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Brij Bhooshan Asst. Professor B.S.A College of Engg. & Technology, Mathura (India)

Copyright by Brij Bhooshan @ 2013

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Please welcome for any correction or misprint in the entire manuscript and your

valuable suggestions kindly mail us [email protected].

1. A room 7m x 4m x 4m is occupied by an air-water vapour mixture at 38°C. The

atmospheric pressure is 1 kgf/cm2 and the relative humidity is 70%. Determine the

humidity ratio, dew-point, mass of air and the mass of water vapour. If the mixture

of air-water vapour is further cooled at constant pressure until the temperature is

10°C, find the amount of water which condensed Assume: Rair/Rvapour = 0.622 and

Rair = 29.2 kg-m/kg-K.

2. Describe a summer air conditioning system with ventilation. An air handling unit

in air conditioning plant supplies a total of 4500 m3/s of dry air which comprises by

weight 20% fresh air at 40° C DBT and 27° C WBT and 80% re-circulated air at 25°

C DBT and 50% RH. The air leaves the cooling coil at 13° C saturated. Calculate

the total cooling load and room.

The following data can be used:

Condition

DBT WBT RH Sp. Hu. Entholpy

°C °C % gm of water vapour

kg of dry air kcal/kg d.a.

Outside 40 27 17.2 20.3

Inside 25 50 10.0 12.15

ADP 13 100 9.4 8.85

Sp volume of air entering the cooling coil = 0.869 m3/kg d.a.

3. A summer air conditioning system is to be designed for a hall using the following




2 Problems of Practices on Psychrometrics

data:

Outside design conditions = 35° C DBT and 27.2°C WBT

Inside design conditions = 25°C DBT and 50% RH

Solar heat gain through walls, roof and floor = 6000 kcal/hr

Solar heat gain through glass = 5500 kcal/hr

Number of occupants = 38

Latent heat gain per person = 90 kcal/hr

Sensible heat gain per person = 75 kcal/hr

Internal lighting load = 20 lamps of 100 W each and 15

flourescent tubes of 80 W each

Sensible heat gain from other sources = 15,000 kcal/hr

Infiltration air = 21 m3/min

40% fresh air and 60% re-circulated air are mixed and passed through the

conditioner coil which has a dew point temperature of 7°C.

Sketch the block diagram of the system and show the processes on skeleton

psychrometric chart, giving enthalpy values at each salient point and calculate-

a) the amount of total air in kg/hr;

b) DBT and RH of supply air in the hall;

c) the capacity of conditioner in tons of refrigeration;

d) By pass factor of the coil.

4. Atmospheric air at 12°C and relative humidity of 25 percent is to be conditioned to

5 kg/kga as it enters an insulated room with a flow rate of 60 cm3/min. Assuming

that the humidifying water is at 12°C determine the

(i) relative humidity;

(ii) temperature of the conditioned air;

(iii) heat transfer per kg of dry air

for the cases of-

(1) constant dry bulb temperature;

(2) constant relative humidity;

(3) adiabatic evaporative process.

Show the three processes on the psychrometric plot.

5. An air-conditioned space is maintained at 27°C dry bulb and 50 percent relative

humidity. The ambient conditions are 40°C dry bulb and 27°C wet bulb. The space

has a sensible heat gain of 14 kW. Air is supplied to the space at 7°C saturated.

Calculate:

(i) mass of moist air supplied to the space in kg/h;

(ii) latent heat gain of space in kW;

(iii) Cooling load of the air washer in kW if 30 per cent of the air supplied to

the space is fresh, the remaining being re-circulated.

6. An air-water vapour mixture at 20°C and 1 atmosphere has a relative humidity of

50%. Calculate:

(i) the partial pressure of water vapour.

(ii) the dew-point.

(iii) the specific humidity.

Also calculate the amount of water vapour condensed per kg of dry are if the

mixture is cooled to 5°C in a constant pressure process.

7. A mixture of air and water vapour at 1.01325 bar.16 °C has dew point of 5 °C.

Calculate the relative humidity and specific humidity.




