Cooling and refrigeration are required in all types of process industries. This article looks at ways to monitor flow to improve efficiency and detect operating problems.
In direct-cooling systems, the medium that requires cooling —that is, air — is in direct contact with the evaporator. Examples include cooling chambers, cooling tunnels and air-conditioning. By contrast, indirect-cooling systems use a secondary medium — for instance, a refrigerant — to transport cooling energy to the point of use. Examples include cooling milk, beer, soup or chocolate using contact cooling with plates, dipping cooling methods or a heat exchanger.
These two methods are used widely in process cooling operations. In breweries, air coolers keep hops at a certain temperature. Direct cooling is done within production processes such as preparing hops for fermentation or beer aging. Dairies also use direct cooling within processes such as pasteurization, yogurt preparation, cream and butter processing.
The chemical industry uses direct and indirect cooling for many different processes. Typical uses include separation of gases, condensation of gases, dehumidification of air, solidification of solute, storage as liquid at low pressure, removal of heat of reaction and cooling for preservation.
The life-science industry uses cooling systems for product cooling and preservation, to cool production tanks and cleaning stations, and for room cooling. Blood plasma and antibiotics are manufactured via a freeze-drying process where water is made to sublime at low pressure and low temperature.
As you can see, refrigeration and cooling energy accounts for a huge proportion of overall energy costs in many industries. A small reduction in energy consumption can lead to significant cost savings. The first step in setting up an effective measurement program is knowledge about system characteristics at different operating conditions.
Gathering Process Information
Assuming that a plant engineer is familiar with cooling systems and knows what needs to be measured to obtain the necessary information, instruments and data-acquisition systems can be employed. Such a measurement system will help determine:
- How efficient the cooling system is.
- How efficient each of the cooling compressors and pumps are.
- If there are any leakages within the cooling system.
- Which loads significantly affect cooling costs.
- How to keep the cooling system at the ideal point of operation.
- How to reduce start/stop cycles.
Analyzing the performance of a cooling system also helps to:
- Benchmark efficiency of cooling systems compared to similar systems or industry values.
- Determine the cost of refrigeration.
- Assess a system’s ability to meet added loads.
- Quantify benefits of system modifications and improvements.
- Verify predicted performance.
Measurement Requirements for Efficient Industrial Refrigeration Operation
Efficient refrigeration requires more than efficient components. It depends mainly on system configuration and operation. As refrigeration systems are typically custom engineered for each application, individual analysis of the supply and demand sides is necessary to find the ideal point of operation.
It is important to define the right measurements to help evaluate cooling system efficiency, system leakages and energy consumption. Typical measurements needed on a cooling system include:
- Volume or mass flow (typically after condenser; i.e., in liquid phase).
- Temperature in liquid line (after condenser and before expansion valve).
- Temperature in gas line (after evaporator).
- Electrical power consumption of compressor, fans and pumps.
- Pressure at condenser outlet, before expansion valve.
- Pressure at evaporator outlet.
This data can be analyzed to find system leaks, save waste heat and calculate load factors.
System leakages can be found through constant monitoring and analysis that detects trends and exposes developing faults. As an example, monitoring suction and discharge pressures can help detect refrigerant leakage. This allows a plant to repair disruptions or leakages to operate the system reliably and efficiently while improving lifetime. It also avoids potential process shutdowns that could occur when leakages keep the system from providing the required cooling. In short, detecting leaks early reduces maintenance and unscheduled downtimes.
Waste-heat-recovery refrigeration systems produce a lot of waste heat that is usable as space or water heating. The measurement of the waste-heat-recovery rate in desuperheaters or condensers is useful to quantify the energy savings due to waste-heat recovery and adjust the system to the most efficient operation.
Apart from efficiency calculations, cooling load data can be used to calculate the load factor, which helps to detect peaks in demand. The data also can be used to reduce start/stop cycles or run the system at the most efficient time such as nighttime operation and buffering cooling energy using cooling storage. In addition, the lifetime of the equipment can be lengthened if the system is running at a constant load.
Making Flow Measurements in Process Cooling Systems
Flow measurements in a cooling system typically are made using:
- Flow in liquid line, temperature of liquid, temperature of gas and pressure of gas and liquid to calculate cooling energy flow in primary circuit.
- Flow and temperature in feed line and temperature in return line to calculate cooling energy flow in secondary circuit (e.g., cold water).
- Flow and temperature in feed line and temperature of return line at consumers.
For flow metering in a primary refrigerant circuit, an ultrasonic flowmeter is recommended because there is virtually no pressure drop. Other types of flowmeters can cause a pressure drop, which can lead to problems in the system. A clamp-on ultrasonic flowmeter works well.
In some cases, a vortex flowmeter can be used. It is important to ensure the wetted materials are selected properly for use with refrigerants such as ammonia (NH3).
For flow metering a heat transfer medium such as water, almost any flowmeter will work, including electromagnetic, Coriolis mass flow and differential pressure devices.
In indirect cooling, measurement of the flow rate of a coolant such as glycol — in a secondary cooling circuit in the return line after a consumer — may be needed. This measurement, along with the temperature differential, can be used to calculate the cooling efficiency.
Other measurements needed include:
- Electrical energy consumption of the compressor (including fans).
- Heat input of absorption chiller.
- Electrical energy consumption of pumps in the distribution system
- Pressure of cooling fluid and process fluid at various points throughout the system to ensure that there are no leaks and, thus, ensure system integrity.
In conclusion, acquiring the data requires installing instrumentation on the various components in a cooling system. Processing the data requires specialty software that can either be developed by the plant or an outside expert. Alternatively, it is available in a data-acquisition system.
Installing instrumentation on process cooling and refrigeration systems and then processing the data with specialized software will help plant operators and maintenance people find problems such as leaks. Analysis of the data can also calculate load factors, detect peaks in demand, reduce start/stop cycles or run the system at the most efficient time.