问题 单项选择题

某车间如下图所示,侧窗进风温度tw=31℃,车间工作区温度tn=35℃,散热有效系数m=0.4,侧窗进风口面积Fj=50m2,天窗排风口面积Fp=36m2,天窗和侧窗流量系数μpj=0.6,该车间自然通风量为下列哪一项空气密度ρt=353/273+t(kg/m3)。()

A.30~32kg/s

B.42~44kg/s

C.50~52kg/s

D.72~74kg/s

答案

参考答案:B

解析:

解法一:《工业通风》、《二版教材》:h2/h1=(F2/F2)2,即:(10-h1)/h1=(50/36)2,得h1=3.34。

《二版教材》:排风温度tp=tw+(tn-tw)/m=31+(35-31)/0.4=41℃,车间内平均温度tnp=(tn+tp)/2=(35+41)/2=38℃。

《二版教材》:

不同温度下空气密度及其修正:标准状况下(0℃,101.35kPa):空气密度ρ1=29/22.4=1.293kg/m3,设计工况P、t下,密度ρ21×[273/(273+t)]×P/101.35,同《二版教材》式(2.4-15):ρt=353/273+t,则题中:ρw=1.293×[273/(273+31)]=1.161kg/m3;ρnp=1.293×[273/(273+38)]=1.135kg/m3;ρp=1.293×[273/(273+41)]=1.124kg/m3

解法二:设进风口余压p,内外密度压=1.161-1.135=0.026kg/m3

进风内外压差P=p,排风内外压差P=p+9.8×10×(0.026)=p+2.548

L=L,并取压差绝对值,0.6×50×[2×(-p)×1.161]0.5=0.6×36×[2×(2.548+p)×1.1242]

解得p=-0.85Pa。所以:L=42.1kg/s。

阅读理解

阅读理解。

     Most people think of racing when they see greyhounds (灰狗) and believe they need lots of exercise.

They can actually be quite lazy! Greyhounds are good at fast races but not long-distance running. They do

need regular exercise but they like to run for a short burst and then get back on the bed or a comfortable

seat. Another misunderstanding is that greyhounds must be aggressive (好斗的) because they are big in size.

In fact greyhounds love people and are gentle with children.

     Greyhounds can live for 12-14 years but usually only race for two or three years, and after that they make

great pets. They don't need a lot of space, don' t make a lot of noise, and don't eat a lot for their size.

     Normally, greyhounds can be as tall as 90 cm. There is, however, a small-sized greyhound, which stands

only 33 cm. Greyhounds come in a variety of colors. Grey and yellowish-brown are the most common.

Others include black, white, blue, red and brown or a mix of these.

     Greyhounds have smooth body coats, low body fat and are very healthy. Because they' re slim (苗条的)

they don't have the leg problems like other dogs the same height. But they do feel the cold, especially since

they would much rather be at home in bed than walking around outside.

1. The text is written mainly _____.

A. to tell people how to raise greyhounds

B. to let people know more about greyhounds

C. to explain why greyhounds are aggressive

D. to describe greyhounds of different colors

2. It can be inferred that greyhounds _____.

A. love big doghouses

B. like staying in bed all day

C. make the best guard dogs

D. need some exercise outdoors

3. Why does the author say that greyhounds make great pets?

A. They are big in size.

B. They live a very long life.

C. They can run races for some time.

D. They are quiet and easy to look after.

4. If you keep a pet greyhound, it is important _____.

A. to keep it slim

B. to keep it warm

C. to take special care of its legs

D. to take it to animal doctors regularly

单项选择题