3 Problems of Practices on Refrigeration & Air-Conditioning By Brij Bhooshan

8. In an air-conditioning plant equipment exists for reheating the air before supply to

a room to be maintained at 25° dbt with 55% R.H. The RSHL= 150,000 W and

RLHL = 122,000 W.

The discharge from cooling coil is 12°C dbt, 95% R.H. Calculate:

(i) the supply air state and rate

(ii) reheating, if any.

9. In an air-conditioning system 30 m3/min of fresh outdoor air is introduced at 43°C

dry-bulb temperature and 30% relative humidity. The remaining air is re-

circulated from the room maintained at 25°C dry-bulb temperature and 50% RH.

The by-pass factor of the cooling coil is 0.15 and apparatus dew point is 11.8°C.

RSH = 100 kW and RLH = 15 kW. Determine (i) humidity ratios for outdoor and

room condition, (ii) OASH and OALH, (iii) ERSH and ERLH, (iv) supply air

temperature, (v) supply air volume flow rate using ERSH and (vi) temperature at

inlet to cooling coil by assuming the density of outdoor air and re-circulated air to

be the same.

10. Given that saturation pressures of water at 25° C and 43° C are 3.1693 kPa and

8.6495 kPa respectively. Show that process on psychometric chart schematically.

Standard atmospheric pressure = 1.01325 bar.

11. The air handling unit of an air conditioning plant supplies a total 4500 cubic

meter/min of dry air which comprises 20 percent fresh air at 40° C DBT and 27° C

WBT and 80 percent of recirculated air at 25° C DBT and 50% RH. The air leaves

the cooling coil at 13° C saturated state. Determine the total cooling load and the

room heat gain.

Take specific volume of air entering the cooling coil as 0.869 m3/kg da.

Condition DBT WBT RH % Sp. humidity Enthalpy

°C °C gwv/kg da kJ/kg da

Outside 40 27 — 17.2 85.0

Inside 25 — 50 10.0 50.8

ADP 13 — 100 9.4 37.0

Note: ADP- Apparatus dew point; wv-water vapour; da- dry air

12. An air conditioning plant is designed to maintain a room at a condition of 20° C dry

bulb temperature and specific humidity 0.0079 kg/kg dry air when the outside

condition is 30° C dry bulb temperature and 40% saturation. The corresponding

heat gains are 18000 W (sensible) and 3600 W (latent). The supply air contains

one-third outside air by mass and the supply temperature is to be 15° C dry bulb

temperature.

The plant consists of mixing chamber for fresh and re-circulated air, an air washer

with chilled spray water with an efficiency of 80%, an after heater battery and a

supply fan. Neglecting temperature changes in fan and ducting, calculate:

(i) the mass flow rate of supply air necessary;

(ii) the specific humidity of the supply air;

(iii) the cooling load on the washer;

(iv) the heating load on the after heater.

13. 10 cu m of atmospheric air at 25 °C DBT and 12°C WBT is flowing per minute

through a duct. Dry saturated steam at 100 °C is injected through the duct. Dry

saturated steam at 100°C is injected into the air stream with a rate of 1.2 kg/min.

(i) What is the temperature of air after mixing the steam?

(ii) What is the relative humidity of air after mixing the steam?

14. A customer complained of poor cooling for an air-conditioning system of 100 TR





capacity. The supplier carried out test on condenser which is water cooled and

noted power input to the motor. The observations made are as under:

Cooling water flow rate: 10 liter/sec

Inlet water temperature: 30°C

Outlet water temperature: 41.12°C

Power input to motor: 120 kW (94.92 % efficiency)

Determine the actual refrigerating capacity and state whether the cooling capacity

is lower, higher or as per specifications.

15. Develop an expression for enthalpy of moist air per unit mass of dry air as under:

h = 1.005 t + w(2500 + 1.88 t) kJ/kg

where t is DBT and w is humidity ratio.

