Calculation of Self-acting temperature controller
EXAMPLE
Selection of temperature controller
A direct acting temperature controller is an independent element of a heating system that does not require additional components and operates without external sources of energy.
The main task of the temperature controller is to control the heating or cooling process of the working environment by shutting off the flow of heat or coolant. The regulating ability is determined by the authority of the valve in the system being controlled, which is why it is recommended to choose a valve taking into account the curve of its regulating characteristic associated with the deviation of the temperature controller's authority from 1. Otherwise, the regulation process may be carried out in a binary mode.
The accuracy of temperature maintenance by the temperature controller depends on the hysteresis and the proportionality zone of the thermosensor, and the speed of reaction to temperature deviations depends on the time constant.
In systems with rapidly changing parameters, it is better to choose 'fast' controllers with a time constant of up to 60 seconds, and in systems with water heaters and heat accumulators, 'slower' controllers will suffice.
It is recommended to choose the thermosensor of the temperature controller in such a way that the maintained temperature is in the middle third of the adjustable range.
Calculation Methodology
The calculation and selection methodology for the temperature controller consists of determining:
- the necessary flow capacity of the controller
- the optimal range of maintained temperatures
- the speed of closing and the accuracy of maintenance
Calculation of throughput capacity
The calculation of the throughput capacity of a temperature regulator Kv is based on data on the flow of the heat carrier through it and the allowable pressure drop. It should be noted that the higher the percentage of losses on the controlled section contributed by the temperature regulator from the available excess pressure, the higher its authority and the smoother the regulation will be.
The above-described algorithm for selecting a temperature regulator, when calculating the deviation of the regulating characteristic of the valve from the authority of 1, by default uses the initial working characteristic - linear.
Calculation of the possibility of cavitation occurrence
Cavitation is the formation of bubbles of steam in the water flow, which occurs when the pressure in it drops below the saturation pressure of water vapor. The Bernoulli equation describes the effect of increasing the flow rate and reducing the pressure in it, which occurs when the passage area is narrowed. The passage area of the temperature regulator is just such a narrowing, the pressure in which can drop to the saturation pressure, and it is the most likely place for cavitation to occur. Vapor bubbles are unstable, they appear sharply and also extinguish sharply, which leads to the erosion of metal particles from the valve gate, which will inevitably cause its premature wear. In addition to wear, cavitation leads to an increase in noise during valve operation.
The main factors that affect the occurrence of cavitation:
- Water temperature - the higher it is, the higher the probability of cavitation.
- Water pressure - in front of the regulating valve, the higher it is, the lower the probability of cavitation.
- Allowable pressure drop - the higher it is, the higher the probability of cavitation. It should be noted here that in the position of the gate close to the closure, the throttled pressure on the temperature regulator tends to excess pressure on the controlled section.
- The cavitation characteristic of the temperature regulator is determined by the peculiarities of the regulating element of the valve. The cavitation coefficient is different for different types of regulators and should be indicated in their technical specifications. However, since most manufacturers do not indicate this value, the algorithm for calculation is based on a range of the most probable cavitation coefficients.
Calculation of temperature regulator for noise generation
High flow velocity in the inlet of the temperature regulator can cause high noise levels. For most rooms where temperature regulators are installed, the permissible noise level is 35-40 dB (A), which corresponds to a velocity in the inlet of the valve of approximately 3 m/s. Therefore, when selecting a temperature regulator, it is recommended not to exceed the specified velocity.
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