature of 176°F (80°C) and a desired condensate temperature of 68°F (20°C). The water constants are:
- Density (ρ) is 0.998 kg/L.
- Molecular weight (MW) is 18.92g/mol.
- Specific heat of liquid (cp) is 4.186kJ/kg K.
- Enthalpy of vaporization (ΔvH°) is 40.657 kJ/mol.
Given those constants and using the equations above, the calculated values are:
- Condensing heat load is 12509 W
- Subcooling heat load is 1393 W.
- Total heat load is 13902 W.
Adding the condensing and subcooling heat load will provide the total heat load required for the process. To size the chiller, add 20 to 30 percent additional capacity to compensate for any efficiency losses. In this case, a system supplying 16.5 to 18 kW will perform properly for the desired distillation process.
Distillation apparatus that routinely utilizes varying solvents and distillation rates such as pilot plants complicates matters. To size a chiller for such an application, take into account all possible scenarios — solvent and distillation rate variations — and select the conditions requiring the largest calculated chiller to perform optimally. In some instances, a reduction in distillation rate might be required in order to achieve proper condensation without overwhelming the chiller’s cooling capacity.
Installing Your Properly Sized Chiller
Once the cooling capacity requirements have been set, it is time to identify the remaining needs of the application. These include the process pump specifications, chiller location, cooling medium, need for remote communication and operation and maintenance.
Process Pump Specifications. A thorough review of the process cooling fluid flow rate and pressure requirements should direct you to an appropriate pump choice. Centrifugal and positive-displacement pumps provide wide flow-rate and pressure capabilities. Consult with the chiller manufacturer when positive-displacement pumps are used to determine if inline filtration of the cooling fluid is needed. Large applications with high pressure or high flow rates should include a pressure-relief bypass, particularly for positive-displacement pumps. A fluid bypass allows for fine adjustment of inline pressure to the cooling application. It also ensures that no damage occurs to the pump should an occlusion occur in the chiller fluid flow path. Additionally, a bypass can aid in periodic system maintenance.
Location. Proper installation, proximity and integration of the chiller to the condenser application can greatly affect the overall process performance. Minimization of heat loss with insulated tubing/piping will maximize efficiency. Utilization of a properly sized fluid-flow circuit with minimal bends will assist in pumping effectiveness. Below are some points to keep in mind when selecting a location:
- Design the fluid-flow path with the most efficient length to minimize external fluid volume.
- Avoid any flow restrictions in the fluid path by using proper inner diameter piping and connectors.
- Maximize thermal efficiency with thorough insulation on all cooling fluid piping, tubing, adapters and connectors.
Cooling Medium. Proper selection of chiller recirculating fluid plays a prominent role in cooling performance and, most importantly, scheduled maintenance. Follow the chiller manufacturer’s recommended maintenance suggestions to optimize efficiency and reduce downtime.
Given its availability and cost, water is by far the most common bath fluid media. Glycol/water mixtures improve chiller efficiency and require regular monitoring. Other fluids such as silicones can be used as well.
It is highly recommended to establish a regular fluid maintenance and replacement protocol.
Remote Communication. Large processes necessitate that external computer systems control all components. Integrating the chiller into a PLC or control system often streamlines operation. Use of a software system:
- Frees operators from continual monitoring and adjustment.
- Reduces foot traffic in the production area.
- Captures product performance data.
Chiller options for external communication include RS232, USB and analog. Controlling software is available to support chiller temperature-profile programming, data capture and monitoring of internal/external temperature and cooling power. Manufacturers also offer plug-and-play drivers for existing integration software such as National Instruments' LabView.
Maintenance. It is important to consult with the chiller manufacturer for suggested maintenance procedures. Air-cooled systems in particular should have the condensers inspected regularly and cleaned, especially in dusty environments. Many manufacturers offer on-site preventive maintenance services to keep the chiller running optimally and identify potential problems before they happen.
Review the manufacturer’s warranty terms and conditions so you are well aware of support during the warranty period. Comprehensive full-service plans should be discussed at the time of purchase. Is it worth the risk? The cost of a comprehensive full-service plan can be easily justified should the chiller break down and stop the production process.
In conclusion, by conducting a thorough analysis and calculation of the condenser application requirements, the properly sized chiller can be identified. This will ensure that the cooling capacity meets the condenser application needs. Optimizing the installation with insulated supply lines maximizes cooling performance. Integration of the chiller into a control system streamlines the process control. Remember to establish and log regular maintenance monitoring of the chiller fluid and perform routine hardware maintenance. Following these suggestions will lead to consistent cooling performance to maximize process output.