16. Moist air enters a chamber at 5°C DBT and 2.5°C WBT at a rate of 90 m3/min. The

barometric pressure is 1.01325 bar. While passing through the chamber, the air

absorbs sensible heat at the rate of 40.7 kW and picks up 40 kg/hr of saturated

steam at 110°C. Determine the dry and wet bulb temperatures of leaving air.

Properties of steam are given below:

Enthalpy of saturated steam at 110°C is 2691.3 kJ/kg.

17. The outdoor summer condition for a Bank for one hundred persons is Tdb = 310 K,

and Twb = 300 K. The required inside conditions are Tdb = 295 K and φ = 60%. The

room sensible heat is 4,00,000 kJ/hr. The room latent heat is 2,00,000 kJ/hr.

Ventilation requirement per person is 0.0047 m3/hr. The By-pass factor is 0.15.

Evaluate:

(i) Grand total heat

(ii) Effective sensible heat factor (ESHF)

(iii) Apparatus dew point

(iv) Volume flow rate of dehumidified air.

18. 20 m3 of air per minute at 30° C DBT and 60% RH is sensibly cooled to 22° C DBT.

Take saturation pressure of water vapour at 30° C and 22° C temperatures to be

0.425 bar and 0.0265 bar respectively. Find heat removed from air. 20 Take

atmospheric air pressure Pb = 1 bar.

19. Illustrate the following processes on psychometric chart with initial state of moist

air as dry bulb temperature equal to 20°C and relative humidity of 50%:

(i) Cooling and humidification

(ii) Heating and humidification

(iii) Humidification at constant dry bulb temperature.

Mention one application each for above humidification processes. If the moist air

leaves the system for the case (iii) above, at 90% relative humidity, determines per

unit mass of dry air:

(i) increase in humidity ratio, AW

(ii) increase in enthalpy, Ah

(iii) increase in dry bulb temperature, At

(iv) Determine Sensible Heat Factor (SHF) for this process.

20. It is desired to maintain a room at temperature of 20°C when outside temperature

is 30°C. The volume of this room is 300 m3. The pressure in the room and outside is

1 bar.

The air in the room is renewed completely in 1 hour. Calculate the mass of air that

the air conditioning system pumps into the room.

(i) Compute maximum possible CO.P. of this A/C system.





(ii) When this A/C system is switched off, the temperature inside the room

reaches 21°C in 20 minutes. Calculate the amount of heat transferred

from the surroundings to the room.

(iii) Calculate power required by this system.

21. Cold water bottles kept in a room often start condensing atmospheric moisture.

What is the minimum temperature to which water bottle can be cooled without any

dripping of moisture from its surface when kept in a room at 25°C. DBT and 60%

RH? If it is desired that a water bottle at 10°C should also not condense moisture

on its surface, what should be the RH in the room keeping the DBT the same?

22. In a winter heating air conditioning system 1 kg/s of ambient air at 7°C DBT, 80%

RH is sensibly heated, then adiabatically saturated to a high humidity of RH -

90%, and then further sensibly heated to the desired outlet state of 25°C DBT and

50% RH. Determine the temperature of air at various state points and show all the

processes on a psychometric chart. Also determine the amount of moisture added

per hour in the adiabatic saturator.

23. An air-conditioning system (see figure) operates at a total pressure of 1 atm. It consists of a

heating section and a humidifier that supplies wet steam (saturated) at 100°C. Air enters

the heating section at 10°C and 70 percent relative humidity at the rate of 35 m3/min. It

leaves at 20°C and relative 60% relative humidity, Determine

(i) temperature and relative humidity of air when it leaves the heating section,

(ii) the rate of heat transfer to the heating section, and

(iii) The rate at which water is added to the air in the humidifying section.

Also draw skeleton Psychometric chart representation showing the process.

24. In the event of failure of heaters in a spacecraft, heat is lost by radiation at the rate of 100 kJ/hr while

electronic instruments generate 75 kJ/hr inside the spacecraft. Initially the air inside the spacecraft is at 1 bar,

25°C with a volume of 10 m3. How long it will take to reach air temperature of 0°C?

