Das Kältemittel fließt durch den Kältekreis.

Because German very often forms compound nouns by joining shorter words into one longer word.

• Kältemittel = Kälte
  • Mittel
    Literally, something like cold-agent or cooling medium. In technical English, this is usually refrigerant.
• Kältekreis = Kälte
  • Kreis
    Literally cooling circuit or refrigeration cycle/circuit.

This is completely normal in German. English often uses separate words where German prefers one compound word.

These are two different things happening:

1. das Kältemittel is the subject of the sentence, so it stays in the nominative case.
2. den Kältekreis comes after durch, and durch takes the accusative case, so der Kältekreis changes to den Kältekreis.

So:

• nominative: der Kältekreis
• accusative: den Kältekreis

And:

• nominative/accusative neuter: das Kältemittel

Yes. Durch is an accusative preposition.

So you say:

• durch den Kältekreis
• durch die Leitung
• durch das Rohr

A useful rule is: after durch, use the accusative.

Because German main clauses normally follow the verb-second rule.

In this sentence:

• Das Kältemittel = first element
• fließt = second element
• durch den Kältekreis = the rest

So the pattern is:

Subject + verb + other information

That is the normal word order for a simple German statement.

You could also move another element to the front, but then the verb still stays second:

• Durch den Kältekreis fließt das Kältemittel.

That sounds more marked or stylistic, but it is still correct.

Because the subject is das Kältemittel, which is third person singular: it flows.

The infinitive is fließen = to flow.

Present tense conjugation:

• ich fließe
• du fließt
• er/sie/es fließt
• wir fließen
• ihr fließt
• sie/Sie fließen

Since das Kältemittel is grammatically es, the correct form is fließt.

Yes. Fließen is a strong verb.

That matters especially in the past tense:

• present: fließen
• simple past: floss
• past participle: geflossen

Examples:

• Das Kältemittel fließt ...
• Das Kältemittel floss ...
• Das Kältemittel ist durch den Kältekreis geflossen.

Fließt is pronounced roughly like fleest in English, but with a clear German ie sound, which is a long ee sound.

About the spelling:

• ß is used after a long vowel or diphthong in standard German spelling.
• In fließt, the ie is long, so you get ß.

Compare:

• fließt with ß
• Fluss with ss

So the spelling helps show vowel length.

Kalt is an adjective meaning cold.
Kälte is a noun meaning cold or cooling/coldness, depending on context.

In compound nouns, German often uses the noun form, not the adjective. That is why you get:

• Kältemittel
• Kältekreis

not:

• Kaltmittel
• Kaltkreis

So Kälte- here is the normal compound form.

Yes, that is the literal structure:

• durch = through
• den Kältekreis = the cooling/refrigeration circuit

But depending on context, English might use a more natural technical phrasing such as:

• The refrigerant flows through the refrigeration circuit.
• The refrigerant circulates through the cooling circuit.

So the literal meaning is straightforward, but the best English translation may depend on the technical context.

You could, but it would mean something slightly different.

• durch den Kältekreis = through the circuit
  This emphasizes movement along or through it.
• im Kältekreis = in the circuit
  This emphasizes location inside the system.

So if you want the idea of flowing along the circuit, durch den Kältekreis is better.

In German, all nouns are capitalized.

So in this sentence:

• Kältemittel is a noun
• Kältekreis is a noun

That is why they begin with capital letters.

The verb fließt is not capitalized because it is not a noun.

Here it is singular.

You can tell because:

• the article is das
• the verb is fließt = singular

The plural would be:

• die Kältemittel

Then the verb would change:

• Die Kältemittel fließen durch den Kältekreis.

It is mostly technical German.

• fließt is a normal everyday verb
• Kältemittel and Kältekreis are more technical terms, especially in refrigeration, air-conditioning, or thermodynamics

So the grammar is basic and common, but the vocabulary is specialized.