1、制(zhi)冷劑熱(re)物理(li)特(te)性(xing)的(de)限制(zhi)

　　恒溫(wen)(wen)(wen)恒濕(shi)試(shi)驗機中(zhong)單級制(zhi)冷(leng)(leng)循環基本上(shang)采用(yong)(yong)的中(zhong)溫(wen)(wen)(wen)制(zhi)冷(leng)(leng)劑(ji)是(shi)R404A，在(zai)(zai)一(yi)個大(da)氣(qi)壓下(xia)其蒸(zheng)(zheng)發(fa)溫(wen)(wen)(wen)度(du)(du)是(shi)-46.5℃(R22/-40.7℃)，但空(kong)氣(qi)冷(leng)(leng)卻式冷(leng)(leng)凝器(qi)傳熱溫(wen)(wen)(wen)差(cha)通(tong)常取(qu)10℃左右(在(zai)(zai)強制(zhi)送風散熱循環下(xia)，蒸(zheng)(zheng)發(fa)器(qi)和(he)內箱的溫(wen)(wen)(wen)差(cha))，就是(shi)說箱內只能制(zhi)取(qu)-36.5℃的低(di)(di)(di)溫(wen)(wen)(wen)，當然，通(tong)過調(diao)低(di)(di)(di)壓縮機的蒸(zheng)(zheng)發(fa)壓力，可以將R404A制(zhi)冷(leng)(leng)劑(ji)的低(di)(di)(di)蒸(zheng)(zheng)發(fa)溫(wen)(wen)(wen)度(du)(du)降低(di)(di)(di)到-50℃;所以要獲(huo)取(qu)-50℃及以下(xia)的低(di)(di)(di)溫(wen)(wen)(wen)時(shi)必須采用(yong)(yong)中(zhong)溫(wen)(wen)(wen)制(zhi)冷(leng)(leng)劑(ji)與低(di)(di)(di)溫(wen)(wen)(wen)制(zhi)冷(leng)(leng)劑(ji)復疊(die)式的制(zhi)冷(leng)(leng)循環，制(zhi)取(qu)-50℃~-80℃的低(di)(di)(di)溫(wen)(wen)(wen)，低(di)(di)(di)溫(wen)(wen)(wen)制(zhi)冷(leng)(leng)劑(ji)一(yi)般選用(yong)(yong)R23它在(zai)(zai)一(yi)個大(da)氣(qi)壓下(xia)的蒸(zheng)(zheng)發(fa)溫(wen)(wen)(wen)度(du)(du)是(shi)-81.7℃。

　　2、單級壓(ya)縮蒸氣制冷(leng)循環壓(ya)比的限制

　　單級(ji)蒸(zheng)氣壓(ya)(ya)(ya)(ya)(ya)縮式制(zhi)(zhi)冷機(ji)的低蒸(zheng)發溫(wen)度(du),主要取決(jue)于(yu)它的冷凝(ning)壓(ya)(ya)(ya)(ya)(ya)力(li)(li)(li)及壓(ya)(ya)(ya)(ya)(ya)縮比，制(zhi)(zhi)冷劑的冷凝(ning)壓(ya)(ya)(ya)(ya)(ya)力(li)(li)(li)由制(zhi)(zhi)冷劑的類別和環(huan)境介質(如空氣或(huo)水)的溫(wen)度(du)決(jue)定，在(zai)通常(chang)情況下(xia)，它處于(yu)0.7~1.8Mpa范圍(wei)內，壓(ya)(ya)(ya)(ya)(ya)縮比與冷凝(ning)壓(ya)(ya)(ya)(ya)(ya)力(li)(li)(li)和蒸(zheng)發壓(ya)(ya)(ya)(ya)(ya)力(li)(li)(li)有關，當冷凝(ning)壓(ya)(ya)(ya)(ya)(ya)力(li)(li)(li)一(yi)定時(shi),隨著蒸(zheng)發溫(wen)度(du)的降低，蒸(zheng)發壓(ya)(ya)(ya)(ya)(ya)力(li)(li)(li)也(ye)相應(ying)下(xia)降，因而使壓(ya)(ya)(ya)(ya)(ya)縮比上升，它將(jiang)引(yin)起壓(ya)(ya)(ya)(ya)(ya)縮機(ji)排氣溫(wen)度(du)的升高，潤滑油變稀(xi)，使潤滑條件變壞，嚴重時(shi)甚(shen)至會(hui)出現(xian)結炭和拉(la)缸現(xian)象;另一(yi)方面,壓(ya)(ya)(ya)(ya)(ya)縮比的增大將(jiang)導致壓(ya)(ya)(ya)(ya)(ya)縮機(ji)的輸(shu)氣系數(shu)降低，制(zhi)(zhi)冷量減(jian)少(shao)，實際壓(ya)(ya)(ya)(ya)(ya)縮過程偏離等熵過程越(yue)遠(yuan)，壓(ya)(ya)(ya)(ya)(ya)縮機(ji)功耗增加，制(zhi)(zhi)冷系數(shu)下(xia)降，經濟(ji)性降低，將(jiang)出現(xian)以下(xia)一(yi)些(xie)影響。