25. What is the significance of by-pass factor? For a heating coil, derive an expression

of by-pass factor. Find the expression for efficiency also for heating coil.

26. 100 m3 of air per minute at 15 °C DBT and 80% relative humidity is sensibly

heated until its temperature becomes 22 °C. Saturation pressures of water vapour

at 15 °C and 22 °C are 0.017 bar and 0.02645 bar respectively. Find heat added to

air per minute. Take atmospheric pressure =1.013 bar.

27. Show that the enthalpy of humid air per kg of dry air is given by

ℎ = 𝐶𝑝𝑚 × 𝐷𝐵𝑇 + 2500𝑤

where CPm = humid air specific heat = (1.005 + l.88w), w = specific humidity kg/kg

of dry air, hfg = 2500 kJ/kg at 0 °C for water and DBT = dry-bulb temperature.

28. The air handling unit in an AC plant supplies a total of 4500 cmm of dry air which

comprises by weight 20% fresh air at 40 °C DBT, 27 °C WBT and 80% recirculated

air at 25 °C DBT and 50% RH. Air leaves the cooling coil at 13 °C saturated.

Calculate total cooling load and room heat gain:





Condition DBT WBT RH Sp. humidity Enthalpy

(°C) (°C) (%) (kg/kg of dry air) (kJ/kg of dry air)

Outside air 40 27 17.2 85

Room air 25 50 10.0 50.8

ADP 13 100 9.4 37.0

29. In a drier operating at steady state atmospheric air at 30°C and 101.325 kPa with

a relative humidity of 40% is first heated to 110°C at constant pressure. The

heated air is then allowed to pass over the material to be dried and the air leaves

the drier at 70°C with a relative humidity of 0.5. If it is required to remove 60

kg/min of moisture from the material, determine the mass flow rate of dry air

required in kg/min and the rate of heat transfer to the air as it passes through the

heating unit. Use hg 1110bar

C = 2696.12 kJ/kg.

30. Air enters a cooling coil at 30 °C, 75% relative humidity. The apparatus dew point

is 12 °C and bypass factor is 0.15. Find the temperature and humidity ratio at

outlet of cooling coil, if mass flow rate of air is 10 kga/s, find the condensate rate

and cooling capacity of cooling coil. The partial pressures of water vapour at 12 °C

and 30 °C are 1.4017 and 4.2431 kPa respectively. Atmospheric pressure is 101.325

kPa. Enthalpy of condensate at 12 °C = 50.24 kJ/kg.

31. Define thermodynamic wet bulb temperature t* and show that humidity ratio may

be expressed as

*

*

)(

*)(005.1*

fg

fg

hth

tthWW

where enthalpy of moist air is expressed as h = 1.005 (t) + hg (t).

32. The air at condition 17°C and relative humidity 60% is passed over a cooling coil at

the rate of 0.5 m3/sec. The temperature of cooling coil is 6°C. Calculate the rate of

water vapour condensed. Assume that the barometric pressure is 1.01325 bar and

the air leaving the coil is at 9°C and 90% relative humidity. Determine also its by-

pass factor.

33. Dry bulb temperature, t and wet bulb temperature, tw of moist air measured by a

sling type psychrometer are 40° C and 26° C respectively. Total pressure p of moist

air is 1.02 bar.

For a sling type psychrometer:

Pw = Pws PA(ta tw)

where ta - dry bulb temperature, tw - measured wet-bulb temperature, P - total

pressure, A is a constant = 7 × 10–4 (1/°C), Pw - partial pressure of water vapour in

moist air Pws - partial pressure of water vapour in saturated air at measured wet

bulb temperature tw.

Assume gas constant for water vapour 461 J/kg-K. Determine using only steam

tables and properties of dry air.

(i) specific humidity,

(ii) relative humidity,

(iii) dew point temperature,

(iv) specific enthalpy,

(v) degree of saturation,

(vi) specific volume, and

(vii) define thermodynamic wet bulb temperature and discuss the procedure for

determining the thermodynamic wet bulb temperature in the present case.





34.

