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Chillers for Condensing Applications

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 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:
  • Solvent
  • Rate of evaporation (ΔVt)
  • Mixture temperature
  • Condensate temperature
Solvent properties and the desired distillation rate factor into the following calculations.
qcondensing= ΔvH° • ΔVt r/ MW;
qsubcooling= ΔVt rcp • (TmixtureTcondensate);
where

qcondensing
= heat load from condensing the vapor
ΔvH° = enthalpy of vaporization of the solvent at the mixture temperature
r = density of the solvent
MW = molecular weight of the solvent
ΔVt = rate of evaporation
cp = constant-pressure specific heat of the solvent
Tmixture = mixture temperature
Tcondensate = condensate temperature
 
Example:  a water distillation process desiring a rate of evaporation (ΔvH°) of 20 L/h with a mixture temperature of 80 °C with a desired condensate temperature of 20 °C.

Water constants:
     Density (r):                                           0.998 kg/L
     Molecular weight (MW)                           18.92 g/mol
     Specific Heat, Liquid (cP)                      4.186 kJ/kg K
     Enthalpy of Vaporization (ΔvH°)              40.657 kJ/mol
 
Calculated values
     Condensing Heat Load:             12509 W
     Sub-cooling Heat Load:             1393 W
Total Heat Load:                       13902 W
 
Adding the condensing and sub-cooling heat load will provide the total heat load required for the process. To size the chiller add 20-30% additional capacity 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 utilize varying solvents and / or distillation rates, such as pilot plants, complicates matters. 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 cooling capacity of the chiller.
 
Installation Requirements
 
Once the cooling capacity requirement specifications have been set identify the remaining needs of the application.
 
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 a wide range of flow rate and pressure capabilities. Consult with the chiller manufacturer when positive displacement pumps are used to determine if in-line filtration of the cooling fluid is needed. Large applications with high pressure and / or high flow rates should include a pressure-relief bypass, particularly for positive displacement pumps. A fluid by-pass allows for fine adjustment of in-line pressure to the cooling application, ensures that no damage occurs to the pump should an occlusion occur in the chiller fluid flow path. Additionally a by-pass 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 pumping effectiveness. Some pointers:
  • Design the fluid flow path with the most efficient length / minimizing external fluid volume.
  • Avoid any flow restrictions in the fluid path by using proper ID piping and connectors.
  • Maximize thermal efficiency with thorough insulation on all cooling fluid piping / tubing, adapters and connectors.
 
Cooling Medium
 
Choice of the chiller recirculating fluid plays a prominent role in cooling performance and, most importantly, scheduled maintenance. Follow the chiller manufacturers recommended maintenance suggestions to optimize efficiency and reduce downtime. Water is by far the most common bath fluid media given its availability and cost. Glycol / water mixtures improve chiller efficiency and require regular monitoring. Other fluids, such as silicones, can be used as well. Some pointers for these fluid options:

Fluid
Monitor Comment
Water pH / hardness within chiller limits
  particulates filter if necessary, can cause issues with positive displacement pumps
  scale if present consult chiller manufacturer for recommended descaling agents
  chlorine / iron high levels can cause pitting / corrosion of stainless steel
  algae add algaecide compatible with chiller components
Glycol / water ratio level proper ratio ensures optimum chiller efficiency
  pH within chiller limits
  scale if present consult chiller manufacturer for recommended descaling agents
Silicone water content sub-ambient chiller operation in humid environments can condense atmospheric moisture in silicone fluid
 
It is highly recommended to establish a regular fluid maintenance and replacement protocol.
 
Remote Communication
 
Large processes necessitate external computer systems control of all components. Integration of the chiller into a PLC or control system streamlines operation. Use of a software system frees operators from continual monitoring and adjustment, reduces foot traffic in the production area and captures product performance data. Chiller options for external communication include RS232, USB, analog, etc. Controlling software supports 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
 
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 which will keep the chiller running optimally and identify potential problems before they happen. Review the manufacturer’s warranty terms and conditions so that you are well aware of support during the warranty period. Comprehensive full-service plans should be discussed at 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.
 
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.

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.