　　3、任何制冷(leng)劑,蒸發(fa)溫度(du)越(yue)低(di)(di),則蒸發(fa)壓力(li)也(ye)就越(yue)低(di)(di).過(guo)低(di)(di)的(de)蒸發(fa)壓力(li),有時可能造成(cheng)壓縮機難以吸(xi)氣,或者(zhe)使外界的(de)空氣進入制冷(leng)系統.

　　4、當(dang)蒸發(fa)溫度過低時,某些(xie)常用制冷劑已達凝固(gu)溫度,無法實(shi)現制冷劑的流(liu)動,循環.

　　5、蒸發壓(ya)力降低,制(zhi)冷劑(ji)的比體(ti)(ti)積增大,制(zhi)冷劑(ji)的質量流量減少,制(zhi)冷量大大下降.為了獲(huo)得所需制(zhi)冷量,必(bi)須增大吸氣容積,使壓(ya)縮機體(ti)(ti)積過于龐(pang)大.

　　6、壓縮機線圈散熱(re)的(de)限制

　　單級(ji)壓(ya)(ya)(ya)縮(suo)(suo)機(ji)工作時，在做(zuo)-35℃左(zuo)右，因為壓(ya)(ya)(ya)縮(suo)(suo)機(ji)的(de)線圈(quan)是(shi)(shi)旋空(kong)在壓(ya)(ya)(ya)縮(suo)(suo)機(ji)中間的(de)，這就(jiu)產生(sheng)(sheng)一(yi)個問(wen)題(ti)，-35℃時，壓(ya)(ya)(ya)縮(suo)(suo)機(ji)的(de)低壓(ya)(ya)(ya)是(shi)(shi)為負數值，也就(jiu)是(shi)(shi)產生(sheng)(sheng)了一(yi)個真(zhen)空(kong)度，這樣線圈(quan)的(de)頂端熱(re)量就(jiu)沒有(you)辦(ban)法散(san)去，這樣就(jiu)壓(ya)(ya)(ya)縮(suo)(suo)機(ji)表面是(shi)(shi)十分涼，可是(shi)(shi)實(shi)際上內部，他的(de)溫度是(shi)(shi)很高(gao)的(de)(因為真(zhen)空(kong)是(shi)(shi)的(de)隔熱(re)介質)。

　　由上可(ke)見:恒溫恒濕試驗針對-40℃機型可(ke)以(yi)采用單級制(zhi)冷(leng)循(xun)環，也可(ke)以(yi)采用復疊式制(zhi)冷(leng)循(xun)環系統(tong)，但(dan)單級制(zhi)冷(leng)循(xun)環是(shi)(shi)靠調小壓(ya)(ya)縮機的膨脹閥開啟度，減小制(zhi)冷(leng)劑流(liu)量(liang)限(xian)流(liu)來(lai)(lai)調低蒸發(fa)壓(ya)(ya)力(li)(約(yue)0.7個大(da)氣壓(ya)(ya))，從而獲得更低的蒸發(fa)溫度的，這樣的設計(ji)是(shi)(shi)以(yi)犧牲系統(tong)的制(zhi)冷(leng)量(liang)來(lai)(lai)達到的(制(zhi)冷(leng)量(liang)約(yue)只有標準的0.7~0.8)，導致(zhi)制(zhi)冷(leng)效(xiao)率低并(bing)加(jia)大(da)了(le)壓(ya)(ya)縮機的負載，而且易(yi)引起(qi)壓(ya)(ya)縮機線圈過熱，影響了(le)壓(ya)(ya)縮機的壽命。