Show that

w

w

PP

PW 622.0 and

)622.0(

)622.0(

WW

WW

S

S

where Pw and P are partial pressure of water vapour and total pressure

respectively, WS is humidity ratio at saturation and φ is relative humidity.

Molecular weights of air and water vapour are 28.966 and 18.016 kg/kg-mole

respectively.

35. In an air-conditioned space, 5 kg/s of fresh air at 45 °C, 30% relative humidity

mixes with 45 kg/s of re-circulated air at 25 °C, 50 RH. The mixed air flows over a

cooling coil whose apparatus dew point is 12 °C and bypass factor is 0.15.

Determine the conditions at the outlet of cooling coil RSH and RLH.

The saturation pressures of water vapour at required temperatures are

12 °C 0.014016 bar

25 °C 0.03166 bar

45 °C 0.09584 bar

Empirical equation for enthalpy of moist air is

h = l.005t + W (2500 + l.88t) kJ/kg, P = 1.01325 bars.

36. Calculate the following properties of moist air at dry bulb temperature = 30°C,

total pressure = 1.01325 bar and relative humidity = 55% -

(i) Humidity ratio; (ii) Degree of saturation; (iii) Dew point temperature (iv)

Enthalpy of moist air and (v) Specific volume

Assume moist air to be a perfect gas mixture. Molecular weight of air Ma = 29,

Molecular weight of water, Mw = 18. Universal gas constant = 8.314 kJ/kg mole-K,

Specific heat of dry air = 1.005 kJ/kg-K.

Thermodynamic properties of saturated water and steam are as follows:

tsat Psat vg hf hg

(°C) (kPa) m3/kg kJ/kg kJ/kg

0.01 0.611 206.2 0 2501.6

20 2.34 57.8 83.9 2538.2

25 3.17 43.4 104.8 2547.3

30 4.246 32.9 125.7 2556.4

Assume specific heat of superheated vapour = 1.88 kJ/ kg-K.

36. In a solar water-heating system, a flat-plate solar collector with no cover plate is

used to collect the solar radiation to heat water. The surface emissivity of the

absorber is 0.15 while the solar absorptivity is 0.96. At a given time of the day, the

absorber temperature is 120 °C when the solar irradiation is 800 W/m2, the

effective sky temperature is 6 °C. The ambient temperature is 27 °C. Assume that

the heat transfer convection coefficient for the calm dry condition is given by

h = 0.23 (TS T∞)1/3 W/m2-K

Assume steady-state condition, bottom surface is well-insulated and the absorber

surface is diffused. Work out the following:

(i) Sketch the system and its control volume

(ii) The useful heat removal rate in W/m2 from the collector

(iii) The efficiency of the collector

(iv) State effect on collector efficiency with reasoning, if the cover plate is installed.

37. For an office of 6m × 3m × 4m size, the inside and outside conditions are dbt =

22°C, φ = 55% (h = 45 kJ/kg) and dbt = 40 °C, wbt = 26 °C (h = 80.5 kJ/kg and v =





0.94 m3/kg of dry air) respectively. Further, it may be presumed that the office has

structural load of 6000 kJ/h, 5 tube lights each of 40 W ratings, 13.5 air changes

per 24 hour for the infiltration load, 71 × 10–1 m3/s per person of ventilation, 10

persons occupancy and each occupant releases 500 kJ/h.

Work out the capacity of a window air-conditioner that will suffice to achieve the

desired objective.

38. A steam power plant employs a wet type cooling tower which receives warm water

at 30 °C at the rate of l.2 kg per kg of air. Air enters the tower at the dbt of 20 °C

and relative humidity of 60% and leaves it at a dbt of 28 °C and 90% relative

humidity. Make-up water is added at 20 °C. Calculate -

(i) the temperature of water leaving the tower;

(ii) the approach and range of the cooling tower;

(iii) the fraction of water evaporated.

