Bad idea to use a too big heat exchanger?

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Discussion Overview

The discussion revolves around the implications of using a heat exchanger that is larger than necessary in a water-based central heating system. Participants explore the potential drawbacks and benefits of oversized heat exchangers in this context, considering various applications and operational conditions.

Discussion Character

  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant expresses concern that using a 150 kW heat exchanger for a system that requires only 40 kW may be problematic and seeks reasons why this could be a bad idea.
  • Another participant suggests that the impact of size depends on the application, noting potential benefits such as lower pressure drop and more efficient heat exchange.
  • A specific example is provided where an oversized heating coil in an outside air handling unit could lead to low fluid velocity and uneven surface temperatures, potentially causing freezing issues.
  • A participant clarifies the specifics of their system, indicating that the heat source is a district heating system with a supply temperature of 120°C, while the radiator system operates at 70-50°C.
  • Another viewpoint suggests that for water-to-water applications, a larger heat exchanger may be advantageous, provided that pumps and control valves are appropriately sized.

Areas of Agreement / Disagreement

Participants express differing views on the implications of using an oversized heat exchanger, with no consensus reached on whether it is inherently a bad idea. Some highlight potential benefits while others caution against specific operational issues.

Contextual Notes

Participants have not fully defined the parameters of "too big" or the specific conditions under which a heat exchanger operates, leading to varying interpretations of its effects. The discussion also lacks a detailed exploration of the implications of pump and valve sizing in relation to heat exchanger size.

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I'm constructing a water based central heating system, and in it is a plate heat exchanger. People often tell me not to be scrimpy regarding its effect when picking one. But then I've heard others say that having a 150 kW heat exchanger - when the effect needed will never rise above 40 kW - is perhaps an even worse idea. Could someone give me some point about why it's a bad idea so I can relay?
 
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It depends on the application. Often it is a significant benefit (lower pressure drop, more efficient heat exchange). When you say a "water based central heating system", what exactly do you mean? What is the heat source that heats the water via the plate heat exchanger?

An example where too big is a bad idea is a heating coil in a 100% outside air handling unit. Too big means low fluid velocity, means uneven surface temperature, means you can freeze the water in the coil even when it comes in very hot.
 
russ_watters said:
When you say a "water based central heating system", what exactly do you mean?
The hottest side of the exchanger comes from a district heating system (120°C supply). The other side consist of a normal radiator system (supply/return 70-50°C).
 
Oh, just water to water: Bigger is better. Just make sure you appropriately size any pumps or control valves .
 

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