HI;
Citric acid is white crystalline powder. Its like you said -- Sugar is sugar.
And Citric acid is citric acid. Its the std. However you can have different grades in citric acid. based on its purity.
The pharmaceutical grade is very pure but the food grade is right behind it followed by industrial grade. Many of the food grade citric acid may well qualify for pharmaceutical grade. A pharmaceutical grade is identified by letters USP at the end of a chemical. It stands for United States Pharmacopoea - Which is the official US Gov book on drug and pharmaceutical standards.
For cooling sys flush food grade is fine.
Now I know I can use my pharmacist's knowledge somewhere.
MVK