For inlet (1) and outlet (2) air

twb1 = 15.2 °C

twb2 = 26.7 °C

h1 = 43 kJ/kg dry air

h2 = 83.5 kJ/kg dry air

w1 = 0.00881 kg water vapour/kg dry air

w2 = 0.02132 kg water vapour/kg dry air

hv = 84 kJ/kg

39. Cold air is to be supplied for a library at a rate of 100 m3/min. The air is to enter

the building at 20°C with 40% relative humidity. Suppose the atmospheric air is

available at 35°C with 70% relative humidity suggest a suitable air-conditioning

system and estimate the rates of cooling and heating required.

40. Define Effective Sensible Heat Factor (ESHF). How can ADP be determined with

its help?

The following data refer to a public hall:

Total sensible heat = 105 kJ/h

Total latent heat = 60000 kJ/h

The room design condition Tdb = 27 °C and = 60% .

The temperature rise of air (i.e., difference between the room design condition and

supply air to the room) is 8 °C.

Find the SHF, show the process on the psychrometric chart and also calculate the

volume of air supplied to room using approximate expression based on sensible

heat and enthalpy drop.

41. In an industrial evaporative cooling application with summer outdoor design

conditions of 40°C DBT, 27°C WBT; the indoor is to be maintained at a maximum

relative humidity of 55%. The room sensible heat is 581.5 kW. All outdoor air must

be used. Find the room dry bulb temperature and supply air quantity as a function

of humidifying efficiencies of 80 and 100%. Comment on the DBT in the room and

supply air quantity in both these cases.

42. A laboratory having an unusually large latent heat gain is required to be air-

conditioned.

The design conditions and the loads are as under:

Summer outdoor design conditions : 40° DBT, 27°C WBT

Indoor design conditions : 25°C DBT, 50% RH

Room Sensible Heat (RSH) : 34.9 kW

Room Latent Heat (RLH) : 18.6 kW

The ventilation air requirement is 85 m3/min. Determine the following:





(i) Ventilation load

(ii) Room Sensible Heat Factor (RSHF) and Effective Sensible Heat Factor

(ESHF)

(iii) Apparatus Dew Point (ADP) and amount of reheat for economical design

(iv) Supply air quantity

(v) Condition of air entering and leaving coil and supply air temperature.

(vi) Grand Total Heat.

Assume a suitable bypass factor. Solve this problem using psychrometric chart.

43. The moist air at 25 °C, total pressure of 100 kPa and 50% RH is compressed to 50

°C, total pressure of 300 kPa and then cooled at constant pressure. At what

temperature will water begin to condense? Show the process on psychometric

chart.

44. An air-conditioned room is maintained at 25 °C DBT and 50 percent RH, The

ambient conditions are 40 °C DBT and 27 °C WBT. The air-handling unit supplies

a total of 4500 cmm of dry air which comprises, by weight, 20 percent fresh air and

80 percent recirculated air at the room conditions. The air leaves the cooling coil at

13 °C saturated state. Calculate:

(i) Fresh air load

(ii) Room heat gain

(iii) Total cooling load.

Draw the configuration and show the process on psychrometric diagram.

45. It is desired to maintain a hall at 23°C DBT and 60% R.H. The atmospheric

conditions of air are 42°C DBT and 22% R.H. Suggest an air-conditioning system

and explain the working of it. Represent the process on psychrometric chart and

express equations for capacity of the equipment used.

46. A drying room is to be maintained at 32°C and 30% R.H. The sensible heat gain to

the room is 150000 kJ/hr. The moisture to be evaporated from the objects during

drying is 18 kg/hr. If there is no direct heat source to provide for evaporation in the

room, calculate the state and rate of supply air at 15°C d b.t.

47. Moist air is heated by steam condensing inside the tubes of a heating coil as shown

by Figure. Part of the air passes through the coil and part is by-passed around the

coil. Barometric pressure is 1 bar.

Determine:

(i) the air per min (in 2) which by-pass the coil and

(ii) the heat added by the coil

48. Air at 0C and 95% R.H. has to be heated and humidified to 25C and 40% R.H. by

preheating, adiabatic saturation in a recirculated water air-washer and then

reheating to final state. Calculate heating required in two heaters, make-up water

required in washer and temperature of air washer.

