載冷劑是一種液體，它被用做熱傳遞的流體，而且當(dāng)?shù)玫交蚴崃坑植桓淖兤錉顟B(tài)時(shí)，它可以改變溫度。對(duì)于低溫制冷劑，就是需要一種冰點(diǎn)比水的還低的載冷劑。在這一章中，將討論各部分設(shè)計(jì)過(guò)程中要考慮的問(wèn)題、所要求的系統(tǒng)參數(shù)，以及載冷劑的應(yīng)用。（相關(guān)內(nèi)容可查閱1997年《ASHRAE Handbook—Fundamentals》的第二、三、十九、二十和三十三章）。

載冷劑的選擇
為了保證最大組成部件的可靠性及使用壽命，在任意溫度、壓力下，載冷劑與系統(tǒng)中的其它物質(zhì)必須都能夠共存，還應(yīng)符合環(huán)境要求、遵守應(yīng)用中的安全規(guī)則。而且，使用和貯存也要比較經(jīng)濟(jì)。
載冷劑應(yīng)具有-50F的最低冰點(diǎn)溫度，最好可以把它置于-150F的最低溫度下。當(dāng)載冷劑降至系統(tǒng)中的最低溫度時(shí)，它的溫度就足夠低了，可以達(dá)到滿意的熱傳遞效果和合理的壓降了。
A secondary coolant is a liquid that is used as a heat transfer fluid and that changes temperature as it gains or loses heat energy without changing into another phase. For the lower temperatures of refrigeration, this requires a coolant with a freezing point below that of water. In this chapter, the design considerations for components system performance requirements, and applications for secondary coolants are discussed. Related information can be found in Chapters 2,3,19,20, and 33 of the 1997 ASHRAE Handbook—Fundamentals.

COOLANT SELECTION
A secondary coolant must be compatible with the other materials in the system at the pressures and temperatures encountered for maximum component reliability and operating life. The coolant should also be compatible with the environment and the applicable safety regulations, and it should be economical to use and replace.
The coolant should have a minimum freezing point of 50F below and preferably 150F below the lowest temperature to which it will be exposed. When subjected to the lowest temperature in the system, the viscosity of the coolant should be low enough to allow satisfactory heat transfer and reasonable pressure drop.
The vapor pressure of the coolant should not exceed that allowed at the maximum temperature encountered. To avoid a vacuum in a low vapor pressure secondary coolant system, the coolant can be pressurized with pressure-regulated dry nitrogen in the expansion tank. However some special secondary coolant such as those used for computer circuit cooling have a high solubility for nitrogen and must therefore be isolated from the nitrogen with a suitable diaphragm.