Process Description
Purpose
The purpose of the Cooling Tower is to remove low-level heat from a process and exhaust it to the atmosphere by exchanging the heat with ambient air through direct contacting of the air and warm cooling circulating water returned from the process. An overview of the simulated cooling system is provided in Schematic 2: Overview that follows this section.
Major Equipment
The cooling system mainly consists of the Cooling Tower, CT-101, two Cooling Water Pumps P-101A/B and three process heat exchangers, E-201, E-211 and E-221 of varying heat removal capacities.
Process Overview
Cooled water is collected in a basin at the bottom of the Cooling Tower and is pumped to a supply header which feeds the three process heat exchangers. The cooling water picks up heat from the process heat exchangers by heat transfer across the surface of metal tubes in shell-and-tube heat exchangers. Warm water from the heat exchangers is collected in a return header and is routed to the top of the Cooling Tower. The warm return water falls to the basin and is cooled by contact with air drawn through the tower by a motor-driven fan on top of the Cooling Tower. The internal packing in the Cooling Tower maximizes contacting between the air and the water to ensure maximum heat exchange. Also, evaporation of a portion of the return water occurs in the Cooling Tower.
Importance
Cooling towers are an extremely important system in most continuous processes. A cooling water system allows more efficient use of plot space compared to using air-cooled exchangers to directly cool a process stream. Process heat exchangers based on cooling water are usually significantly smaller than air-cooled exchangers for the same heat transfer rate and they do not require additional fan and louver systems. The overall capital cost and the operating cost is most often lower for a cooling water system, especially when multiple process streams need to be cooled. Also, lower cooling temperatures can be attained by using a cooling water system in hotter weather than by directly using air-cooled exchangers. Lower temperatures are a result of the evaporative effect within the cooling tower as ambient air directly contacts the water.
Instrumentation
Basic Controls
This section describes the controls and instruments of the Cooling Tower. Note that there are no special controls in the Cooling Tower process; all controllers are single loop configurations.
The flow rate of makeup water to the Cooling Tower’s basin is controlled by FIC-104. The temperature of the makeup water is assumed to be the same as ambient conditions. The level of the Cooling Tower’s basin is indicated on LI-101.
The filtration system flow is indicated on FI-102 and the filter pressure drop is indicated on PDI-103.
Acid solution flow is controlled by FIC-105 and base solution flow is controlled by FIC-106. These solutions are added to the filtration system’s return line to the basin. When adding these chemicals, the filter system should be in operation to avoid locally high/low pH situations in the return line.
The supply header pressure is controlled by PIC-101 which sends excess cooling water pumped by P-101A/B back to the Cooling Tower. The setpoint is normally 50 PSIG. The total flow of cooling water sent to the supply header is indicated on FI-101 and the temperature of the supply water is indicated on TI-101.
Blowdown is taken off the discharge of the Cooling Water Pumps P-101A/B and controlled by FIC-107.
The return header pressure is indicated on PI-102 and the return water temperature is indicated on TI-102. Return water is directed to the top of the Cooling Tower with HIC-103 or to the basin with HIC-102.
The opening position of the louvers of the Cooling Tower is adjusted with HIC-108. Maximum air flow is at 100% output. At 0% output, air flow will be at a minimum but will be non-zero to avoid starving the Cooling Tower’s fan. HIC-108 is used to adjust the cooling water supply temperature. However, in very cold weather regulation of the supply temperature to a desired value may not be possible. In this case, some of the return cooling water can be bypassed around the Cooling Tower by opening HIC-102.
Process Heat Exchangers
All three process heat exchangers, E-201, E-211 and E-221 are identically instrumented as follows:
- FIC-201/211/221 control the process flows through the tube side of the respective heat exchangers.
- TIC-202/212/222 control the process outlet temperatures by adjusting the cooling water control valve. Note that these valves have a mechanical minimum stop of 10% opening.
- Cooling water flow rates are indicated on FI-202/212/222.
- Process inlet temperatures are indicated on TI-201/211/221.
- Cooling water outlet temperatures are indicated on TI-203/213/223.