49. A small auditorium is required to be maintained at 22°C dry bulb temperature and

70% relative humidity. The ambient conditions are at 30°C dry bulb temperature

and 75% relative humidity. The amount of free air circulated is 200 m3/min. The





required conditions are achieved by first cooling and dehumidifying through a

cooling coil having apparatus dew point of 14⁰C and then by heating. With the help

of psychrometric chart find.

(i) The cooling capacity of cooling coil in tons of refrigeration and its bypass

factor.

(ii) The amount of water vapour removed by the cooling coil in kg/h.

Assuming the bypass factor as 0.2. determine the capacity of the heating coil in kW

and its surface temperature.

50. Outside air at 35°C, 60% RH is passed over a cooling coil having an apparatus dew

point of 10°C and 0.06 bypass factor. The cooled air is then supplied to a room

which is to be maintained at 22°C dbt and 60% RH. The supply air rate 4500 kg/hr.

Estimate the sensible and latent heat loads of the room and the heat removed in

the cooling coil, in kW.

51. Atmospheric air at 40° C dry bulb temperature and 55% relative humidity is

passed over a cooling coil at the rate of 600 m3/min. At exit from the coil the air is

saturated and its temperature is 8° C. The condensate also leaves at 8° C

Determine the amount of condensate leaving the coil per minute and the

refrigeration required in kW. The specific heat of superheated steam may be

assumed to be 1.88 kJ/kg. K. Solve the problem without the use of Psychrometic

chart.

52. The properties of air in a room are temperature 35° C and relative humidity 70

percent. Determine

(i) the partial pressure of air,

(ii) the humidity ratio,

(iii) the saturation ratio,

(iv) dew point temperature,

(v) density of the air-vapour mixture,

(vi) the enthalpy of mixture per kg of dry air.

Assume room pressure to be 1.0132 bar and the specific heat for the super-heated

vapour as 1.863 kJ/kg K. Solve the problem with the help of steam tables.

53. 300 m3/min of air at 10° c dry bulb temperature and 90% relative humidity is to be

heated and humidified to 35°C drybulb temperature and 22.5°C wet bulb

temperature. The required conditions are achieved by heating, humidifying and

then again by heating. The relative humidity of the air coming out of the

humidifier is 90%. Find

(i) the heating capacity of the first heater and the by-pass factor if the surface

temperature of the coil is 40° C

(ii) the capacity of the humidifier in kg/hour

(iii) the heating capacity of the second heater and the coil surface temperature if

the bypass factor is 0.5, and

(iv) the humidifying efficiency of air washer (humidifier).

54. The following date refers to design of an air conditioning system of a theatre:

Total seating capacity: 600 persons

Sensible heat per person: 200 kJ/hr

Latent heat per person: 130 kJ/hr

Solar heat gain through glass, doors and walls: 2,50,000 kJ/hr

Outside design condition: 40° C DBT, 27° C WBT

Inside design condition: 24° C DBT, 50% RH

Apparatus dew point temperature of cooling oil: 4°C

50% of return air is recirculated and it is mixed with fresh air before entering the





cooling coil. Calculate

(i) the bypass factor of the cooling coil.

(ii) the mass flow rate of supply air to the theatre, and,

(iii) the refrigeration capacity of the coil in tons of refrigeration.

55. An air conditioning system is designed under the following conditions:

Outdoor conditions = 30°C dbt, 75% R.H.

Required indoor conditions = 22°C dbt, 70% R.H.

Amount of free air circulated = 3.33 m3/s

Coil dew point temperature = 14° C

The required condition is achieved first by cooling and dehumidification, and then

by heating. Estimate (i) the capacity of the cooling coil in tonnes, (ii) the capacity of

the heating coil in KW, and (iii) the amount of water vapour removed in kg/s.

56. 100 m3 per minute outdoor air at 43.3° C d.b.t. and 37% relative humidity is mixed

with 200 m3 per minute of air at 38.2°C d.b.t. and 24.5°C wet bulb temperature.

