Теплообменник заменили, с тем чтобы избежать промерзания и затопления.

This is a very common Russian pattern: 3rd person plural with no stated subject.

So Теплообменник заменили literally looks like They replaced the heat exchanger, but in real usage it often means:

• The heat exchanger was replaced
• Someone replaced the heat exchanger
• They replaced the heat exchanger in the vague English sense where they means unspecified people

Russian often uses this structure where English would prefer a passive.

Both are possible, but they feel a bit different.

• Теплообменник заменили = neutral, natural, action-focused, common in reports and technical writing
• Теплообменник был заменён = more explicitly passive, sometimes a bit heavier or more formal

Russian often prefers an active-looking sentence with an omitted agent instead of a passive construction. So this sentence is very idiomatic.

Because it is the direct object of заменили, so it should be in the accusative case.

But for a masculine inanimate singular noun, the accusative is the same as the nominative.

So:

• nominative: теплообменник
• accusative: теплообменник

That is why the form stays the same.

С тем чтобы is a formal way to express purpose. It means:

• in order to
• so as to
• with the aim of

So the second part of the sentence explains why the heat exchanger was replaced.

This expression is more formal and bookish than plain чтобы.

For example:

• заменили, чтобы избежать... = more everyday
• заменили, с тем чтобы избежать... = more formal, official, technical

Because after с тем чтобы, Russian is expressing a goal or purpose, not a completed action.

So:

• заменили = the completed main action
• избежать = what they wanted to achieve by doing it

In English this is just like in order to avoid.
The infinitive is the normal form for that idea.

Because the verb избежать takes the genitive case.

This is something you simply have to learn with the verb:

• избежать чего? = to avoid what?

So:

• избежать промерзания
• избежать затопления

not accusative forms.

This is a very important pattern:

• избежать ошибки
• избежать проблем
• избежать аварии

Промерзание is a noun formed from the verb промерзать / промёрзнуть. In technical or building-related contexts, it means something like:

• freezing through
• deep freezing
• freezing of a structure, pipe, system, or ground

It is more technical than just cold or freezing weather.
In this sentence it likely refers to unwanted freezing of equipment, pipes, or surrounding structures.

Because Russian technical and formal style often likes abstract action nouns.

So instead of saying something more verbal like:

• чтобы ничего не промёрзло и не было затопления
• or чтобы не промёрзло и не затопило

the sentence uses compact technical nouns:

• промерзание
• затопление

This gives the sentence a more official, engineering-style tone.

Because this part functions as a purpose clause attached to the main clause.

Main clause:

• Теплообменник заменили

Purpose:

• с тем чтобы избежать промерзания и затопления

Russian normally separates that kind of added purpose phrase with a comma.

Also, with multi-word conjunctions like с тем чтобы, punctuation can vary a little in other contexts, and you may sometimes see с тем, чтобы in other texts. But in your sentence, the comma before the whole expression is normal.

Because the sentence presents both actions as whole, goal-oriented events.

• заменили is perfective because the replacement was completed
• избежать is perfective because the intended result is to successfully prevent those outcomes

If you changed the aspect, the meaning would shift:

• заменяли would sound more like an ongoing or repeated process
• избегать would sound more like a general habit of avoiding something, not a single targeted result

So the perfective forms fit the sentence very well.

A native speaker would understand it immediately, but in everyday speech they would probably use a simpler version such as:

• Теплообменник заменили, чтобы избежать промерзания и затопления.

That says the same thing in a more ordinary way.

So the original sentence sounds natural, but it has a formal, written, technical flavor rather than a casual spoken one.