Heat Exchanger Control & BTU Calculation

Overview

Heat Exchanger Control & BTU CalculationThe US1000-11 Digital Indicating Controller can be programmed to provide two functions, as a heat exchanger temperature controller and a BTU calculator. Why perform both functions?

The temperature control is used to maintain a constant product temperature on the discharge side of the heat exchanger. A flow meter is installed in the inlet to perform a feed forward action, compensating for demand changes and allowing optimum outlet temperature control.

A BTU calculation is performed by placing temperature elements in both the inlet and outlet piping of the heat exchanger. Using the difference of these two temperatures and multiplying by the flow rate, a BTU calculation is obtained. This can be used to identify possible fouling or scaling lowering efficiency of the heat exchanger. Additionally, if individual operating units are "charged" for their energy consumption, they become more energy conscious and cost effective.

Product Temperature Control

The outlet temperature of the heat exchanger is the process variable (PV). This is the input of the closed loop PID controller. The product flow is measured and the signal transmitted to the US1000 as a feed forward input. As the flow increases or decreases, the control output to the heating medium valve is varied according to this change in flow and optimum temperature control can be achieved. Refer to the diagram above.

BTU Calculation

The BTU (British Thermal Unit) is universally accepted as a unit of heat transfer measurement. The BTU is defined as the quantity of heat required to raise the temperature of one pound of water 1°F. The equation for heat transfer to a liquid is as follows:

Q=W(T2-T1)Cp

Q=Heat transfer per unit time (BTU/hr)
W=Mass flow rate (lb/hr)
T2=Outlet temperature (°F)
T1= Inlet temperature (°F)
Cp=Specific heat capacity (1.0 BTU/lb for water)

The US1000-11 has custom computation capabilities using a library of function blocks. In the equation above, a subtraction block determines the ΔT of the temperature inputs. A multiplication block is used for determining the product of the product flow and ΔT. Another multiplication block can used on that result if the specific heat differs from 1.0 BTU/lb (water).

The BTU's can be shown in a user defined display on the US1000-11 as instantaneous BTU transfer rate. Additionally, this BTU data can be retransmitted as a 4-20mADC signal to a recording device. The BTU data can be used to:

  1. Determine billing for the user of the heat exchanger. This can provide a fair method of energy allocation cost.
  2. A decrease in BTU transfer rate can indicate that the heat exchanger surface tubes are scaling. Recording this data can provide a guideline for preventive maintenance to clean the exchanger before failure may occur.

Summary

The US1000-11 Digital Loop Controller was designed as a high performance PID controller. Its universal design allows numerous process control applications to be configured using the same hardware platform. Custom computation capabilities allow special calculations to be performed, e.g., BTU transfer rate as described here.

Industrias

Productos y Soluciones Relacionadas

US1000

72×144×100 mm sized Digital Indicating Controller has clear bar graph display for PV, SV and MV. It comes with powerful 14 control modes, including optional Custom Computation and dual loop control. The universal input, auto-tuning, "SUPER" functions are standard. Position proportional control and RS485 communication are optionally available.

Single Loop Controllers

Single loop controllers receive temperature, flow rate, pressure, and other types of measurement readings from sensors and send corresponding instructions to control elements such as valves in order to maintain these values within a desired range.

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