The mixed air is dehumidified first by a cooling coil having by-pass factor of 0.32

and apparatus dew point of 15° C and then by a chemical dehumidifier. Air leaves

the chemical dehumidifier at 30°C dry bulb temperature. Air is then passed over a

cooling coil whose surface temperature is 15° C and by-pass factor 0.26. Calculate:

(i) Capacity of two cooling coils in tons of refrigeration

(ii) Capacity of coil dehumidifier in Kg/min.

(iii) Capacity of chemical dehumidifier in Kg/min.

(iv) Total humidifying capacity of the system in Kg/min.

57. Air enters a chamber at 5°C dry bulb temperature (DBT) and 2.5°C wet bulb

temperature at the rate of 100 m3/min and at the pressure of 1 bar. While passing

through the chamber, the air absorbs 50 kW heat and picks up 35 kg/hr of

saturated steam at 110°C. Show the process on a psychrometric chart and find the

dry and wet bulb temperatures of the leaving air. At 110°C, enthalpy of saturated

steam is 2691.3 kJ/kg.

58. In an air-conditioned space, 50 kga/s of fresh air at 45'IV C dry-bulb temperature

and 30% relative humidity is introduced. The room air at 25°C dry-bulb

temperature and 50% relative humidity is recirculated at 450 kga/s. The mixed air

flows over a cooling coil which has apparatus dew point of 12⁰C and bypass factor

of 0.15. Determine the conditions at outlet of cooling coil, RSH, RLH, the cooling

load of coil and the condensate rate.

The saturation pressure of water at required temperatures are

t°C pws (bar)

12 0.014016

25 0.03166

45 0.09584

h = 1.005 t + W (2500 + 1.88 t) kJ/kga

t is in °C and kga refers to kg of dry air.

59. A spray cooling coil is used to operate under the following conditions –

Air inlet 28°DBT, 21°WBT, Air outlet 10°DBT, 6°WBT. Total air flow rate 2000

m3/mt. Chilled water inlet and outlet temperatures are 7°C and 12°C. Find

(a) Cooling load on the coil

(b) Water flow rate through the coil.

Properties of moist air from psychometric chart are as follows:

DBT WBT Sp. humidity Sp. vol. Enthalpy





gm/kg m3/kg kJ/kg

28 21.0 12.95 0.87 61.0

10 6.0 4.2 0.81 21.0

60. Calculate all the psychrometric properties of air at 1 bar and 25 °C dbt and 15 °C

wbt.

The following properties of water may be assumed:

Temp. Sat. pressure Sp. volume of vapour Enthalpy (kJ/kg)

(°C) (bar) (m3/kg) Sat. liq. Sat. vap.

25 0.03166 43.40 - 2547.3

15 0.01703 77.98 62.94 -

10 0.01078 - - -

0 0.01002 - - -

The following expression may be used, if necessary:

328.1325.12854

8.1)(,

,

dbt

satv

sattvsatvt

wbtdbtPPPP

61. One kg of air at 35°C DBT and 60% RH is mixed with 2 kg of air at 20°C DBT and

13°C dew point temperature. Calculate the specific humidity, temperature and

enthalpy of the mixture.

Assume specific heat of steam as 1.88 kJ/kg K and the following properties may be

used:

Temperature Saturation pressure Enthalpy of saturated steam

°C Ps bar hg kJ/kg

13 0.0150 2525.4

20 0.0234 2538.2

26 0.0336 2549.1

27 0.0356 2550.9

35 0.0563 2565.4

62. The following data relates to an air-conditioned space:

Outdoor condition: 38°C DBT/50% RH

Room condition: 24°C DBT/50% RH

Sensible heat load: 24 Kw

Latent heat load: 6 kW

Bypass factor of the cooling coil: 0.16

If the ventilation requirement is such that on mass flow rate basis 20% fresh air is

introduced and 80% supply air is recirculated, determine

(i) Supply air flow rate

(ii) Outside air sensible heat

(iii) Outside air latent heat

(iv) Grand total heat

(v) Effective room sensible heat factor.

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